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First published: February 2011 Production Reference: 1160211
Published by Packt Publishing Ltd. 32 Lincoln Road Olton Birmingham, B27 6PA, UK. ISBN 978-1-849514-48-4 www.packtpub.com Cover Image by Charwak A (charwak86@gmail.com)
Credits
Author Jason Morris Reviewers David J. Groom Martin Skans Acquisition Editor Chaitanya Apte Development Editor Reshma Sundaresan Technical Editor Harshit Shah Copy Editor Neha Shetty Indexer Tejal Daruwale Editorial Team Leader Akshara Aware Project Team Leader Priya Mukherji Project Coordinator Shubhanjan Chatterjee Proofreader Joel T. Johnson Graphics Nilesh R. Mohite Production Coordinators Kruthika Bangera Aparna Bhagat Cover Work Kruthika Bangera
About the Author
Jason Morris has worked on software as diverse as fruit tracking systems, insurance systems, and travel search and booking engines. He has been writing software for as long as he can remember. He is currently working as a Software Architect for Travelstart in South Africa. He works on multiple front-end and middleware systems, leveraging a variety of Java based technologies. The people I'd like to thank most for their direct, or indirect help in writing this book are my wife Caron Morris, my father Mike Morris, my mom Jayne Morris, and the rest of my family for their love and support. I'd also like to thank Wayne, Stuart, Angela, and James, and everyone on my team at Travelstart. Finally a very big thanks to Martin Skans for his invaluable input.
About the Reviewer
Martin Skans graduated from Lund University in Sweden, with a Master's degree in
Computer Science. After a couple of years in the online marketing industry, he moved on to become a developer for Travelstart, an online travel agency. He relocated to Cape Town and is currently working on Travelstart's African travel platform which has been recently launched for the mobile market.
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Table of Contents
Preface Chapter 1: Developing a Simple Activity
Developing our first example Creating the project structure Time for action – setting up the Android SDK Time for action – starting a new project Examining the Android project layout Time for action – running the example project The screen layout The layout XML file
Resource selection qualifiers
1 11
11 12 12 13 14 14 15 16
16
Time for action – setting up the question activity Populating a View and a ViewGroup Time for action – asking a question Time for action – adding a space for answers Time for action – adding more buttons Defining common dimensions Limitations of the layout XML format Populating the QuestionActivity Time for action – writing more Java code Dynamically creating widgets Time for action – putting the questions on the screen Handling events in Android Summary
18 19 19 21 23 25 27 29 30 32 32 34 36
Chapter 2: Presenting Data for Views
Listing and selecting data ListView choice modes
No selection mode – CHOICE_MODE_NONE Single selection mode – CHOICE_MODE_SINGLE Multiple selection mode – CHOICE_MODE_MULTIPLE
37
38 38
38 39 40
Table of Contents
Adding header and footer widgets Creating a simple ListView Time for action – creating a fast food menu Styling the standard ListAdapters
Defining standard dimensions
40 41 41 43
43
Time for action – improving the restaurant list Creating custom adapters
Creating a menu for The Burger Place
44 47
47
Time for action – creating a Burger item layout Time for action – presenting Burger objects Creating TheBurgerPlaceActivity class Time for action – implementing TheBurgerPlaceActivity Registering and starting TheBurgerPlaceActivity Using the ExpandableListView class Creating ExpandableListAdapter implementations Using the GridView class Time for action – creating the fruit icon Displaying icons in a GridView Time for action – building the fruit menu Time for action – creating the FourBucketsActivity Summary
48 50 52 53 54 56 57 58 59 60 61 62 64
Chapter 3: Developing with Specialized Android Widgets
Creating a restaurant review application Time for action – creating the robotic review project structure Building a TabActivity Creating tab icons Android tabs and icons Implementing the ReviewActivity Time for action – writing the ReviewActivity class Time for action – creating the Review layout Working with switcher classes Time for action – turning on the TextSwitcher Creating a simple photo gallery Time for action – building the Photos tab Creating a thumbnail widget Implementing a GalleryAdapter Time for action – the GalleryAdapter Time for action – making the gallery work Building the reservation tab Time for action – implementing the reservation layout Time for action – initializing the reservation tab
[ ii ]
Time for action – listening to the SeekBar Time for action – selecting date and time Creating complex layouts with Include, Merge, and ViewStubs Using Include tags Merging layouts Using the ViewStub class Summary
92 93 96 97 97 99 100
Chapter 4: Leveraging Activities and Intents
Exploring the Activity class Using Bundle objects Time for action – building an example game: "guess my number" Creating and consuming intents Defining Intent actions Passing data in an Intent Adding extra data to an Intent Using advanced Intent features Getting data back from an Intent Time for action – viewing phone book contacts Summary
103
104 105 106 110 111 112 112 113 113 114 118
Chapter 5: Developing Non-linear Layouts
Time for action – creating a layouts example project FrameLayout Common uses Time for action – developing a FrameLayout example Table Layout Common uses Using TableLayout for a memory game Time for action – developing a simple memory game AbsoluteLayout/Custom Layouts Developing your own Layouts Time for action – creating a custom layout Using the CircleLayout Time for action – finishing the CircleLayout example RelativeLayout Common uses Integrating the RelativeLayout Time for action – creating a contact editor Time for action – integration with the layout example SlidingDrawer Common uses
[ iii ]
Creating a SlidingDrawer example Time for action – creating a SlidingDrawer Time for action – sliding drawer integration Summary
147 147 148 150
Chapter 6: Validating and Handling Input Data
Dealing with undesirable input Correctly labeling input Signaling undesirable input Recovering from undesirable input Giving users direct feedback Avoiding invalid input entirely Capturing date and time Using spinners and ListView for selection
Changing the data set Disabling selections
153
153 154 154 155 155 156 156 159
159 159
Capturing text input Autocompleting text input Building activities for results Generic filtering search Activity Time for action – creating the ListItemSelectionActivity Time for action – creating an ArrayAdapter Time for action – creating the CursorAdapter Time for action – setting up the ListView Time for action – filtering the list Time for action – returning the selection Using the ListItemSelectionActivity Summary
160 160 162 162 163 164 165 169 170 171 172 174
Chapter 7: Animating Widgets and Layouts
Using standard Android animations Time for action – animating a news feed Using flipper and switcher widgets Using the ImageSwitcher and TextSwitcher implementations Animating layout widgets Time for action – animating a GridView Creating Custom Animations Time for action – writing a custom animation Time for action – making a Button vanish Summary
175
176 176 181 182 182 183 187 188 189 192
Chapter 8: Designing Content-centric Activities
Considering design options when displaying content on an Android device
[ iv ]
193
194
Table of Contents
Considering user behavior
Drawing user attention
195
196
Displaying content with the WebView class Using a WebView object Time for action – creating a recipe viewer application Taking WebView further Creating relative layouts for content display Taking full advantage of RelativeLayout Considering Android layout constraints Styling TextView objects Time for action – developing specialized content views Developing an online music store Designing the music store Developing the music store Time for action – building a track item Time for action – developing the main user interface layout Time for action – developing the main user interface Java code Summary
Chapter 9: Styling Android Applications
Working with style resources Using shape resources How shapes behave
Rendering lines
227
228 230 231
231
Time for action – drawing a broken line Rendering rectangles Time for action – creating a rounded border Rendering ovals Time for action – applying a gradient to an oval shape Rendering rings Time for action – rendering a spinner ring Defining layers Stretching using nine-patch images Creating nine-patch images Using bitmap images in Android Handling different screen sizes Handling different screen densities Handling configuration changes Providing landscape layouts Providing text input on a landscape layout Altering screen content Summary
[v]
Chapter 10: Building an Application Theme
Creating a basic calculator layout Designing a standard calculator Time for action – building the standard calculator Building the calculator styling Time for action – creating the button images Time for action – styling the calculator buttons Time for action – styling the display Scientific landscape layout Defining string resources for the scientific layout Styling the scientific layout Building the scientific layout Time for action – coding the scientific layout Handling the Activity restart Supporting hardware keyboards Adding in display animations Time for action – animating the display Summary
Preface
On 9th January, 2007, Apple officially launched the iPhone, and the world of user interface design shifted. While tablet PCs had been around for a while, the iPhone was the first device to give so many people a portable touchscreen, and people loved it. Just over a year later, Google and the Open Handset Alliance announced Android which in many ways is the direct competitor to iPhone. What is it about touchscreen phones that we love? The answer is simple—feedback. Touchscreens offer a way to directly manipulate on-screen objects, which in the past had to be driven through a keyboard, mouse, joystick, or other input device. The touchscreen model of direct manipulation has a large impact on the way we think about our user interfaces as developers, and changes the expectations a user has for the application. Touchscreen devices require us to stop thinking in terms of forms, and start thinking about object-oriented user interfaces. Android is used as the primary operating system for a rapidly expanding range of consumer electronics, including:
Smartphones Netbooks Tablets Some desktop systems
While all of these devices have different purposes and specifications, all of them run Android. This is unlike many other operating environments which are almost always have a special purpose. The services and the APIs they provide to developers generally reflect their target hardware. Android on the other hand makes the assumption that a single application may be required to run on many different types of devices, with very different hardware capabilities and specifications, and makes it as easy as possible for developers to handle the differences between these devices simply and elegantly.
Preface
New challenges
As Android and the touchscreen devices it powers become increasingly common, they will bring a new set of challenges to user interface design and development: You generally don't have a mouse You may have more than one pointing device You often don't have a keyboard Any keyboard that does exist may be a software keyboard A software keyboard may consume some of your application's screenspace
The software keyboard reduces the amount of screen space available to your application, and in much the same vein, if there is a hardware keyboard present it may or may not always be exposed to the user. Therefore, not only are different Android devices different, but they may also appear to change features while your application is running.
The rule of finger
Most Android devices have touchscreens (although this is not a requirement). The first restriction placed on any touchscreen user interface is the size of the human forefinger, which of course varies widely from one person to another. If a widget is too small on the screen, it won't be clear what the user is trying to touch. You'll notice that most Android widgets take up plenty of space, and have more than the normal amount of padding around them. On a touchscreen device, you can't rely on pixel-perfect precision. You need to make sure that when the user touches a widget, they make contact, and they don't accidentally touch another widget.
The magic touch
Another impact touchscreens have on user interface design is that an application and all the widgets that it uses must be entirely self-explanatory (even more than usual). Far too often, we substitute good user interface planning and design with a roll-over or tooltip to indicate a widget's function. On a touchscreen device, there is no mouse or pointing device. The first interaction it has with the user is when they touch it, and they will expect something to happen.
A touchy subject Most Android devices have a touchscreen, but it's not a requirement. The quality of a touchscreen also varies wildly from device to device. The category of touchscreens and their capabilities will also vary from one device to the next, depending on the intended use of the device and often its intended market segment.
[2]
Preface
A smaller view on the world
Most Android devices are small, and as a result have smaller screens and generally fewer pixels than a normal PC or laptop. This lack of size limits the size of the widgets. Widgets must be big enough to touch safely, but we also need to pack as much information onto the screen as possible. So don't give your users information that they don't want, and also avoid asking them for information you don't need.
Classic user interface principals
Here are some core guidelines which every user interface should follow. These guidelines are what will keep your users happy, and ensure your application is successful. Throughout the rest of the book, we'll be walking through these guidelines with practical examples of improvements that can be made to a user interface.
Consistency
This is the cornerstone of good user interface design. A button should look like a button. Make sure that the layout of each screen has a relationship with every other screen in your application. People often mistake this principle for "stick to the platform look and feel". Look and feel is important, consistency mostly applies to the layout and overall experience of the application, rather than the color scheme.
Recycling your interface
The easiest way to maintain a consistent user interface, is to recycle as much of it as possible. At first glance, this suggestion looks merely like a "good object-oriented" practice. However, a closer look will reveal ways to reuse graphical widgets in ways you hadn't thought of. By changing the visibility of various widgets, or you can reuse an edit screen to view list items of the intended type.
Simplicity
This is especially important in a phone-based application. Often, when a user encounters a new application, it's because they are looking for something. They may not have the time (or more often patience) to learn a new user interface. Make sure that your application asks for as little as possible, and guides the user to the exact information they want in as few steps as possible.
[3]
Preface
The Zen approach
Generally, when you are using a mobile device, your time is limited. You may also be using an application in less-than-ideal circumstances (perhaps, in a train). The lesser information a user needs to give an application, and the lesser they need to absorb from it, the better. Stripping away options and information also leads to a shorter learning-curve.
Android's hidden menu
A very useful feature of Android is the hidden menu structure. The menu is only visible when the user presses the "Menu" button, which would generally mean, they're looking for something that isn't currently on the screen. Typically, a user shouldn't need to open a menu. However, it's a good way of hiding advanced features until they are needed.
Feedback
Feedback is what makes a touchscreen device exciting. When you drag an object, it sticks to your finger across the screen until you let go of it. When the users puts their finger on your application, they expect some reaction. However, you don't want to get in their way—instead of showing an error message when they touch a button, disable the button until it's valid to use, or don't show it at all.
Location and navigation
When you're in a place you've never been to previously, it's easy to get disoriented, or lost. The same is true for a piece of software. Just because the application makes sense to you, the developer, it doesn't mean it seems logical to your user. Adding transition animations, breadcrumbs, and progress gauges help the user to identify where in the application they are, and what's happening.
The road to recovery
A common way to tell users that something is wrong on a desktop application, or on the web is to open an error dialog. On a mobile device, people want smoother use of an application. While in a normal application you may inform the user that they selected an invalid option, in a mobile application, you generally want to make sure they can't select that option in the first place. Also, don't make them scroll through huge lists of options. Instead, allow them to filter through the list using an auto-complete or something similar. When something goes wrong, be nice, and be helpful—don't tell the user, "I couldn't find any flights for your search". Instead tell them, "There were no available flights for your search, but if you're prepared to leave a day earlier, here is a list of the available flights". Always make sure your user can take another step forward without having to go "Back" (although the option to go backwards should always exist).
[4]
Preface
The Android way
The Android platform is in many ways similar to developing applications for the web. There are many devices, made by many manufactures, with different capabilities and specifications. Yet as a developer, you will want your users to have the most consistent experience possible. Unlike a web browser, Android has built-in mechanisms for coping with these differences, and even leveraging them. We'll be looking at Android from the point of view of a user rather than having a purely development-centric approach. We'll cover topics such as:
What user interface elements Android provides How an Android application is assembled Different types of Android layouts Presenting various types of data to the user Customising of existing Android widgets Tricks and tools to keep user interfaces looking great Integration between applications
We're about to take a jump into building user interfaces for Android devices—all Android devices, from the highest speed CPU to the smallest screen.
What this book covers
Chapter 1, Developing a Simple Activity introduces the basics of building an Android application, starting with a simple user interface. It also covers the various options available to you when implementing your design as code. Chapter 2, Views With Adapters shows us how to leverage Adapter-based widgets, Android's answer to the Model-View-Controller (MVC) structure. Learn about these widgets, and where they will best serve you. Chapter 3, Specialized Android Views takes a close look at some of the more specialized widgets that the Android platform provides, and how they relate to the mundane widgets. This chapter covers widgets such as the gallery and rating-bar, and how they can be used and styled. Chapter 4, Activities and Intents discusses more about how Android runs your application, and from that point-of-view, how best to write its user interfaces. This chapter takes a look at how to make sure that your application will behave the way users expect it to, with minimal effort on your part.
[5]
Preface
Chapter 5, Non-Linear Layouts takes a look at some of the advanced layout techniques which Android offers. It talks about the best way to present different screens to the user while taking into account the wide discrepancy in the screens on Android devices. Chapter 6, Input and Validation provides tips regarding taking input from a user, and how to keep this experience as painless as possible. This chapter investigates the different input widgets Android provides and how to configure them best, depending on the situation. Also, when everything else fails, how best to inform your users that what they are doing is wrong. Chapter 7, Animating Widgets and Layouts will inform the reader as to where, when, why, and how to animate your Android user interfaces. It also sheds light on what kind of animations are provided by default, how to compose them together, and how to build your own. This chapter looks at the importance of animations in a mobile user interface and demonstrates how complex animations are made easy by Android. Chapter 8, Content-centric Design details how to go about designing the screen layout, when presenting the user with information on the screen. This chapter looks at the pros and cons of some of the different display techniques which Android offers. Chapter 9, Styling Android Applications shows us how to keep the look of our entire application consistent, in order to make our application easier to use. Chapter 10, Building an Application Theme looks at the design process, and how applicationwide themes can be applied to help your application stand out.
What you need for this book
Please have a look at "System Requirements" mentioned on the Andriod Developers website at http://developer.android.com/sdk/requirements.html. The code for this book was tested on Ubuntu Linux 10.04 and Mac OS X.
This book is aimed at developers with at least some Java experience who want to build applications on the Android platform. It will also be of use to people who have developed applications on the Android platform and would like to gain additional knowledge about its user interface design. It will also be a helpful reference for the numerous widgets and resource structures that the Android platform provides.
Who this book is for
[6]
Preface
This book will also be helpful to:
Java developers learning Android development MIDP developers looking to broaden their skill-set iPhone developers wanting to port applications Entrepreneurial Android developers wanting to widen their user base
Conventions
In this book, you will find several headings appearing frequently. To give clear instructions of how to complete a procedure or task, we use:
Time for action – heading
1. 2.
Open the res/layout/main.xml layout resource in an editor or IDE. Remove the default content within the LinearLayout element.
Instructions often need some extra explanation so that they make sense, so they are followed with:
What just happened?
This heading explains the working of tasks or instructions that you have just completed. You will also find some other learning aids in the book, including:
Pop quiz – heading
These are short multiple choice questions intended to help you test your own understanding.
Have a go hero – heading
These set practical challenges and give you ideas for experimenting with what you have learned. You will also find a number of styles of text that distinguish between different kinds of information. Here are some examples of these styles, and an explanation of their meaning. Code words in text are shown as follows: "We'll start off by creating a selector Activity, and a simple NewsFeedActivity".
[7]
Preface
A block of code is set as follows:
When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:
Any command-line input or output is written as follows: android create project -n AnimationExamples -p AnimationExamples -k com. packtpub.animations -a AnimationSelector -t 3
New terms and important words are shown in bold. Words that you see on the screen, in menus or dialog boxes for example, appear in the text like this: "Generally users are more inclined to feel a sense of trust if they pick the Buy Music button and are not suddenly whisked off to their web browser".
Warnings or important notes appear in a box like this.
[8]
Preface
Tips and tricks appear like this.
Reader feedback
Feedback from our readers is always welcome. Let us know what you think about this book—what you liked or may have disliked. Reader feedback is important for us to develop titles that you really get the most out of. To send us general feedback, simply send an e-mail to feedback@packtpub.com, and mention the book title via the subject of your message. If there is a book that you need and would like to see us publish, please send us a note in the SUGGEST A TITLE form on www.packtpub.com or e-mail suggest@packtpub.com. If there is a topic that you have expertise in and you are interested in either writing or contributing to a book, see our author guide on www.packtpub.com/authors.
Customer support
Now that you are the proud owner of a Packt book, we have a number of things to help you to get the most from your purchase.
Downloading the example code for this book You can download the example code files for all Packt books you have purchased from your account at http://www.PacktPub.com. If you purchased this book elsewhere, you can visit http://www.PacktPub.com/support and register to have the files e-mailed directly to you.
Errata
Although we have taken every care to ensure the accuracy of our content, mistakes do happen. If you find a mistake in one of our books—maybe a mistake in the text or the code—we would be grateful if you would report this to us. By doing so, you can save other readers from frustration and help us improve subsequent versions of this book. If you find any errata, please report them by visiting http://www.packtpub.com/support, selecting your book, clicking on the errata submission form link, and entering the details of your errata. Once your errata are verified, your submission will be accepted and the errata will be uploaded on our website, or added to any list of existing errata, under the Errata section of that title. Any existing errata can be viewed by selecting your title from http://www.packtpub.com/support.
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Preface
Piracy
Piracy of copyright material on the Internet is an ongoing problem across all media. At Packt, we take the protection of our copyright and licenses very seriously. If you come across any illegal copies of our works, in any form, on the Internet, please provide us with the location address or website name immediately so that we can pursue a remedy. Please contact us at copyright@packtpub.com with a link to the suspected pirated material. We appreciate your help in protecting our authors, and our ability to bring you valuable content.
Questions
You can contact us at questions@packtpub.com if you are having a problem with any aspect of the book, and we will do our best to address it.
[ 10 ]
Developing a Simple Activity
In the world of Android, an Activity is the point at which you make contact with your users. It's a screen where you capture and present information to the user. You can construct your Activity screens by using either: XML layout files or hard-coded Java.
1
To begin our tour of Android user interfaces, we need a user interface to start with. In this chapter, we will begin with a simple Activity. We will:
Create a new Android project Build the Activity layout in an application resource file Tie the resource file to an Activity class Dynamically populate the Activity with a series of multiple-choice questions
Developing our first example
For our first example, we're going to write a multiple-choice question and answer Activity. We could use it for applications such as "Who wants to be a millionaire?", or "What type of a monkey are you?". This example will pose questions in order to answer a very important question: "What should I have to eat?" As the user answers the questions, this application will filter a database of food ideas. The user can exit the process at any time to view a list of suggested meals, or just wait until the application runs out of questions to ask them. Since it's a user interface example, we'll skip building filters and recipe databases. We'll just ask our user food preference-related questions. For each question, we have a list of preset answers which the user can select from (that is, multiple-choice questions). Each answer they give will allow us to narrow the list of suitable recipes.
Developing a Simple Activity
Creating the project structure
Before we can start writing code, we need a project structure. An Android project is made up of far more than just its Java code—there are also manifest files, resources, icons, and more. In order to keep things easy, we use the default Android toolset and project structure. You can download the latest version of the Android SDK for your favorite operating system from http://developer.android.com. A single Android SDK may be used to develop against any number of target Android versions. You will need to follow the installation instructions on the website at http://developer.android.com/sdk/installing.html to install the latest SDK "starter package" and one or more platform targets. Most of the examples in this book will work on Android 1.5 and higher. The Android website also maintains a very useful chart where you can see what the most popular versions of Android are.
Time for action – setting up the Android SDK
Once you have downloaded the Android SDK archive for your operating system, you'll need to install it and then download at least one Android Platform package. Open a command-line or console and complete the following steps:
1. 2. 3. 4. 5. 6.
Extract the Android SDK archive. Change directory to the root of the unpackaged Android SDK. Change directory to the tools directory of the Android SDK. Update the SDK by running the following command: android update sdk
Create a new Virtual Device by going to the Virtual Devices screen and clicking on the New button. Name the new Virtual Device default. Specify its target as the most recent version of Android downloaded by the SDK. Set the size of the SD Card to 4096 MiB. Click on the Create AVD button.
What just happened?
The above command tells the new Android SDK installation to look for available packages and install them. This includes installing a Platform Package. Each Platform Package that you install can be used to create an Android Virtual Device (AVD). Each AVD you create is much like buying a new device on which tests can be performed, each with its own configuration and data. These are virtual machines that the Android emulator will run your software on when you wish to test.
[ 12 ]
Chapter 1
Time for action – starting a new project
The Android SDK provides a handy command-line tool named android which can be used to generate the skeleton of a new project. You'll find it under the tools directory of your Android SDK. It's capable of creating a basic directory structure and a build.xml file (for Apache Ant) to help get you started with your Android application development. You will need to make sure that the tools directory is in your executable path for this to work. Open a command-line or console.
1. 2. 3.
Create a new directory in your home directory or desktop named AndroidUIExamples. You should use this directory for each of the examples in this book. Change the directory to the new AndroidUIExamples. Run the following command: android create project -n KitchenDroid -p KitchenDroid -k com.packtpub. kitchendroid -a QuestionActivity -t 3
What just happened
We just created a skeleton project. In the preceding command line, we used the following options to specify the structure of the new project:
Option -n -p -k -a Description Gives the project a name, in our case, KitchenDroid. This is really just an internal identifier for the project. Gives the base directory for the project. In this case use the same name as that of the project. The android tool will create this directory for you. Specifies the root Java package for the application. This is a fairly important concept since it defines our unique namespace on the Android client devices. Gives the tool a name for a "main" Activity class. This class will be populated with a skeleton layout XML, and serves as a base point to build your application from. The skeleton project will be pre-configured to load this Activity when it's started.
If you run the command android list targets and it presents you with an empty list of possible targets, then you have not downloaded any of the Android Platform packages. You can generally run the android tool by itself and use its graphical interface to download and install Android Platform packages. The previous example uses API Level 3 which corresponds to Android Platform version 1.5.
[ 13 ]
Developing a Simple Activity
Examining the Android project layout
A typical Android project has almost as many directories and files as an enterprise Java project. Android is as much of a framework as it is an operating environment. In some ways, you can think of Android as an application container designed for running on phones and other limited devices. As part of the new project structure, you will have the following important files and directories:
Folder name bin gen res src AndroidManifest.xml Description Your binary files will be placed in this directory by the compiler. Source code generated by various Android tools. Application resources go here, to be compiled and packaged with your application. The default Java source code directory, where the build script will look for source code to compile. Your application descriptor, similar to a web.xml file.
Resource Types and Files Most types of application resources (placed in the res directory) receive special handling by the Android application packager. This means these files consume less space than they usually would (since XML is compiled into a binary format instead of being left as plain text). You access resources in various ways, but always through an Android API (which decodes them into their original form for you). Each subdirectory of res indicates a different file format. Therefore, you cannot put files directly into the root res directory since the package tool won't know how to handle it (and you'll get a compile error). If you need to access a file in its raw state, put it in the res/raw directory. Files in the raw directory are copied byte-for-byte into your application package.
Time for action – running the example project
The android tool has given us a minimal example of an Android project, basically a "Hello World" application.
1. 2. 3.
In your console or command-line, change directory to KitchenDroid. To build and sign the project, run: ant debug
You will need to start the emulator with the default AVD you created earlier: emulator -avd default [ 14 ]
Chapter 1
4. 5.
Now install your application in the emulator: ant install
In the emulator, open the Android menu and, you should see an icon named QuestionActivity in the menu. Click on this icon.
What just happened?
The Android emulator is a full hardware emulator including the ARM CPU, hosting the entire Android operating system stack. This means software running under the emulator will run exactly how it will on bare-metal hardware (although the speed may vary). When you use Ant to deploy your Android applications, you will need to use the install Ant target. The install Ant target looks for a running emulator and then installs the application archive on its virtual memory. It's useful to note that Ant will not start the emulator for you. Instead, it will emit an error and the build will fail.
Application Signatures Every Android application package is digitally signed. The signature is used to identify you as a developer of the application, and establish permissions for the application. It's also used to establish permissions between applications. You will generally use a self-signed certificate, since Android doesn't require that you use a certificate authority. However, all applications must be signed in order for them to be run by the Android system.
The screen layout
While Android allows you to create a screen layout in either Java code, or by declaring the layout in an XML file, we will declare the screen layout in an XML file. This is an important decision for several reasons. The first is that, using the Android widgets in Java code requires several lines of code for each widget (a declaration/construction line, several lines invoking setters, and finally adding the widget to its parent), while a widget declared in XML takes up only one XML tag.
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The second reason for keeping layouts as XML is that it's compacted into a special Android XML format when it's stored in the APK file. Therefore your application uses less space on the device, takes less time to download, and its in-memory size is also smaller since less byte code needs to be loaded. The XML is also validated by the Android resource packing tool during compilation, and so is subject to the same type safety as Java code. The third reason XML layouts are a "good idea" is that they are subject to the same resource selection process as all the other external resources. This means that a layout can be varied based on any of the defined properties, such as language, screen orientation and size, and even the time of day. This means that you can add new variations on the same layout in the future, simply by adding new XML files, and without the need to change any of your Java code.
The layout XML file
All XML layout files must be placed in the /res/layout directory of your Android project in order for the Android packaging tools to find them. Each XML file will result in a resource variable of the same name. For example, if we name our file /res/layout/ main.xml, then we can access it in Java as R.layout.main. Since we are building the screen layout as a resource file, it will be loaded by the application resource loader (having been compiled by the resource compiler). A resource is subject to a selection process, so while there is only one resource that the application loads, there may be multiple possible versions of the same resource available in the application package. This selection process is also what Android internationalization is built on top of. If we wanted to build a different version of the user interface layout for several different types of touchscreens, Android defines three different types of touchscreen properties for us: notouch, stylus, and finger. This roughly translates to: no touchscreen, resistive touchscreen, and capacitive touchscreen. If we wanted to define a more keyboard-driven user interface for devices without a touchscreen (notouch), we write a new layout XML file named /res/layout-notouch/main.xml. When we load the resource in our Activity code, the resource selector will pick the notouch version of the screen if the device we're running on doesn't have a touchscreen.
Resource selection qualifiers
Here is a list of commonly used qualifiers (property names) that will be taken into account when Android selects a resource file to load. This table is ordered by precedence, with the most important properties at the top.
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Name
Description
Examples
API Level
MCC and MNC The mobile-country-code (MCC) and mobile-networkcode (MNC). These can be used to determine which mobile operator and country the SIM card in the device is tied to. The mobile-network-code optionally follows the mobilecountry-code, but cannot be used on its own (you must always specify country-code first). Language and Language and region codes are probably the most region codes commonly used resource properties. This is generally how you localize your application to the user language preferences. These values are standard ISO language and region codes, and are not case-sensitive. You cannot specify a region without a country code (similar to java.util. Locale). Screen size There are only three variations of this property: small, medium, and large. The value is based on the amount of screen space that can be used:
Small: QVGA (320×240 pixel) low-density type screens; Medium: WQVGA low-density, HVGA (480x360 pixels) medium-density, and WVGA high-density type screens; Large: VGA (640x480 pixels) or WVGA mediumdensity type screens long notlong land port night notnight 4
Screen aspect This is the aspect type of the screen, based on the way the device would "normally" be used. This value doesn't change based on the orientation of the device. Screen orientation Night mode Used to determine whether the device is currently in portrait (port) or landscape (land) mode. This is only available on devices that can detect their orientation. This value simply changes with the time of day.
1
8
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Developing a Simple Activity
Name
Description
Examples ldpi mdpi hdpi nodpi
API Level 4
Screen density The DPI of the device screen. There are four possible (DPI) values for this property:
ldpi: Low-density, approximately 120dpi; mdpi: Medium-density, approximately 160dpi; hdpi: High-density, approximately 240dpi; nodpi: Can be used for bitmap resources that shouldn't be scaled to match the screen density
Keyboard status
keysexposed What sort of keyboard is available on this device? This keyshidden attribute shouldn't be used to determine whether the keyssoft device has a hardware keyboard, but instead whether a keyboard (or software keyboard) is currently visible to the user.
1
Time for action – setting up the question activity
To kick things off we're going to be working with Android's simplest layout called: LinearLayout. Unlike Java AWT or Swing, Android layout managers are defined as specific container types. Thus a LinearLayout is much like a Panel with a built-in LayoutManager. If you've worked with GWT, you'll be quite familiar with this concept. We'll lay out the screen in a simple top-to-bottom structure (which LinearLayout is perfect for).
1. 2. 3.
Open the file in the /res/layout directory of your project named main.xml in you favorite IDE or text editor. Delete any template XML code. Copy the following XML code into the file:
What just happened?
We just removed the "Hello World" example, and put in an entirely empty layout structure which will serve as the platform for us to build the rest of the user interface upon. As you can see, Android has a special XML namespace for its resources.
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Chapter 1
All resource types in Android use the same XML namespace.
We declare our root element as LinearLayout. This element corresponds directly to the class android.widget.LinearLayout. Each element or attribute prefixed with the Android namespace corresponds to an attribute that is interpreted by the Android resource compiler. The AAPT (Android Asset Packaging Tool) will generate an R.java file into your root (or primary) package. This file contains all of the Java variables used to reference your various application resources. In our case, we have the main.xml package in the /res/layout directory. This file becomes an R.layout.main variable with a constant value assigned as its identification.
Populating a View and a ViewGroup
A widget in Android is called a View, while a container (such as LinearLayout) is a ViewGroup. We have an empty ViewGroup now, but we need to start populating it in order to build up our user interface. While it is possible to nest a ViewGroup inside another ViewGroup object, an Activity has only one root View—so a layout XML file may have only one root View.
Time for action – asking a question
In order to ask our user a question, you will need to add a TextView to the top of your layout. A TextView is a bit like a Label or JLabel. It's also the base class for many other Android View widgets that display text. We want it to take up all of the available horizontal space, but only enough vertical space for our question to fit. We populate the TextView with Please wait... as its default text. Later, on we will replace this with a dynamically selected question.
1. 2. 3.
Go back to your main.xml file. Between the and create a element, ending it with the empty element /> syntax since elements representing View objects are not allowed to have child elements. Give the TextView element an ID attribute: android:id="@+id/question" [ 19 ]
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4.
Change the layout width and height attributes to fill_parent and wrap_ content respectively (the same as the LinearLayout element): android:layout_width="fill_parent" android:layout_height="wrap_content"
5. 6. 7.
Give the TextView some placeholder text so we can see it on the screen: android:text="Please wait..."
Reinstall the application using Apache Ant from your project root folder: ant install
Run the application again in the emulator and it should look like the following screenshot:
The code for the TextView should end up looking something like this:
What just happened
In the preceding example, we used fill_parent and wrap_content as values for the layout width and height attributes. The fill_parent value is a special value that is always equal to the parent size. If it's used as the value for the android:layout_width attribute (as in our example), then it's the width of the parent view. If it's used in the android:layout_height attribute, it would be equal to the height of the parent view instead. The value wrap_content can be used much like a preferred size in Java AWT or Swing. It says to the View object, "Take as much space as you need to, but no more". The only valid place to use these special attribute values is in the android:layout_width and android:layout_height attributes. Anywhere else will result in a compiler error.
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Chapter 1
We will need to access this TextView in our Java code later, in order to invoke its setText method (which directly corresponds to the android:text attribute we used for the placeholder text). A Java reference to a resource variable is created by assigning the resource an ID. In this example, the ID is declared here as @+id/question. The AAPT will generate an int value as an identifier for each resource of id as part of your R class. The ID attribute is also needed for accessing resources from another resource file.
Time for action – adding a space for answers
While posing a question to the user is all very fine and well, we need to give them some way to answer that question. We have several possibilities at our disposal: We could use a RadioGroup with a RadioButton for each possible answer, or a ListView with an item for each answer. However, to minimize the required interaction, and make things as clear as possible, we use one Button for each possible answer. However, this complicates things slightly, since you can't declare a variable number of Button objects in your layout XML file. Instead, we will declare a new LinearLayout and populate it with Button objects in the Java code.
1.
Under the TextView where we pose our question, you will need to add a element. While this element would normally have child elements, in our case, the number of possible answers is varied, so we leave it as an empty element. By default, a LinearLayout will place its child View objects horizontally alongside each other. However, we want each child View to be vertically below each other, so you'll need to set the orientation attribute of the LinearLayout: android:orientation="vertical" 2.
3.
We will need to populate the new ViewGroup (LinearLayout) later in our Java code, so give it an ID: answers: android:id="@+id/answers" 4. 5.
Like our TextView and root LinearLayout, make the width fill_parent: android:layout_width="fill_parent" Make the height wrap_content so that it doesn't take up more space than all the buttons it will be populated with: android:layout_height="wrap_content" [ 21 ]
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The resulting code should look like this:
What just happened?
You may have noticed that for this example, we have no content in our new LinearLayout. This may seem a little unusual, but in this case, we want to populate it with a variable number of buttons—one for each possible answer to our multiple-choice questions. However, for the next part of the example we need some simple content Button widgets in this LinearLayout so that we can see the entire screen layout in action. Use the following code in your layout resource file to add Yes!, No!, and Maybe? Button widgets to the LinearLayout:
In Android XML layout resources, any View classes extending from the ViewGroup class are considered containers. Adding widgets to them is as simple as nesting those View elements inside the element of your ViewGroup (as opposed to closing it with no child XML elements).
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Chapter 1
The following is a screenshot of the preceding Yes!, No!, Maybe? options:
Time for action – adding more buttons
We have two additional buttons to add to the screen layout. One will allow the user to skip the current question; the other will allow them to look at the short list of meals that we have filtered through so far (based on the questions they have already answered).
1.
Start by creating an empty element below our answers ViewGroup (but still within the root LinearLayout element). Assign it the ID skip, so that we can reference it in Java: android:id="@+id/skip" 2. 3. 4.
Create some padding between the answers and the new button by using a margin: android:layout_marginTop="12sp" Give it the display label Skip Question: android:text="Skip Question"
Like all of the previous widgets, the width should be fill_parent and the height should be wrap_content: android:layout_width="fill_parent" android:layout_height="wrap_content"
5. 6. 7.
Now create another empty element below the Skip Question button The ID for the new button should be view: android:id="@+id/view" We want this button to display the text: Feed Me!: android:text="Feed Me!"
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Developing a Simple Activity
8.
Again, put a little space between the Skip Question button, and the new Feed Me! button: android:layout_marginTop="12sp" 9.
Finally, set the width and height of the Feed Me! button as with the other elements we've created so far: android:layout_width="fill_parent" android:layout_height="wrap_content"
When you've completed these two buttons, your layout XML file should now end with:
What just happened
Separation of unrelated user interface objects is a very important part of user interface design. Groups of items can be separated by whitespace, a border, or a box. In our case, we chose to use whitespace, as space also helps make the user interface feel cleaner. We created our whitespace by using a margin above each of the buttons. Margins and padding work exactly the same way as they (should) do in CSS. A margin is spacing outside of the widget, while padding is spacing inside the widget. In Android, a margin is the concern of the ViewGroup, and so its attribute name is prefixed with layout_. Because padding is the responsibility of a View object, the padding attribute has no such prefix:
The previous code would create extra space between the edge of the Button and the text in the middle of it, as well as retaining the margin above the button.
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Chapter 1
All of the measurements in the preceding example are specified in the sp unit, which is short for "scale independent pixels". Much like CSS, you suffix your measurement numbers with the unit of size that you are specifying the measurement in. Android recognizes the following measurements:
Unit suffix px Full name Pixel Description and uses Exactly one pixel of the device screen. This unit is the most common when writing desktop applications, but with the wide variety of phone screen sizes, it becomes much harder to use. One inch (or the closest approximation). This is based on the physical size of the screen. This is great if you need to work with real world measurements, but again, because of the variations in the size of a device screen, it is not always very useful. Another real world measurement, made to the closest approximation. This is just a metric version of inches: 25.4 millimeters in 1 inch. Points are 1/72 of an inch in size. Much like millimeters and inches, they are very useful for sizing things against real-world sizes. They are also commonly used for sizing fonts, and so work well relative to font sizes. A single DP is the same size as a single pixel is for a 160 dpi screen. This size is not always a direct ratio, not always precise, but is a best approximation for the current screen. Much like the dp unit, it is a pixel scaled according to the user selected font size. This is possibly the best unit to use, as it's based on a user-selected parameter. The user may have increased the font size because they find the screen hard to read. Using an sp unit ensures that your user interface scales with it.
in
Inch
mm
Millimeters
pt
Points
dp or dip
Densityindependentpixels Scaleindependent pixels
sp
Defining common dimensions
Android also allows you to define your own dimension values as resource constants (note: dimensions, not measurements). This can be useful when you want several view widgets to be the same size, or to define a common font size. Files containing dimension declarations are placed in the /res/values directory in your project. While the actual file name isn't significant, a common name is dimens.xml. Dimensions can technically be included with other value types (that is, strings), but this is not recommended since it makes it harder to trace the dimension that are being applied at runtime.
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One advantage of having your dimensions in their own file as opposed to being declared inline) is that you can then localize them based on the size of the screen. This makes screenresolution-significant scales (such as pixels) much more useful. For example, you can place a dimens.xml file with different values into /res/values-320x240 and another version of the same dimensions into /res/values-640x480. A dimensions resource file is a simple values file (much like strings.xml), but dimensions are defined with the tag: 160px
To access this as a size in a layout XML file, you use a resource reference (much the same way as you access a resource string):
Building a list of common dimensions comes in handy when you want to build complex layouts that will look good on many different screens since it avoids the need to build several different layout XML files.
Have a go hero – improve the styling
Now we have the most basic structure for this user interface, but it doesn't really look too great. Other than the margins between the answer buttons, and the Skip Question and Feed Me! buttons, you can't really tell them apart. We need to let the user know that these buttons all do different things. We also need to draw more attention to the question, especially if they don't have a lot of time to squint at their screen. You may need the Android documentation, which can be found online at http://developer.android.com/reference/. We have a question at the top of our screen, but as you can see in the previous screenshots, it doesn't stand out much. Therefore, it's not really very clear to the user what they need to do (especially the first time they use the application). Try making the following styling changes to the question TextView at the top of our screen. These will only require you to add some attributes to its XML element: 1. Center the text. 2. Make the text bold. 3. Change the text size to 24sp. 4. Add 12sp spacing between the bottom of the question and the answer buttons
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Chapter 1
The Feed Me! button is also very important. This is the button that gives the user access to the list of suggested recipes that the application has filtered based on their answers, so it should look good. The following styling should help the Feed Me! button to stand out nicely (hint: Button extends TextView): 1. Make the text size 18sp. 2. Change the text color to a nice red #9d1111. 3. Style the text as bold. 4. Add a text shadow: x=0, y=-3, radius=1.5, and color=white ("#fff"). When you've finished styling the screen, it should look like the following screenshot:
Limitations of the layout XML format
One of the most obvious limitations of the layout XML format is that you can't dynamically populate part of the Activity based on external variables—there are no loops or methods in the XML file. In our example, this limitation shows itself in the form of our empty LinearLayout. Because each question has any number of possible answers, we need a varying number of buttons in the group. For our purposes, we will create the Button objects and put them into the LinearLayout as part of the Java code.
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The other place the XML layout format falls down is dynamically referencing external resources. This can be seen in our example, where we put placeholder text in the android:text attribute on the TextView element—question. We could have referenced an external string using the following syntax:
This will effectively reference a static variable from the strings.xml file. It's not suitable for a dynamically selected question, which will change each time we initialize the Activity.
Pop quiz
1. What reason do you have for writing your layouts in XML instead of in pure Java code? a. Android can read the layout file externally for optimization. b. The layout becomes part of the resource selection process. c. Your users could download new layouts from the App Store. d. The layout can have custom themes applied to it. 2. How would we make the text of the Next Question button bold? a. Use the android:typeface attribute. b. Create a custom Button implementation. c. Add a CSS attribute: style="font-weight: bold". d. Use the android:textStyle attribute. 3. What would happen if we changed the LinearLayout from vertical orientation, to horizontal? a. The layout would turn on its side. b. All of the widgets would be squashed together on the screen. c. Only the question TextView would be visible on the screen. d. The question, and possibly some other View objects may be visible on the screen depending on the number of pixels available. e. The layout would overflow, causing the widgets to appear next to each other, over several lines.
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Populating the QuestionActivity
We have a basic user interface, but right now, it's static. We may want to ask our user many different questions, each of which have different answers. We may also want to vary which questions we ask in some way or another. In short, we need some Java code to populate the layout with a question and some possible answers. Our questions are made up of two parts:
The question A list of possible answers
In this example, we will make use of string array resources to store all of the question and answer data. We will use one string array to list the question identifiers, and then one string array for each question and its answers. The advantages of this approach are very similar to the advantages of using a layout XML file instead of hard-coding it. The res/values directory of your project will have an auto-generated strings.xml file. This file contains string and string-array resources that you want your application to use. Here is the start of our strings.xml file, with two questions to ask the user: Kitchen Droid vegetarian size Are you a Vegetarian? Yes No I\'m a vegan How much do you feel like eating? A large meal Just a nice single serving of food Some finger foods Just a snack
The first item of each question array (vegetarian and size) is the question itself, while each following item is an answer.
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Time for action – writing more Java code
1. 2. 3.
Open the QuestionActivity.java file in an editor or IDE. Import the Android Resources class below the package declaration: import android.content.res.Resources;
In order to start asking the questions from your strings.xml file, you'll need a method to look in the questions and find the name of the array that contains the current question. This is not normally something you need to do with application resources—their identifiers are generally known to you through the R class. In this case however, we want to work in the order defined in the questions , making things a little bit more difficult: private int getQuestionID(Resources res, int index) {
4.
We can now look at the questions string-array, which contains the identifying name of each question (our index string-array):
String[] questions = res.getStringArray(R.array.questions);
5.
We have the array of questions, and we need to find the identifier value. This is much like using R.array.vegetarian for the vegetarian question, except that it's a dynamic lookup, and therefore much slower than normal. In general, the following line is not recommended, but in our case it's very useful: return res.getIdentifier( questions[index], "array", "com.packtpub.kitchendroid");
6.
The QuestionActivity class will display several questions to the user. We want the application to "play nice" with the phone and its environment. For that reason, each question will be posed in a new instance of QuestionActivity (allowing the device to control the display of our Activity). However, this method raises an important question: How do we know the index of the question to pose to the user? The answer: Our Intent. An Activity is started with an Intent object, and each Intent object may carry any amount of "extra" information (similar to request attributes in the HttpServletRequest interface) for the Activity to use, sort of like arguments to a main method. So, an Intent is also like a HashMap, containing special data for the Activity to use. In our case we use an integer property named KitchenDroid.Question: private int getQuestionIndex() { return getIntent().getIntExtra("KitchenDroid.Question", 0); } [ 30 ]
Chapter 1
These two methods form the basis for populating our question screen and navigating our way through a defined list of questions. When complete, they should look like this: private static int getQuestionID( final Resources res, final int index) { final String[] questions = res.getStringArray(R.array.questions); return res.getIdentifier( questions[index], "array", "com.packtpub.kitchendroid"); } private int getQuestionIndex() { return getIntent().getIntExtra("KitchenDroid.Question", 0); }
What just happened
The getQuestionID method is pretty straight forward. In our code we use R.array. questions to access the which identifies all of the questions we are going to ask the user. Each question has a name in the form of a String, and a corresponding resource identification number in the form of an int. In the getQuestionID method, we make use of the Resources.getIdentifier method, which looks for the resource identifier (the integer value) for a given resource name. The second parameter of the method is the type of resource to look up. This parameter is normally an inner class to the generated R class. Finally, we pass the base package that the resource is found in. Instead of all three of these parameters, you could also look up the resource by its full resource name: return res.getIdentifier( "com.packtpub.kitchendroid:array/" + questions[index], null, null);
The getQuestionIndex method tells us where in the questions we currently are, and thus, which question to ask the user. This is based on the "extra" information in the Intent that triggered the Activity. The getIntent() method provides you with access to the Intent that triggered your Activity. Each Intent may have any amount of "extra" data, and that data may be any "primitive" or "serializable" type. Here we fetch the KitchenDroid.Question extra integer value from our Intent, substituting a value of 0 if it has not been set (that is, the default value). If the user taps our icon in the menu, Android won't have specified that value, so we start from the first question.
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Dynamically creating widgets
Up to this point we've only used the layout XML file to populate our screen. In some cases, this is just not enough. In this simple example, we want the user to have a list of buttons that they can touch to answer the questions posed to them. We could pre-create some buttons and name them button1, button2, and so on, but that means limiting the number of possible answers. In order to create buttons from our resources, we need to do it in Java. We created a ViewGroup earlier (in the form of the LinearLayout that we named answers). This is where we will add our dynamically created buttons.
Time for action – putting the questions on the screen
Your application now knows where to find the questions to ask, and knows which question it should be asking. Now it needs to put the question on the screen, and allow the user to select an answer.
1. 2. 3. 4.
Open the main.xml file in your editor or IDE. Remove the Yes!, No!, and Maybe? Button elements from the layout resource. Open the QuestionActivity.java file in an editor or IDE. We will need a new class field to hold the dynamically-created Button objects (for reference): private Button[] buttons;
5.
In order to keep things neat, create a new private method to put the questions on the screen: initQuestionScreen: private void initQuestionScreen() {
6.
In this method, we assume that the layout XML file has already been loaded into the Activity screen (that is, it will be invoked after we setContentView in onCreate). This means that we can look up parts of the layout as Java objects. We'll need both the TextView named question and the LinearLayout named answers:
TextView question = (TextView)findViewById(R.id.question); ViewGroup answers = (ViewGroup)findViewById(R.id.answers);
7.
These two variables need to be populated with the question and its possible answers. For that we need the (from our strings.xml file) which contains that data, so we need to know the resource identifier for the current question. Then we can fetch the actual array of data:
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The first element of a question string array is the question to pose to the user. The following setText call is exactly the same as specifying an android:text attribute in your layout XML file: question.setText(quesionData[0]); 9.
We then need to create an empty array to store references to our Button objects: int answerCount = quesionData.length – 1; buttons = new Button[answerCount];
10.
Now we're ready to populate the screen. A for loop over each of the answer values indexed according to our arrays: for(int i = 0; i < answerCount; i++) {
11. 12.
Get each answer from the array, skipping the question string at index zero:
String answer = quesionData[i + 1];
Create a Button object for the answer and set its label:
Button button = new Button(this); button.setText(answer);
13.
Finally, we add the new Button to our answers object (ViewGroup), and reference it in our buttons array (where we'll need it later): answers.addView(button); buttons[i] = button;
14.
Having done that, just after the setContentView calls in onCreate, we need to invoke our new initQuestionScreen method.
What just happened?
The findViewById method traverses the tree of View objects looking for a specific identifying integer value. By default, any resource declared with an android:id attribute in its resource file will have an associated ID. You could also assign an ID by hand using the View.setId method. Unlike many other user interface APIs, the Android user interface API is geared towards XML development than pure Java development. A perfect example of this fact is that the View subclasses have three different constructors, two of which are designed for use with the XML parsing API. Instead of being able to populate the Button label in a constructor (as with most other UI APIs), we are forced to first construct the object, and then use setText to define its label.
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What you do pass into the constructor of every View object is a Context object. In the preceding example you pass the Activity object into the constructor of the answer Button objects as this. The Activity class inherits from the Context class. The Context object is used by the View and ViewGroup objects to load the application resources and services that they require in order to function correctly. You can now try running the application, in which case you'll be greeted with the following screen. You may have noticed that there is additional styling in this screenshot. If you don't have this, you may want to backtrack a little to the previous Have a go hero section.
Handling events in Android
Android user interface events work in much the same way as a Swing event-listener or a GWT event-handler. Depending on the type of event you wish to receive, you implement an interface and pass an instance to the widget you wish to receive events from. In our case we have Button widgets that fire click-events when they are touched by the user. The event-listener interfaces are declared in many of the Android classes, so there isn't a single place you can go look for them. Also, unlike most event-listener systems, many widgets may only have one of any given event-listeners. You can identify an event-listener interface by the fact that their class names are prefixed with On (much like HTML event attributes). In order to listen for click-events on a widget, you would set its OnClickListener using the View.setOnClickListener method. The following code snippet shows how a click-listener might be added to a Button object to show a Toast. A Toast is a small pop-up box which is displayed briefly to give the user some information:
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The preceding event-listener is declared as an anonymous inner class, which is okay when you are passing similar event-listeners to many different widgets. However, most of the time you'll want to be listening for events on widgets you've declared in an XML layout resource. In these cases it's better to either have your Activity class implement the required interfaces, or create specialized classes for different event-driven actions. While Android devices are very powerful, they are still limited when compared to a desktop computer or laptop. Therefore, you should avoid creating unnecessary objects in order to conserve memory. By placing as many event-listener methods in objects that will already be created, you lower the overhead required.
Pop quiz
1. When you declare an object in a layout XML file, how do you retrieve its Java object? a. The object will be declared in the R class. b. Using the Activity.findViewById method. c. By using the Resources.getLayout method. d. The object will be injected into a field in the Activity class. 2. What is the "best" way of listening for events in an Android application? a. Declaring the listeners as anonymous inner classes. b. Create a separate event listener class for each Activity. c. Implement the event-listening interfaces in the Activity class. 3. Why do you pass this Activity into the constructors of View objects (that is, new Button(this)). a. It defines the Activity screen they will be displayed on. b. It's where event messages will be sent to. c. It's how the View will reference its operating environment.
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Developing a Simple Activity
Summary
Android comes with some great tools to create and test applications, even if you don't have an Android device handy. That said, there's no replacement for actually touching your application. It's part of what makes Android such a compelling platform, the way it feels and responds (and the emulator just doesn't convey that). One of the most important tools in an Android developer's arsenal is the resource selection system. With it you can build highly dynamic applications that respond to changes in the devices, and thus, the user environment. Changing the screen layout based on the orientation of the device, or when the user slides out the phone's QWERTY keyboard, lets them know that you've taken their preferences into account when building your application. When building user interfaces in Android, it's strongly recommended to build at least the layout structure in an XML file. The XML layout files are not only considered as application resources, but Android also strongly favors building XML user interfaces over writing Java code. Sometimes, however, a layout XML file isn't enough, and you need to build parts of the user interface in Java. In this case it's a good idea to define at least a skeleton layout as XML (if possible), and then place the dynamically created View objects into the layout by using marker IDs and containers (much like dynamically adding to an HTML document in JavaScript). When building a user interface, think carefully about the look and feel of the outcome. In our example, we use Button objects for the answers to questions. We could have used RadioButton objects instead, but then the user would have needed to select an option, and then touch a Next Question button, requiring two touches. We could also have used a List (which interacts nicely with the fact that it needs to be dynamically populated), however, a List doesn't indicate an "action" to the user quite the way a Button does. When coding layouts, be careful with the measurement units that you use. It's strongly recommend that you stick to using sp for most purposes—if you can't use one of the special fill_parent or wrap_content values. Other values are highly dependent on the size of screen, and won't respond to the user preferences. You can make use of the resource selection process to build different screen designs for small, medium, or large screens. You could also define your own measurement unit and base it on the screen size. Always think about how your user will interact with your application, and how much (or little) time they are likely to have with it. Keeping each screen simple and responsive keeps your users happy. Now that we've learned how to create a skeleton Android project, and a simple Activiy, we can focus on the more subtle problems and solutions of Android user interface design. In the next chapter, we will focus on working with data-driven widgets. Android has several widgets designed specifically for displaying and selecting from more complex data structures. These widgets form the basis of data-driven applications such as an address book or a calendar application.
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Presenting Data for Views
In the first chapter we covered the basic creation of a project, and how to put together a simple user interface. We backed our first Activity with enough code to dynamically generate some buttons that the user can use to answer our multiple-choice questions. So now we can capture some data, but what about displaying data? One large advantage of software is its ability to present and filter very large volumes of data quickly and in an easy-to-read format. In this chapter we will look at a series of Android widgets that are designed exclusively for presenting data.
2
Most Android data-centric classes are built on top of Adapter objects, and thus extend the AdapterView class. An Adapter can be thought of as a cross between a Swing Model, and a renderer (or presenter). An Adapter object is used to create View objects for data objects that your software needs to display to the user. This pattern allows the software to maintain and work with a data-model and only create a graphical View for each of the data objects when one is actually needed. This doesn't just help conserve memory, but it is also more logical from a development point of view. As a developer you work with your own data objects instead of trying to keep your data in graphical widgets (which are often not the most robust of structures). The most common AdapterView classes you'll encounter are: ListView, Spinner, and GridView. In this chapter we'll introduce the ListView class and GridView, and explore the various ways they can be used and how they can be styled.
Presenting Data for Views
Listing and selecting data
The ListView class is probably the most common way to display lists of data. It's backed by a ListAdapter object, which is responsible for both holding the data and rendering the data objects in a View. A ListView includes built-in scrolling, so there's no need to wrap it in a ScrollView.
ListView choice modes
The ListView class allows for three basic modes of item selection, as defined by its constants: CHOICE_MODE_NONE, CHOICE_MODE_SINGLE, and CHOICE_MODE_MULTIPLE. The mode for a ListView can be set by using the android:choiceMode attribute in your layout XML file, or by using the ListView.setChoiceMode method in Java.
Choice modes and items The choice mode of a ListView changes the way the ListView structure behaves, but not the way it looks. The look of a ListView is defined mostly by the ListAdapter, which provides View objects for each of the items that should appear in the ListView.
No selection mode – CHOICE_MODE_NONE
On a desktop system, this would make no sense—a list that doesn't allow the user to choose anything? However, it's the default mode of an Android ListView. The reason is it makes sense when your user is navigating by touch. The default mode of a ListView allows the user to tap on one of the elements, and trigger an action. As a result of this behavior, there's no need for a "Next" button, or anything similar. So the default mode for a ListView is to act like a menu. The following screenshot is a default ListView object displaying a list of different strings from a String array Java object, taken from one of the default ApiDemos examples in Android SDK.
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Chapter 2
Single selection mode – CHOICE_MODE_SINGLE
In this mode, the ListView acts more like a desktop List widget. It has the notion of the current selection, and tapping on a list item does nothing more than selecting it. This behavior is nice for things like configuration or settings, where the user expects the application to remember his or her current selection. Another place a single selection list becomes useful is when there are other interactive widgets on the screen. However, be careful not to put too much information in a single Activity. It's quite common for a ListView to occupy almost an entire screen.
Single-choice selection: It doesn't directly change the way your list items appear. The look and feel of your list items is defined entirely by the ListAdapter object.
Android does, however, provide a collection of sensible defaults in the system resources. In the android package you will find an R class. It's a programmatic way to access the system's default resources. If you wanted to create a single-choice ListView with a of colors in it, you could use the following code: list.setAdapter(new ArrayAdapter( this, android.R.layout.simple_list_item_single_choice, getResources().getStringArray(R.array.colors)));
In this case we use the provided ArrayAdapter class from the android.widget package. In the second parameter we referenced the Android layout resource named simple_list_ item_single_choice. This resource is defined by the Android system as a default way to display items in a ListView with CHOICE_MODE_SINGLE. Most typically this is a label with a RadioButton for each object in the ListAdapter.
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Presenting Data for Views
Multiple selection mode – CHOICE_MODE_MULTIPLE
In multi-selection mode, the ListView replaces the radio buttons of single-selection mode with normal checkboxes. This design structure is often used on desktops and web-based systems as well. Checkboxes are easily recognized by users, and make it easy to go back and turn options off again. If you wish to use a standard ListAdapter, Android provides you with the android.R.layout.simple_list_item_multiple_choice resource as a useful default: A label with a CheckBox for each object in the ListAdapter.
Adding header and footer widgets
Headers and footers in a ListView allow you to put additional widgets at the top and bottom of the List. The header and footer widgets are by default treated as though they are items in a list (as though they come from your ListAdapter). This means that you will be able to select them as though they are data elements in the List structure. A very simple example of a header item could be:
TextView header = new TextView(this); header.setText("Header View"); list.addHeaderView(header);
Often you don't want your headers and footers to be items in the ListView, but instead a label or group of labels identifying parts of the ListView, or providing other information. In this case you need to tell the ListView that your header or footer views are not selectable list items. This can be done by using the extended implementation of addHeaderView or addFooterView:
TextView footer = new TextView(this); footer.setText("Footer View"); list.addFooterView(footer, null, false);
The ListView class integrates headers and footers so tightly into the list structure that you can also provide an Object that it will return from the AdapterView. getItemAtPosition(index) method. In our previous example we have provided null. Each header item will offset the index of subsequent views by one (as though you are adding new items to the ListView). The third parameter tells the ListView whether the header or footer should be counted as a selectable list item (in our previous example it shouldn't).
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Chapter 2
If you are used to desktop widgets, the header and footer widgets on an Android ListView will have a bit of a surprise for you. They will scroll with the rest of the list items, and won't stay attached to the top and bottom of the ListView object.
Creating a simple ListView
To introduce the ListView class, we'll start a new example which will be enhanced by various subsequent sections of this chapter. The first Activity we will create will use a simple ListView populated from a resource.
Time for action – creating a fast food menu
To continue with the food and eating theme, let's build a simple application that allows us to order various types of fast food, and get it delivered! The user will first select where they want to order from, and then select the various foodstuffs that they want to eat.
1.
Create a new android project using the Android command-line tool: android create project -n DeliveryDroid -p DeliveryDroid -k com. packtpub.deliverydroid -a SelectRestaurantActivity -t 3
2. 3.
Open the /res/values/strings.xml file in your favorite editor or IDE. Create a string-array structure listing the various fast-food restaurants our users can order from: The Burger Place Mick's Pizza Four Buckets \'o Fruit Sam\'s Sushi
4. 5. 6. 7. 8.
Open the /res/layout/main.xml file in your favorite editor or IDE. Remove any widget that is inside the default LinearLayout. Add a new element. Assign the element an ID of restaurant:
Assign the width and height of the ListView to fill_parent: android:layout_width="fill_parent" android:layout_height="fill_parent"
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Presenting Data for Views
9.
Since we have a string-array resource of the content we want to populate the ListView with, we can reference it directly in our layout XML file: android:entries="@array/restaurants" 10.
When you've completed the specified steps, you should have a main.xml layout file that looks like the following:
What just happened
If you install your application into the emulator, and run it, you'll be presented with a screen where you can select from the list of restaurants specified in your string-array resource. Notice that the choiceMode on the ListView is left as CHOICE_MODE_NONE, making this into a more direct menu where the user selects their restaurant and is instantly transported to its menu.
In this example, we used the android:entries attribute in the layout XML file to specify a reference to a string-array resource with our desired list items in it. Normally, using an AdapterView requires you to create an Adapter object to create View objects for each of the data objects.
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Chapter 2
Using the android:entries attribute allows you to specify the data contents of the ListView from your layout resource, instead of requiring you to write the normal Java code associated with an AdapterView. It, however, does have two disadvantages to be aware of:
The View objects created by the generated ListAdapter will always be the system-specified defaults, and so cannot be easily themed. You cannot define data objects that will be represented in the ListView. Since string-arrays are easily localized, your application will rely on the index locations of items to determine what they indicate.
You may notice that at the top of the screenshot, the label Where should we order from? is not the application default. The label for an Activity is defined in the AndroidManifest.xml file as follows:
Styling the standard ListAdapters
The standard ListAdapter implementations require each item be represented in a TextView item. The default single-choice and multiple-choice items are built using a CheckedTextView, and while there are plenty of other TextView implementations in Android, it does limit our options a bit. However, the standard ListAdapter implementations are very convenient and provide solid implementations for the most common listing requirements. Since a ListView with CHOICE_MODE_NONE is a lot like a menu, wouldn't it be nice to change the items into Button objects instead of normal TextView items? Technically, a ListView can contain any widget that extends TextView. However, some implementations are more suitable than others (for example, a ToggleButtonView won't maintain the specified text-value when the user touches it).
Defining standard dimensions
In this example we'll be creating various menus for the application. In order to maintain a consistent look and feel, we should define a set of standard dimensions which will be used in each of our layout files. This way we can redefine the sizes for different types of screens. There's nothing more frustrating for a user than only being able to see a partial item because it's been sized bigger than their screen. Create a new resource file to contain the dimensions. The file should be named res/ values/dimens.xml. Copy the following code into the new XML file:
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Presenting Data for Views 48sp 52sp 45sp 24sp 15dp
We declare two height dimensions for the list items: item_outer_height and item_ inner_height. The item_outer_height will be the height of the list items, while the item_inner_height is the height of any View object contained inside the list item. The padding dimension at the end of the file is used to define a standard amount of whitespace between two visual elements. This is defined as dp so it will remain constant based on the DPI of the screen (instead of scaling according to the font size preferences of the user).
Sizing of interactive items In this styling, you'll notice that the item_outer_height and menu_item_ height are 48sp and 52sp, which makes the items in the ListView rather large. The standard size of a list view item in Android is 48sp. The height of a list item is critical. If your users have large fingers, they will struggle to tap on their target list item if you make them too small. This is a general "good practice" for Android user interface design. If the user needs to touch it, make it big.
Time for action – improving the restaurant list
The list of restaurants we put together earlier is nice, but it's a menu. In order to further emphasize the menu, the text should stand out more. In order to style a ListView with a standard ListAdapter implementation, you will need to specify the ListAdapter object in your Java code.
1. 2.
Create a new file in the res/layout directory named menu_item.xml. Create the root XML element as a TextView:
3.
Import the Android resource XML namespace: xmlns:android="http://schemas.android.com/apk/res/android" [ 44 ]
Chapter 2
4. 5. 6. 7.
Center the text in the TextView widget by setting its gravity: android:gravity="center|center_vertical" We assign the textSize of the TextView to our standard item_text_size: android:textSize="@dimen/item_text_size" The default color of the text of TextView is a bit gray, we want it to be white: android:textColor="#ffffff" We want the width of the TextView to be the same as the ListView that contains it. Since this is for our main menu, its height is menu_item_height: android:layout_width="fill_parent" android:layout_height="@dimen/menu_item_height"
8. 9.
Now that we have a styled TextView resource, we can incorporate it into our menu. Open the SelectRestaurantActivity.java file. In the onCreate method, after you use setContentView, we need a reference to the ListView we created earlier in main.xml:
ListView restaurants = (ListView)findViewById(R.id.restaurant);
10.
Set the restaurants ListAdapter to a new ArrayAdapter containing the stringarray of restaurants we created in our values.xml file: restaurants.setAdapter(new ArrayAdapter( this, R.layout.menu_item, getResources().getStringArray(R.array.restaurants)));
What just happened
We first created a new layout XML resource containing the styled TextView that we wanted to be used for each list item in our restaurant's ListView. The menu_item.xml file you wrote should contain the following code:
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Presenting Data for Views
Unlike our previous layout resources, menu_item.xml contained no ViewGroup (such as LinearLayout). This is due to the fact that the ArrayAdapter will attempt to cast the root View of the menu_item.xml file to a TextView. So, if we nested the TextView in a ViewGroup of some sort, we'd get a ClassCastException. We also created an ArrayAdapter instance to reference both our menu_item XML resource, and the string-array of restaurants we created earlier. This action eliminates the use of the android:entries attribute on the ListView in the main.xml layout XML resource. If you want, you can remove that attribute. Your onCreate method in SelectRestaurantActivity should now look as follows: public void onCreate(final Bundle icicle) { super.onCreate(icicle); setContentView(R.layout.main); final ListView restaurants = (ListView) findViewById(R.id.restaurant); restaurants.setAdapter(new ArrayAdapter( this, R.layout.menu_item, getResources().getStringArray(R.array.restaurants))); }
Try re-installing the application into the emulator with Apache Ant, and you'll now be greeted by a screen that looks a lot more like a menu:
Have a go hero – developing a multiple-choice question application
Try going back to the multiple-choice question application we wrote in Chapter 1, Developing a Simple Activity. It uses LinearLayout and Button objects to display the possible answers to the questions, but it also uses string-arrays for the answers. Try modifying the application to:
Use a ListView instead of a LinearLayout Style the ListView with Button objects, as we styled our restaurant menu with TextView objects
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Chapter 2
Make sure you have some margin between the Button list items so that they're not too close to each other
Creating custom adapters
When we want to order food, we often want to order more than one of the same item. The ListView implementation, and the standard ListAdapter implementations allow for us to select a Cheese Burger item, but not for us to request 3 Cheese Burgers. In order to display a menu of different foods that the user can order in multiple quantities, we need a customized ListAdapter implementation.
Creating a menu for The Burger Place
For each restaurant in our main menu, we are going to build a separate Activity class. In reality, this is not a great idea, but it allows us to investigate different ways of organizing and displaying the menu data. Our first stop is The Burger Place, for which we present the user with a list of burgers, and let them tap the ones they want on the screen. Each time they tap a list item, they order another burger. We will display the number of burgers they are ordering in bold to the left of the burger's name. Next to burgers that they aren't ordering, there should be no number (this allows the user to see what they are ordering at a quick glance).
The Burger class In order to display the menu, we need a simple Burger data object. The Burger class will hold a name to be displayed in the menu, and the number of Burger the user is ordering. Create a Burger.java file in the root package of your project with the following code: class Burger { final String name; int count = 0; public Burger(String name) { this.name = name; } }
You'll notice that there are no getters and setters in the preceding code, and that both the name and count fields are declared as package-protected. In versions of Android prior to 2.2, methods incurred a heavy expense when compared to a straight field lookup. Since this class will be a small part of the rendering procedure (we will be extracting data from it for display), we should make sure we incur as little expense as possible.
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Presenting Data for Views
Time for action – creating a Burger item layout
The first thing to do in order to create a nice looking menu for The Burger Place is to design the menu items. This is done in much the same way as the styling of the restaurant list with a layout XML resource. However, since we will be building the ListAdapter ourselves this time, we are not forced to use a single TextView, but can instead build a more complex layout.
1. 2.
Create a new XML file in the res/layout directory named burger_item.xml. This file will be used for each burger in the ListView. Declare the root of the layout as a horizontal LinearLayout (note the height, which will be the height of each item in the ListView):
3.
Next, declare a TextView, which we will use as a counter for the number of burgers being ordered. We will later access this through its ID:
4.
The counter text size is exactly the same as all of the other list items in the application. However, it should be bold, so it can be easily identified and read: android:textSize="@dimen/item_text_size" android:textStyle="bold"
5.
We also want the counter to be square, so set the width and height exactly the same: android:layout_width="@dimen/item_inner_height" android:layout_height="@dimen/item_inner_height"
6. 7. 8.
We also want to center the text inside the counter: android:gravity="center|center_vertical" We'll also need a text space to display the name of the burger:
The text size is standard: android:textSize="@dimen/item_text_size" [ 48 ]
Chapter 2
9. 10.
We want a little bit of space between the counter and the text label: android:layout_marginLeft="@dimen/padding" The label's width should fill the ListView, but we want the size of both TextView objects to be the same: android:layout_width="fill_parent" android:layout_height="@dimen/item_inner_height"
11.
The text of the label should be centered vertically, to match the location of the counter. However, the label should be left-aligned: android:gravity="left|center_vertical" What just happened?
You've just built a very nice LinearLayout ViewGroup which will be rendered for each of the burgers we sell from The Burger Place. Since the counter TextView is a separate object from the label, it can be independently styled and managed. This makes things much more flexible going forward if we want to apply additional styles to them independently. Your complete burger_item.xml file should now appear as follows:
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Presenting Data for Views
Time for action – presenting Burger objects
The standard ListAdapter classes work well if your data objects are either strings or easily represented as strings. In order to display our Burger objects nicely on the screen, we need to write a custom ListAdapter class. Fortunately, Android provides us with a nice skeleton class for ListAdapter implementations named BaseAdapter.
1.
Create a new class named BurgerAdapter, and have it extend from the android. widget.BaseAdapter class: class BurgerAdapter extends BaseAdapter {
2.
An Adapter is part of the presentation layer, but is also the underlying model of the ListView. In the BurgerAdapter we store an array of Burger objects which we assign in the constructor: private final Burger[] burgers; BurgerAdapter(Burger... burgers) { this.burgers = burders; }
3.
Implement the Adapter.getCount() and Adapter.getItem(int) methods directly on top of the array of Burger objects: public int getCount() { return burgers.length; } public Object getItem(int index) { return burgers[index]; }
4.
An Adapter is also expected to provide identifiers for the various items, we will just return their index: public long getItemId(int index) { return index; }
5.
When an Adapter is asked for a View of a list item, it may be given an existing View object that could be reused. We will implement a simple method to handle this case, and if required, inflate the burger_item.xml file we wrote earlier using the LayoutInflator class from the android.view package: private ViewGroup getViewGroup(View reuse, ViewGroup parent) { if(reuse instanceof ViewGroup) { return (ViewGroup)reuse; } [ 50 ]
The most important method for us in the BurgerAdapter is the getView method. This is where the ListView will ask us for a View object to represent each list item it needs to display: public View getView(int index, View reuse, ViewGroup parent) {
7.
In order to fetch the correct View for a given item, you'll first need to use the getViewGroup method to ensure you have the burger_item.xml ViewGroup to display the Burger item in:
ViewGroup item = getViewGroup(reuse, parent); TextView counter = (TextView)item.findViewById(R.id.counter); TextView label = (TextView)item.findViewById(R.id.text);
8.
We'll be populating these two TextView objects with the data from the Burger object at the requested index. The counter widget needs to be hidden from the user if the current count is zero:
Burger burger = burgers[index]; counter.setVisibility( burger.count == 0 ? View.INVISIBLE : View.VISIBLE); counter.setText(Integer.toString(burger.count)); label.setText(burger.name); return item;
What just happened?
We just wrote a custom Adapter class to present an array of Burger objects to the user in a ListView. When a ListView invokes the Adapter.getView method, it will attempt to pass in the View object that was returned from a previous call to Adapter.getView. A View object will be created for each item in the ListView. However, when the data displayed by the ListView changes, the ListView will ask the ListAdapter to reuse each of the View objects it generated the first time around. It's important to try and honor this behavior, since it has a direct impact on the responsiveness of your application. In our preceding example, we implemented the getViewGroup method so that it would take this requirement into account.
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Presenting Data for Views
The getViewGroup method is also used to inflate the burger_item.xml file we wrote. We do this using a LayoutInflator object, which is exactly how the Activity. setContentView(int) method loads XML layout resources. The Context object which we fetch from our parent ViewGroup (which will generally be the ListView) defines where we will load the layout resource from. If the user hasn't selected a Burger, we hide the counter TextView using the View.setVisibility method. In AWT and Swing, the setVisible method takes a Boolean parameter, whereas in Android, setVisibility takes an int value. The reason for this is that Android treats visibility as part of the layout process. In our case we want the counter to disappear, but still take up its space in the layout, which will keep the text labels left-aligned with each other. If we wanted the counter to vanish and take up no space, we could use: counter.setVisibility(burger.count == 0 ? View.GONE : View.VISIBLE);
ListView objects will automatically handle the highlighting of a selected item. This includes when the user holds their finger on the item, and when they use a track-pad or directional buttons to navigate the ListView. When an item is highlighted, its background generally changes color, according to standard UI conventions.
However, using widgets in a ListView that in some way directly captures user input (that is, a Button or EditText) will cause the ListView to stop showing the selection highlighting for that widget. In fact, it will stop the ListView from registering OnItemClick events completely.
Custom separators in a ListView If you override the isEnabled(int index) method of ListAdapter, you can strategically disable specified items in the ListView. A common use of this is to turn certain items into logical separators. For example, a section separator in an alphabetically sorted list, containing the first letter of all items in the next "section".
Creating TheBurgerPlaceActivity class
In order to put the Burger menu on the screen, and to allow the user to order items, we need a new Activity class. We need to know when the user touches the items in the list, for which we will need to implement the OnItemClickListener interface. When a specific event occurs (in this case the user touches a specific item in the ListView), objects registered as listeners will have a related method invoked with the details of the event that occurred. Android provides a simple ListActivity class to provide some default layout and utility methods for this scenario.
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Time for action – implementing TheBurgerPlaceActivity
In order to present a ListView of Burger objects with the BurgerAdapter class, we will need to create an Activity implementation for The Burger Place. The new Activity will also be responsible for listening to "touch" or "click" events on the items in the ListView. When the user touches one of the items, we need to update the model and ListView to reflect that the user has ordered another Burger.
1.
Create a new class in the root package of your project named TheBurgerPlaceActivity, and make sure it extends ListActivity: public class TheBurgerPlaceActivity extends ListActivity {
2. 3. 4.
Override the Activity.onCreate method. Invoke the super.onCreate to allow normal Android startup. Create an instance of BurgerAdapter with some Burger objects, and set it as the ListAdapter for the ListActivity code to use: setListAdapter(new BurgerAdapter( new Burger("Plain old Burger"), new Burger("Cheese Burger"), new Burger("Chicken Burger"), new Burger("Breakfast Burger"), new Burger("Hawaiian Burger"), new Burger("Fish Burger"), new Burger("Vegatarian Burger"), new Burger("Lamb Burger"), new Burger("Rare Tuna Steak Burger")));
5.
Finally, implement the onListItemClicked method with the following code: protected void onListItemClick( ListView parent, View item, int index, long id) { BurgerAdapter burgers = (BurgerAdapter) parent.getAdapter(); Burger burger = (Burger)burgers.getItem(index); burger.count++; burgers.notifyDataSetInvalidated(); }
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Presenting Data for Views
What just happened?
This implementation of TheBurgerPlaceActivity has a simple hard-coded list of Burger objects to display to the user and creates a BurgerAdapter to turn these objects into the burger_item View objects which we created earlier. When the user taps a list item, we increment the count of the related Burger object in the onItemClick method. We then call notifyDataSetInvalidated() on the BurgerAdapter. This method will inform the ListView that the underlying data has changed. When the data changes, the ListView will re-invoke the Adapter.getView method for each item in the ListView. The items in a ListView are represented by effectively static View objects. This means that the Adapter must be allowed to update or recreate that View when the data model is updated. A common alternative is to fetch the View representing your updated data, and update it directly.
Registering and starting TheBurgerPlaceActivity
In order to start the new Activity class from our restaurant menu, you will need to register it in the AndroidManifest.xml file. First, open the AndroidManifest.xml file in an editor or IDE, and copy the following code into the ... block:
To start the Activity, you'll need to go back to SelectRestaurantActivity and implement the OnItemClickListener interface. After setting the Adapter on the restaurants ListView, set SelectRestaurantActivity as the OnItemClickListener of the restaurants ListView. You can start TheBurgerPlaceActivity using an Intent object in the onItemClick method. Your SelectRestaurantActivity class should now look like the following code snippet: public class SelectRestaurantActivity extends Activity implements OnItemClickListener { @Override public void onCreate(Bundle icicle) { super.onCreate(icicle); setContentView(R.layout.main); [ 54 ]
Chapter 2 ListView restaurants = (ListView) findViewById(R.id.restaurant); restaurants.setAdapter(new ArrayAdapter( this, R.layout.menu_item, getResources().getStringArray(R.array.restaurants))); restaurants.setOnItemClickListener(this); } public void onItemClick( AdapterView parent, View item, int index, long id) { switch(index) { case 0: startActivity(new Intent( this, TheBurgerPlaceActivity.class)); break; } } }
When you reinstall the application and start it up in the emulator, you'll be able to navigate to The Burger Place and place an order for burgers. Pressing the hardware "Back" button in The Burger Place menu will take you back to the restaurant menu.
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Presenting Data for Views
Pop quiz
1. Setting the choice mode on a ListView object to CHOICE_MODE_SINGLE will: a. Add a RadioButton to each item. b. Do nothing (this is the default). c. Make the ListView track a "selected" item. 2. A ListAdapter defines how a ListView displays its items. When will it be asked to reuse a View for an item object? a. When the data model is invalidated or changed. b. On every item, for rubber-stamping. c. When the ListView redraws itself. 3. When a ListView is scrollable, header and footer objects will be positioned: a. Above and below the scrolling items. b. Horizontally alongside each other, above and below the scrolling items. c. Scrolling with the other items.
Using the ExpandableListView class
The ListView class is great for displaying small to medium amounts of data, but there are times when it will flood your user with far too much information. Think about an email application. If your user is a heavy email user, or subscribes to a few mailing lists, they may well have several hundred emails in a folder. Even though they may not need to scroll beyond the first few, seeing the scrollbar shrink to a few pixels in size doesn't have a good psychological effect on your user. In desktop mail clients, you will often group the email list by time: Today, yesterday, this week, this month, and forever (or something similar). Android includes the ExpandableListView for this type of grouping. Each item is nested inside a group, and a group can be displayed or hidden by the user. It's a bit like a tree view, but always nested to exactly one level (you can't display an item outside a group).
Massive ExpandableListView groups There are times where even an ExpandableListView will not be enough to keep the amount of data to a reasonable length. In these cases, consider giving your user the first few items in the group and adding a special View More item at the end. Alternatively, use a ListView for the groups, and a separate Activity for the nested items.
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Creating ExpandableListAdapter implementations
Since the ExpandableList class includes two levels of detail, it can't work against a normal ListAdapter which only handles a single level. Instead, it includes the ExpandableListAdapter which uses two sets of methods: one set for the group level and another set for the item level. When implementing a custom ExpandableListAdapter, it's generally easiest to have your ExpandableListAdapter implementation inherit from the BaseExpandableListAdapter, as it provides implementations for event registration and triggering. The ExpandableListAdapter will place an arrow pointer on the left side of each group item to indicate whether the group is open or closed (much like a dropdown/combobox). The arrow is rendered on top of the group's View object as returned by the ExpandableListAdapter. To stop your group label from being partly obscured by this arrow, you'll need to add padding to your list item View structures. The default padding for a list item is available as the theme parameter expandableListPreferredItemPaddingLeft, which you can make use of: android:paddingLeft= "?android:attr/expandableListPreferredItemPaddingLeft"
In order to keep your ExpandableListView looking consistent, it's a good idea to add the same amount of padding to the normal (child) items of the ExpandableListView (to keep their text aligned with that of their parent group), unless you are putting an item on the left-hand side, such as an icon or checkbox.
Have a go hero - ordering customized pizzas
For the Mick's Pizza example, we're going to create a menu of categorized pizza toppings. Each topping consists of a name, whether it's 'on' or 'off' the pizza, or 'extra' (for example, extra cheese). Use two TextView objects arranged horizontally for each item. The right TextView can hold the name of the topping. The left TextView can be empty when toppings are not included, On when toppings are included, and Extra for toppings that the user wants more than the usual amount. Create an object model with ToppingCatagory objects, containing a name and an array of PizzaTopping objects. You'll want to store some state whether each topping is ordered, and in what quantity. You'll also want to implement a PizzaToppingAdapter class, extending the BaseExpandableListAdapter class. Make use of the default Android simple_ expandable_list_item_1 layout resource for the group label, and a new customized layout resource for the item labels.
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When the user taps on a pizza topping, it changes its status between the three values: Off, On, and Extra.
Using the ListView.getAdapter() method will not return your ExpandableListAdapter implementation, but a wrapper instead. To fetch the original ExpandableListAdapter, you will need to use the getExpandableListAdapter() method. You will also want to make use of the ExpandableListView. OnChildClickListener interface to receive click events.
When your new Activity is complete, you should have a screen which looks something like the following:
Using the GridView class
A GridView is a ListView with a fixed number of columns, arranged left-to-right, top-to-bottom. The standard (un-themed) Android application menu is arranged like a GridView. The GridView class makes use of a ListAdapter in the exact same format as ListView. However, because of its fixed column count, a GridView is very well suited for lists of icons.
Using GridViews effectively A GridView can display significantly more information on a single screen than a ListView, at the expense of not being able to show as much text information. From a usability point of view, icons are often easier to work with than text. Icons can be recognized more quickly than text, thanks to their colors. When you have information that can be represented using icons, it's a good idea to display it as such. However, remember that icons need to be unique within a single screen preferably within the entire application.
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Chapter 2
For our next example, we're going to build the Four Buckets 'o Fruit menu, using GridView. The GridView will have an icon for each item on the menu, and the name of the item below the icon. So, when complete, it will look much like the standard Android application menu. This next example will focus less on the implementation of the ListAdapter, since it's largely the same as the ListAdapter we built for The Burger Place.
Icons on touchscreen devices It's important to think about icons on a touchscreen device. They need to be even more self-explanatory than usual, or be accompanied by some text. With a touchscreen, it's very hard to provide any sort of contextual help, such as a tool-tip. If the user is touching the object, it's often obscured by their finger and/or hand, making the icon and tool-tip invisible.
Time for action – creating the fruit icon
In order to display the various types of fruits as icons, we will need to create a layout XML file. Each icon in the GridView will be represented as an instance of this layout, in exactly the same way as list items are represented in a ListView. We create each item as an ImageView for the icon, with a TextView below it for the label.
1. 2.
Create a file in the res/layout directory named fruit_item.xml. Declare the root element of the icon as a vertical LinearLayout:
3.
Create the ImageView element that will serve as our icon:
4.
Next, create the TextView element that will serve as the label:
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What just happened?
The fruit_item.xml file is a very simple layout for our menu icons, and could be used for many other types of icons represented as a grid. ImageView objects will, by default, attempt to scale their content to their size. In our previous example, the root LinearLayout has the width and height defined as fill_parent. When placed in a GridView as a single item, using fill_parent as a size will cause the LinearLayout to fill the space provided for that grid item (not the entire GridView).
Displaying icons in a GridView
We need an object model and ListAdapter in order to display the fruits to the user in a GridView. The adapter is fairly straightforward at this point. It's a normal ListAdapter implementation built on top of an item class and the layout XML we defined for the icons. For each item of fruit, we will need an object holding both the fruit's name and icon. Create a FruitItem class in the root package with the following code: class FruitItem { final String name; final int image; FruitItem(String name, int image) { this.name = name; this.image = image; } }
In the preceding code, we referenced the icon image for the fruit as an integer. When we reference application resources and IDs in Android, it's always with an integer. For this example we're assuming that all of the different types of fruit each have an icon as an application resource. Another option would be to hold a reference to a Bitmap object in each FruitItem. However, this would have meant holding the full image in memory when the FruitItem is potentially not on the screen. In order for the Android Asset Packaging Tool to recognize and store the icons, you will need to put them in the res/drawable directory.
Android Image Resources Generally, it's considered a good practice in Android to store bitmap images as PNG files. Since you will be accessing these files from your code, make sure they have Java-friendly filenames. The PNG format (unlike JPG) is lossless, can have various different color depths, and correctly handles transparency. This generally makes it a great image format on the whole.
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Time for action – building the fruit menu
For the Four Buckets 'o Fruit menu, we're going to need a ListAdapter implementation to render the FruitItem objects into the fruit_item.xml layout resources. We'll also need a layout resource for the GridView which we will load in our new Activity class.
1. 2. 3.
Create a new class named FruitAdapter extending BaseAdapter in the root package of the project.
FruitAdapter needs to hold and represent an array of FruitItem objects. Implement the class using the same structure as the BurgerAdapter.
In the ListAdapter.getView method, set the label and icon as defined in the fruit_item.xml layout resource:
FruitItem item = items[index]; TextView text = ((TextView)view.findViewById(R.id.text)); ImageView image = ((ImageView)view.findViewById(R.id.icon)); text.setText(item.name); image.setImageResource(item.image);
4. 5.
Create a new layout resource to hold the GridView that we will use for the Four Buckets 'o Fruit menu, and name it res/layout/four_buckets.xml. Populate the new layout resource with a three column GridView:
What just happened?
The new four_buckets.xml layout resource has nothing but a GridView. This is unlike the other layout resources we've written so far, especially since the GridView has no ID. For this example, the fruit menu Activity will contain nothing but the GridView, so there's no need for an ID reference or layout structure. We also specified horizontal and vertical spacing of 5dip. A GridView object's default is to have no spacing between its cells, which makes for fairly squashed content. In order to space things out a bit, we ask for some whitespace between each of the cells.
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Time for action – creating the FourBucketsActivity
Since we are working with a layout resource with only a GridView, and no ID reference, we're going to walk through the creation of the Activity step-by-step. Unlike previous Activity implementations, we will need a direct reference to the GridView defined in four_buckets.xml, and this means loading it manually.
1. 2.
Start by creating a new class in your project's root package: public class FourBucketsActivity extends Activity {
Override the onCreate method, and invoke the super implementation: protected void onCreate(final Bundle istate) { super.onCreate(istate);
3. 4.
Get the LayoutInflator instance for your Activity object:
LayoutInflater inflater = getLayoutInflater();
Inflate the four_buckets.xml resource and cast its contents directly to a GridView object:
GridView view = (GridView)inflater.inflate( R.layout.four_buckets, null);
5.
Set the ListAdapter of the view object to a new instance of the FruitAdapter class, and populate the new FruitAdapter with some FruitItem objects: view.setAdapter(new FruitAdapter( new FruitItem("Apple", R.drawable.apple), new FruitItem("Banana", R.drawable.banana), new FruitItem("Black Berries", R.drawable.blackberry), // and so on
6. 7. 8.
Use setContentView to make the GridView your root View object: setContentView(view); Register your FourBucketsActivity class in your AndroidManifest.xml. Add a case to the SelectRestaurantActivity to start the new FourBucketsActivity when the user selects it.
What just happened?
You just completed the Four Buckets 'o Fruit menu. If you re-install the application into your emulator, you'll now be able to go and order fruits (just be careful to have the 16 ton weight ready in case the delivery guy attacks you).
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If you look through the Activity documentation, you'll notice that while there's a setContentView method, there's no corresponding getContentView method. Take a closer look and you will notice the addContentView method. An Activity object may have any number of View objects attached to it as "content". This precludes any useful implementation of a getContentView method. In order to get around this limitation, we inflated the layout ourselves. The getLayoutInflator() method used is simply a shortcut for LayoutInflator. from(this). Instead of using an ID and findViewById, we simply cast the View returned directly to a GridView, since that's all that our four_buckets.xml file contains (much the same way the ArrayAdapter class works with TextView objects). If we wanted to make things a little more abstract, we could have cast it to an AdapterView, in which case we could have swapped in implementation in the file with a ListView. However,
this wouldn't have been very useful for this example. If you now re-install and run the application, your new FourBucketsActivity will present you with a screen similar to the following one:
Have a go hero – Sam's Sushi
The last restaurant on the menu is Sam's Sushi. Try using the Spinner class along with a GridView to create a composite sushi menu. Place the spinner at the top of the screen, with options for different types of sushi:
Sashimi Maki Roll Nigiri Oshi
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California Roll Fashion Sandwich Hand Roll
Below the Spinner, use a GridView to display icons for each different type of fish that the user can order. Here are some suggestions:
Tuna Yellowtail Snapper Salmon Eel Sea Urchin Squid Shrimp
The Spinner class makes use of the SpinnerAdapter instead of a ListAdapter. The SpinnerAdapter includes an additional View object which represents the drop-down menu. This is most typically a reference to the android.R.layout.simple_ dropdown_item_1line resource. However, for this example, however, you can probably make use of the android:entries attribute on the Spinner XML element.
Summary
Data display is one of the most common requirements of a mobile application, and Android has many different options available. The ListView is probably one of the most commonly used widgets in the standard Android suite, and styling it allows it to be used to display varying amounts of data, from one line menu items to multi-line to-do notes. The GridView is effectively a tabular version of ListView, and is well suited for presenting the user with icon views. Icons have enormous advantages over text, since they can be recognized much more quickly by the user. Icons can also take up significantly less space, and in a GridView, you could easily fit four to six icons in a portrait screen without making the user interface cluttered or more difficult to work it. This also frees up precious screen space for other items to be displayed.
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Building custom Adapter classes not only allows you to take complete control over the styling of the ListView, but also determine where the data comes from, and how it's loaded. You could, for example, load the data directly from a web service by using an Adapter which generates dummy View objects until the web service responds with actual data. Take a good look at the default Adapter implementations, they will generally serve your requirements, especially when coupled with a custom layout resource. In the next chapter, we will take a look at some less generic, more specialized View classes that Android provides. As with almost everything in Android, the defaults may be specific, but they can be customized in any number of ways to fit some very unusual purposes.
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Developing with Specialized Android Widgets
Along with the many generic widgets such as buttons, text fields, and checkboxes, Android also includes a variety of more specialized widgets. While a button is fairly generic, and has use in many situations, a gallery-widget for example, is far more targeted. In this chapter we will start looking at the more specialized Android widgets, where they appear, and how best they can be used.
3
Although these are very specialized View classes, they are very important. As mentioned earlier (and it really can't be stressed enough) one of the cornerstones of good user interface design is consistency. An example is the DatePicker widget. It's certainly not the prettiest date-selector in the world. It's not a calendar widget, so it's sometimes quite difficult for the user to select exactly which date they want (most people think in terms of "next week Tuesday", and not "Tuesday the 17th"). However, the DatePicker is standard! So the user knows exactly how to use it, they don't have to work with a broken calendar implementation. This chapter will work with Android's more specialized View and layout classes:
These classes have very specialized purposes, and some have slight quirks in the way they are implemented. This chapter will explore how and where to use these widgets, and where you need to be careful of their implementation details. We'll also discuss how best to incorporate these elements into an application, and into a layout.
Creating a restaurant review application
In the previous chapter, we built an ordering-in application. In this chapter, we're going to take a look at reviewing restaurants. The application will allow the user to view other people's opinions on the restaurant, a gallery of photos of the restaurant, and finally a section for making an online reservation. We will divide the application into three sections:
Review: Review and ratings information for this restaurant Photos: A photo gallery of the restaurant Reservation: Request a reservation with the restaurant
When building an application where all three of these sections need to be quickly available to the user, the most sensible option available is to place each of the sections in a tab on the screen. This allows the user to switch between the three sections without having all of them on the screen at the same time. This also saves screen real estate giving us more space for each section. The Review tab will include a cycling list of comments that people have made about the restaurant being viewed, and an average "star" rating for the restaurant. Displaying photographs of the restaurant is the job of the Photos tab. We'll provide the user with a thumbnail "track" at the top of the screen, and a view of the selected image consuming the remaining screen space. For the Reservation tab, we will want to capture the user's name and when they would like the reservation to be (date and time). Finally we also need to know for how many people the reservation will be made.
Time for action – creating the robotic review project structure
To start this example we'll need a new project with a new Activity. The new layout and Activity will be a little different from the structures in the previous two chapters. We will need to use the FrameLayout class in order to build a tabbed layout. So to begin, we'll create a new project structure and start off with a skeleton that will later become our tab layout structure. This can be filled with the three content areas.
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1.
Create a new Android project using the Android command-line tool: android create project -n RoboticReview -p RoboticReview -k com. packtpub.roboticreview -a ReviewActivity -t 3
2. 3. 4.
Open the res/layout/main.xml file in an editor or IDE. Clear out the default code (leaving in the XML header). Create a root FrameLayout element:
5.
Inside the new FrameLayout element, add a vertical LinearLayout:
6.
After the LinearLayout, add another empty LinearLayout element:
7.
Then, after the second LinearLayout element, add an empty ScrollView:
The FrameLayout will be used by the Android tab structures as a content area, each of the child elements will become the contents of a tab. In the preceding layout, we've added in two LinearLayout elements for the Review and Photos sections, and a ScrollView for the Reservation tab.
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What just happened?
We've just started the "restaurant review" application, building a skeleton for the user interface. There are several key parts of this main.xml file which we should walk through before continuing the example. First, our root element is a FrameLayout. The FrameLayout anchors all of its children to its own top-left corner. In effect, the two occurrences of LinearLayout and the ScrollView will overlap each other. This structure can be used to form something like a Java AWT CardLayout, which will be used by the TabHost object to display these objects when their relative tab is active. Second, each of the LinearLayout and the ScrollView have an ID. In order to identify them as tab roots, we need to be able to easily access them from our Java code. Tab structures may be designed in XML, but they need to be put together in Java.
Building a TabActivity
In order to continue, we need our Activity class to set up the three tab content elements we declared in our main.xml file as tabs. By preference, all tabs in Android should have an icon. The following is a screenshot of the tabs without their icons:
The following is a screenshot of the tabs with the icons:
Creating tab icons
Android applications have a specific look and feel defined by the default widgets provided by the system. In order to keep all applications consistent for users, there are a set of user interface guidelines that application developers should follow. While it's important to have your application stand out, users will often get frustrated with applications that are not familiar or look out of place (this is one of the reasons why automatically ported applications are often very unpopular).
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Android tabs and icons
When selecting tab icons for your application, it's considered a good practice to include several different versions of the icon for different screen sizes and densities. The anti-aliased corners that look so good on a high-density screen, look terrible on low-density screens. You can also provide entirely different icons for very small screens, instead of loosing all of your icons details. Android tabs appear raised when they are selected, and lowered in the background when they are not selected. The Android tab icons should appear in the "opposite" etching effect to the tab that they are placed in, that is, lowered when they are selected and raised when they are not selected. The icons therefore have two primary states: selected and unselected. In order to switch between these two states, a tab-icon will generally consist of three resource files:
The selected icon image The unselected icon image An XML file describing the icon in terms of its two states
Tab icons are generally simple shapes while the image size is squared (generally at a maximum of 32 x 32 pixels). Different variations of the image should be used for screens of different pixel densities (see Chapter 1, Developing a Simple Activity for "Resource Selection" details). Generally you will use a dark outset image for the selected state, since when a tab is selected, the tab background is light. For the unselected icon, the opposite is true and a light inset image should be used instead. The bitmap images in an Android application should always be in the PNG format. Let's call the selected icon for the Review tab res/drawable/ic_tab_selstar.png, and name the unselected icon file res/drawable/ic_tab_unselstar.png. In order to switch states between these two images automatically, we define a special StateListDrawable as an XML file. Hence the Review icon is actually in a file named res/drawable/review. xml, and it looks like this:
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Note the android:constantSize="true" of the element. By default, Android will assume that each state in the resulting StateListDrawable object will cause the image to be of a different size, which in turn may cause the user interface to re-run its layout calculations. This can be fairly expensive, so it's a good idea to declare that each of your states is exactly of the same size. For this example, we'll be using three tab icons, each with two states. The icons are named review, photos, and book. Each one is composed of three files: A PNG for the selected icon, a PNG for the unselected icon, and an XML file defining the state-selector. From our application, we will only make direct use of the state-selector XML files, leaving the Android APIs to pickup the actual PNG files.
Implementing the ReviewActivity
As usual, we will want to have localized text in our strings.xml file. Open the res/ values/strings.xml file and copy the following code into it: Robotic Review Review Photos Reservations
Time for action – writing the ReviewActivity class
As already said, we will need to set up our tabbed-layout structure in our Java code. Fortunately, Android provides a very useful TabActivity class that does much of the heavy lifting for us, providing us with a ready-made TabHost object with which we can construct the Activity tab structure.
1. 2. 3. 4.
Open the ReviewActivity.java file generated earlier in an editor or IDE. Instead of extending Activity, change the class to inherit TabActivity: public class ReviewActivity extends TabActivity
In the onCreate method, remove the setContentView(R.layout.main) line (generated by the android create project utility) completely. Now start by fetching the TabHost object from your parent class:
TabHost tabs = getTabHost();
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5.
Next, we inflate our layout XML into the content view of the TabHost: getLayoutInflater().inflate( R.layout.main, tabs.getTabContentView(), true);
6. 7.
We'll need access to our other application resources:
Resources resources = getResources();
Now we define a TabSpec for the Review tab:
TabHost.TabSpec details = tabs.newTabSpec("review"). setContent(R.id.review). setIndicator(getString(R.string.review), resources.getDrawable(R.drawable.review));
8. 9.
Define two more TabSpec variables for the Photos and Reservation tabs using the preceding pattern. Add each of the TabSpec objects to our TabHost: tabs.addTab(details); tabs.addTab(gallery); tabs.addTab(reservation);
This concludes the creation of the tab structure for the ReviewActivity class.
What just happened?
We built a very basic tabbed-layout for our new ReviewActivity. When working with tabs, we didn't simply use the Activity.setContentView method, instead we inflated the layout XML file ourselves. Then we made use of the TabHost object provided by the TabActivity class to create three TabSpec objects. A TabSpec is a builder object that enables you to build up the content of your tab, similar to the way you build up text with a StringBuilder. The content of a TabSpec is the content-view that will be attached to the tab on the screen (assigned using the setContent method). In this example, we opted for the simplest option and defined the tab content in our main.xml file. It's also possible to lazy-create the tab content using the TabHost.TabContentFactory interface, or even to put an external Activity (such as the dialer or browser) in the tab by using setContent(Intent). However, for the purposes of this example we used the simplest option. You'll notice that the TabSpec (much like the StringBuilder class) supports chaining of method calls, making it easy and flexible to either set up a tab in a "single shot" approach (as done previously), or build up the TabSpec in stages (that is, while loading from an external service).
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The indicator we assigned to the TabSpec is what will appear on the tab. In the previous case, a string of text and our icon. As of API level 4 (Android version 1.6) it's possible to use a View object as an indicator, allowing complete customization of the tab's look and feel. To keep the example simple (and compatible with earlier versions) we've supplied a String resource as the indicator.
Time for action – creating the Review layout
We've got a skeleton tab structure, but there's nothing in it yet. The first tab is titled Review, and this is where we are going to start. We've just finished enough Java code to load up the tabs and put them on the screen. Now we go back to the main.xml layout file and populate this tab with some widgets that supply the user with review information.
1. 2.
Open res/layout/main.xml in an editor or IDE. Inside the that we named review, add a new TextView that will contain the name of the restaurant:
3.
Below the new TextView, add a new RatingBar, where we will display how other people have rated the restaurant:
4.
Keeping this first tab simple, we add a TextSwitcher where we can display other people's comments about the restaurant:
The Review tab only has three widgets in this example, but more could easily be added to allow the user to input their own reviews.
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What just happened
We just put together the layout for our first tab. The RatingBar that we created has a width of wrap_content, which is really important. If you use fill_parent, then the number of stars visible in the RatingBar will simply be as many as can fit on the screen. If you want control over how many stars appear on your RatingBar, stick to wrap_content, but also make sure that (at least on portrait layouts) the RatingBar has its own horizontal line. If you install the Activity in the emulator now, you won't see anything in either the TextView or the TextSwitcher. The TextSwitcher has no default animations, so we specify the "in" animation as the default fade_in as provided by the android package, while the "out" animation will be fade_out. This syntax is used to access resources that can be found in the android.R class.
Working with switcher classes
The TextSwitcher we've put in place is used to animate between different TextView objects. It's really useful for displaying things like changing stock-prices, news headlines, or in our case, reviews. It inherits from ViewSwitcher which can be used to animate between any two generic View objects. ViewSwitcher extends ViewAnimator which can be used as a sort of animated CardLayout. We want to display a series of comments from past customers, fading between each of them with a short animation. TextSwitcher needs two TextView objects (which it will ask us to create dynamically), for our example. We want these to be in a resource file. For the next part of the example, we'll need some comments. Instead of using a web service or something similar to fetch real comments, this example will load some comments from its application resources. Open the res/values/strings.xml file and add with a few likely comments in it: Just Fantastic Amazing Food What rubbish, the food was too hairy Messy kitchen; call the health inspector.
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Time for action – turning on the TextSwitcher
We want the TextSwitcher to display the next listed comment every five seconds. For this we'll need to employ new resources, and a Handler object. A Handler is a way for Android applications and services to post messages between threads, and can also be used to schedule messages at a point in the future. It's a preferred structure to use over a java. util.Timer since a Handler object will not allocate a new Thread. In our case, a Timer is overkill, as there is only one task we want to schedule.
1. 2.
Create a new XML file in your res/layout directory named review_comment.xml. Copy the following code into the new review_comment.xml file:
3. 4. 5. 6. 7.
Open the ReviewActivity.java file in your editor or IDE. We'll need to be able to load the review_comment resources for the TextSwitcher, so ReviewActivity needs to implement the ViewSwitcher. ViewFactory interface. In order to be update the TextSwitcher, we need to interact with a Handler, and the easiest way to do that here is to also implement Runnable. At the top of the ReviewActivity class, declare a Handler object: private final Handler switchCommentHandler = new Handler();
We'll also want to hold a reference to the TextSwitcher for our run() method when we switch comments: private TextSwitcher switcher;
8.
In order to display the comments, we'll need an array of them, and an index to keep track of which comment the TextSwitcher is displaying: private String[] comments; private int commentIndex = 0;
9.
Now, in the onCreate method, after you add the TabSpec objects to the TabHost, read the comments string-array from the Resources: comments = resources.getStringArray(R.array.comments); [ 76 ]
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10. 11. 12.
Next, find the TextSwitcher and assign it to the switcher field: switcher = (TextSwitcher)findViewById(R.id.reviews);
Tell the TextSwitcher that the ReviewActivity object will be its ViewFactory: switcher.setFactory(this); In order to comply with the ViewFactory specification, we need to write a makeView method. In our case it's really simple—inflate the review_comment resource: public View makeView() { return getLayoutInflater().inflate( R.layout.review_comment, null); }
13.
Override the onStart method so that we can post the first timed event on the Handler object declared earlier: protected void onStart() { super.onStart(); switchCommentHandler.postDelayed(this, 5 * 1000l); }
14.
Similarly, override the onStop method to cancel any future callback: protected void onStop() { super.onStop(); switchCommentHandler.removeCallbacks(this); }
15.
Finally, the run() method alternates the comments in the TextSwitcher, and in the finally block, posts itself back onto the Handler queue in five seconds: public void run() { try { switcher.setText(comments[commentIndex++]); if(commentIndex >= comments.length) { commentIndex = 0; } } finally { switchCommentHandler.postDelayed(this, 5 * 1000l); } }
Using Handler objects instead of creating Thread objects means all of the timed tasks can share the main user interface thread instead of each allocating a separate thread. This reduces the amount of memory and CPU load your application places on the device, and has a direct impact on the application performance and battery life.
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What just happened?
We just built a simple timer structure to update the TextSwitcher with a rotating array of comments. The Handler class is a convenient way to post messages and actions between two application threads. In Android, as with Swing, the user interface is not thread-safe, so inter-thread communication becomes very important. A Handler object attempts to bind itself to the thread it's created in (in the preceding case, the main thread). It's a prerequisite that a thread which creates a Handler object must have an associated Looper object. You can set this up in your own thread by either inheriting the HandlerThread class, or using the Looper.prepare() method. Messages sent to a Handler object will be executed by the Looper associated with the same thread. By sending our ReviewActivity (which implements Runnable) to the Handler object that we had created in the main thread, we know that the ReviewActivity.run() method will be executed on the main thread, regardless of which thread posted it there. In the case of long-running tasks (such as fetching a web page or a long-running calculation), Android provides a class that bares a striking resemblance to the SwingWorker class, named AsyncTask. AsyncTask (like Handler) can be found in the android.os package, and you make use of it by inheritance. AsyncTask is used to allow interaction between a background task and the user interface (in order to update a progress bar or similar requirements).
Creating a simple photo gallery
The use of the word Gallery is a little misleading, it's really a horizontal row of items with a "single item" selection model. For this example we'll be using the Gallery class for what it does best, displaying thumbnails. However, as you'll see, it's capable of displaying scrolling lists of almost anything. Since a Gallery is a spinner, you work with it in much the same way as a Spinner object or a ListView, that is, with an Adapter.
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Time for action – building the Photos tab
Before we can add images to a Gallery, we need the Gallery object on the screen. To start this exercise, we'll add a Gallery object and an ImageView to FrameLayout of our tabs. This will appear under the Photos tab that we created at the beginning of the chapter. We'll stick to a fairly traditional photo gallery model of the sliding thumbnails at the top of the screen, with the full view of the selected image below it.
1. 2.
Open res/layout/main.xml in your editor or IDE. Inside the second LinearLayout, with android:id="@+id/photos", add a new Gallery element to hold the thumbnails:
3.
Gallery objects, by default, squash their contents together, which really doesn't look great in our case. You can add a little padding between the items by using the spacing attribute of Gallery class: android:spacing="5dip" 4.
We also have tabs directly above the Gallery, and we'll have an ImageView directly below it. Again, there won't be any padding, so add some using a margin: android:layout_marginTop="5dip" android:layout_marginBottom="5dip"
5.
Now create an ImageView which we can use to display the full-sized image:
6.
In order to ensure that the full display is scaled correctly, we need to specify the scaleType on the ImageView: android:scaleType="centerInside" The Gallery element provides us with the thumbnail track at the top of the screen. The image selected in the Gallery will be displayed at full-size in the ImageView widget.
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What just happened?
We just populated the second tab with the standard widgets required for a basic photo gallery. This structure is very generic, but is also well known and understood by users. The Gallery class will handle the thumbnails, scrolling, and selection. However, you will need to populate the main ImageView with the selected image, and provide the Gallery object with the thumbnail widgets to display on the screen. The spacing attribute on the Gallery element will add some whitespace, which serves as a simple separator between thumbnails. You could also add a border into each of the thumbnail images, border each ImageView widget you return for a thumbnail, or use a custom widget to create a border.
Creating a thumbnail widget
In order to display the thumbnails in the Gallery object, we will need to create an ImageView object for each thumbnail. We could easily do this in Java code, but as usual, it is preferable to build even the most basic widgets using an XML resource. In this case, create a new XML resource in the res/layout directory. Name the new file gallery_thn.xml and copy the following code into it:
That's right, it has just two lines of XML, but to reiterate, this allows us to customize this widget for many different configurations without editing the Java code. While editing the code might not seem like a problem (the resource needs to be recompiled anyway), you also don't want to end up with a long series of if statements to decide on exactly how you should create the ImageView objects.
Implementing a GalleryAdapter
For the example, we'll stick to using application resources to keep things simple. We'll have two arrays of resource IDs, thumbnails, and the full-size images. An Adapter implementation is expected to provide an identifier for each of the items. In this next example, we're going to provide an identifier as the resource identifier of the full-size image, which gives us easy access to the full-size image in classes outside of the Adapter implementation. While this is an unusual contract, it provides a convenient way for us to pass the image resource around within an already defined structure. In order to display your gallery, you'll need some images to display (mine are sized 480 x 319 pixels). For each of these images, you'll need a thumbnail image to display in the Gallery object. Generally, these should simply be a scaled-down version of the actual image (mine are scaled to 128 x 84 pixels).
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Time for action – the GalleryAdapter
Creating the GalleryAdapter is much like the ListAdapter classes we created in Chapter 2, Presenting Data for Views. The GalleryAdapter however, will use ImageView objects instead of TextView objects. It also binds two lists of resources together instead of using an object model.
1. 2.
Create a new Java class in your project root package named GalleryAdapter. It should extend the BaseAdapter class. Declare an integer array to hold the thumbnail resource IDs: private final int[] thumbnails = new int[]{ R.drawable.curry_view_thn, R.drawable.jai_thn, // your other thumbnails };
3.
Declare an integer array to hold the full-size image resource IDs: private final int[] images = new int[]{ R.drawable.curry_view, R.drawable.jai, // your other full-size images };
4.
The getCount() method is simply the length of the thumbnails array: public int getCount() { return thumbnails.length; }
5.
The getItem(int) method returns the full-size image resource ID: public Object getItem(int index) { return Integer.valueOf(images[index]); }
6.
As mentioned earlier, the getItemId(int) method returns the full-size image resource ID (almost exactly the way that getItem(int) does): public long getItemId(int index) { return images[index]; }
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7.
Finally, the getView(int, View, ViewGroup) method uses a LayoutInflater to read and populate the ImageView which we created in the gallery_thn.xml layout resource: public View getView(int index, View reuse, ViewGroup parent) { ImageView view = (reuse instanceof ImageView) ? (ImageView)reuse : (ImageView)LayoutInflater. from(parent.getContext()). inflate(R.layout.gallery_thn, null); view.setImageResource(thumbnails[index]); return view; }
The Gallery class is a subclass of AdapterView and so functions in the same way as a ListView object. The GalleryAdapter will provide the Gallery object with View objects to display the thumbnails in.
What just happened
Much like the Adapter classes built in the last chapter, the GalleryAdapter will attempt to reuse any View object specified in its getView method. A primary difference however, is that this GalleryAdapter is entirely self-contained, and will always display the same list of images. This example of a GalleryAdapter is extremely simple. You could also build a GalleryAdapter that held bitmap objects instead of resource ID references. You'd then make use of the ImageView.setImageBitmap method instead of ImageView.setImageResource. You could also eliminate the thumbnail images by having the ImageView scale the full-size images into thumbnails. This would just require a modification to the gallery_thn.xml resource file in order to specify the required size of each thumbnail.
The adjustViewBounds attribute tells the ImageView to adjust its own size in a way such that it maintains the aspect ratio of the image it contains. We also change the scaleType attribute to centerInside, which will also retain the aspect ratio of the image when it scales. Finally, we set a maximum width for the ImageView. Using the standard layout_width or layout_height attributes is ignored by the Gallery class, so we instead specify the desired thumbnail size to the ImageView (the layout_width and layout_height attributes are handled by the Gallery, while the maxWidth and maxHeight are handled by the ImageView).
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This would be a standard speed/size trade-off. Having the thumbnail images takes up more application space, but having the ImageView perform the scaling makes the application slower. The scaling algorithm in ImageView will also not be as high-quality as the scaling performed in an image-manipulation application such as Adobe Photoshop. In most cases this won't be a problem, but if you have high detail images, you often get "scaling artifacts" with simpler scaling algorithms.
Time for action – making the gallery work
Now that we've got the GalleryAdapter working, we need to connect the Gallery, the GalleryAdapter, and the ImageView together, so that when a thumbnail is selected, the full-view of that image is displayed in the ImageView object.
1. 2. 3.
Open the ReviewActivity source code in your editor or IDE. Add AdapterView.OnItemSelectedListener to the interfaces that the ReviewActivity implements. Below the declaration of the TextSwitcher, declare a reference to the ImageView which will hold the full-size image: private TextSwitcher switcher; private ImageView photo;
4.
At the end of the onCreate method, find the ImageView named photo and assign it to the reference you just declared: photo = ((ImageView)findViewById(R.id.photo));
5. 6. 7. 8.
Now fetch the Gallery object as declared in the main.xml layout resource:
Gallery photos = ((Gallery)findViewById(R.id.gallery));
Create a new GalleryAdapter and set it on the Gallery object: photos.setAdapter(new GalleryAdapter());
Set the OnItemSelectedListener of the Gallery object to this: photos.setOnItemSelectedListener(this); At the end of the ReviewActivity class, add the onItemSelected method: public void onItemSelected( AdapterView av, View view, int idx, long id) { photo.setImageResource((int)id); }
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9.
OnItemSelectedListener requires an onNothingSelected method as well,
but we don't need it to do anything for this example.
The GalleryAdapter provides the ReviewActivity with the resource to load for the full view of the photo through the id parameter. The id parameter could also be used as an index or identifier for a URL if the image was located on a remote server.
What just happened?
We've now connected the Gallery object to the ImageView where we will display the fullsize image instead of the thumbnail. We've used the item ID as a way to send the resource ID of the full-size image directly to the event listener. This is a fairly strange concept since you'd normally use an object model. However, an object model in this example wouldn't just introduce a new class, it would also require another method call (in order to fetch the image object from the Adapter when the event is triggered). When you specify an Adapter on an AbsSpinner class like Gallery, it will by default attempt to select the first item returned from its new Adapter. This in turn notifies the OnItemSelectedListener object if one has been registered. However, because of the single-threading model used by the Android user interface objects, this event doesn't get fired immediately, but rather some time after we return from the onCreate method. When we call setAdapter(new GalleryAdapter()) on the Gallery object, it schedules a selection change event, which we then receive. The event causes the ReviewActivity class to display the first photo in the GalleryAdapter object. If you now reinstall the application in your emulator, you'll be able to go to the Photos tab and browse through a Gallery of all the images that you had populated the GalleryAdapter with.
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Pop quiz
1. What would happen in the previous example if you substituted OnItemSelectedListener with OnItemClickListener (as done in the ListView examples)? a. The full size won't appear anymore. b. The Gallery will not rotate the thumbnails when they are touched. c. The full-size photo won't appear until a thumbnail is clicked. 2. What is the primary difference between the ScaleType values fitXY and centerInside? a. The fitXY type will anchor the picture to the top-left, while centerInside will center the picture in the ImageView. b. fitXY will cause the picture to distort to the size of the ImageView, while centerInside will maintain the picture's aspect ratio. c. centerInside will cause the larger axis to be cropped in order to fit the picture into the ImageView, while fitXY will scale the picture so that the larger axis is of the same size as the ImageView.
3. What dictates the size of a Gallery object containing ImageView objects when using the wrap_content attribute? a. The width and height of the ImageView objects, as dictated by the size of their content image, or their maxWidth and maxHeight parameters. b. The itemWidth and itemHeight parameters on the Gallery object. c. The LayoutParams set on the ImageView objects (either with the setLayoutParams method, or layout_width/layout_height attributes).
Have a go hero – animations and external sources
Now that you have the basic example working, try improving the user experience a bit. When you touch the images, they should really animate instead of undergoing an instant change. They should also come from an external source instead of application resources. 1. Change the ImageView object of full-size images to an ImageSwitcher, use the standard Android fade-in/fade-out animations. 2. Remove the thumbnail images from the project, and use the ImageView declared in the gallery_thn.xml file to scale the images. 3. Change from a list of application resource IDs to a list of Uri objects so that the images are downloaded from an external website.
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Building the reservation tab
While the Review and Photos tabs of this example have been concerned with displaying information, the Reservation tab will be concerned with capturing the details of a reservation. We really only need three pieces of information:
The name under which the reservation needs to be made The date and time of the reservation How many people the reservation is for
In this part of the example we'll create several widgets which have formatted labels. For example, How Many People: 2, which will update the number of people as the user changes the value. In order to do this simply, we specify that the widget's text (as specified in the layout file) will contain the format to use for display. As part of the initialization procedure, we read the text from the View object and use it to create a format structure. Once we have a format, we populate the View with its initial value.
Time for action – implementing the reservation layout
In our main.xml layout resource, we need to add the View objects which will form the Reservation tab. Currently it consists only of an empty ScrollView, which enables vertically-long layouts to be scrolled by the user if the entire user interface doesn't fit on the screen.
1. 2.
Open the main.xml file in your editor or IDE. Inside the we had created for the Reservation tab earlier. Declare a new vertical LinearLayout element:
3.
Inside the new LinearLayout element, create a TextView to ask the user under what name the reservation should be made:
4.
After the TextView label, create an EditText to allow the user to input the name under which reservation is to be made: [ 86 ]
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5.
Create another TextView label to ask the user how many people will be going. This includes a format element where we will place the number:
6.
Add a SeekBar with which the user can tell us about how many people are going:
7.
Use another TextView to ask the user what date the reservation will be on:
8.
Add a Button to display the date for which the reservation is made. When the user taps this Button, we will ask him to select a new date:
9.
Create another TextView label to ask the time of reservation:
10.
Add another Button to display the time, and allow the user to change it:
11.
Finally add a Button to make the reservation, and add some margin to separate it from the rest of the inputs in the form: [ 87 ]
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Several of the preceding widgets include the format of their labels instead of the label literal, the actual label will be generated and set in the Java code. This is because these labels are subject to change when the user changes date, time, or the number of people expected for the reservation.
What just happened?
In the Reservation tab, we ask the user how many people the reservation is for, and in order to capture their answer, we make use of a SeekBar object. The SeekBar works in much the same way as a JSlider in Swing, and provides the user with a way of selecting the number of people for the reservation, as long as that number is within a range that we define. SeekBar in Android is actually built on top of the ProgressBar class, and so inherits all of its XML attributes, which will seem a little strange at times. Unfortunately, unlike a JSlider or JProgressBar, the SeekBar class has no minimum value, and since you can't make a reservation for 0 people, we work around this by always adding 1 to the selected value of the SeekBar before display. This means that the default value is 1 (setting the displayed value to 2 people).
Most people would make a restaurant reservation for two people, hence the default value of 1.
In the How Many People: label, we put in a %d, which is a printf marker for where we will put the number of people the reservation is being made for. When the SeekBar is manipulated by the user, we'll update the label with the number the user selects using String.format. In the "date" and "time" Button labels, we want to display the currently selected date and time for the reservation. We set the label in the XML file to the format that we want to display this data in, and we'll parse it later with a standard java.text. SimpleDateFormat. What about internationalization in our previous example? Shouldn't we have put the labels in the strings.xml file so that the layout doesn't need to change? The answer is: Yes, if you want to internationalize your user interface. Later, make sure you have all of your display text in an application resource file. However, I strongly recommend fetching the format strings directly from the layout, since it allows you to decouple the format data one additional level. In the preceding layout, you created Button widgets to display the date and time. Why not use a DatePicker and TimePicker object directly? The answer is: They unfortunately don't fit well into normal layouts. They take up a large amount of vertical space, and don't scale horizontally. If we placed a DatePicker and TimePicker inline in this user interface, it would look like the following screenshot on the left, while the actual user interface is the screenshot on the right.
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As you can see, the Button objects give a much cleaner user interface. Thankfully, Android provides us with a DatePickerDialog and TimePickerDialog for just this sort of situation. When the user taps on one of the Button widgets, we'll pop up the appropriate dialog and then update the selected Button label when he approves. While the use of a Button and Dialog adds at least two more touches to the user interface, it dramatically improves the look and feel of the application. User interfaces that are not properly aligned will irritate users, even if they can't tell why it's irritating. Screens that users find annoying or irritating are screens that they will avoid, or worse—simply uninstall.
Time for action – initializing the reservation tab
In the Reservation tab we made use of formatted labels. These labels shouldn't be displayed to the user as-is, but need to be populated with data before we let the user see them. For this, we need to go to our Java code again and build some functionality to remember the format, and populate the label.
1. 2.
Open the ReviewActivity Java source in your editor or IDE. Below of all the fields you've declared so far, we need to add some more for the Reservations tab. Declare a String to remember the formatting of the How Many People: label: private String peopleLabelFormat;
3.
Then declare a reference to the How Many People: label: private TextView peopleLabel;
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4. 5. 6. 7. 8.
Declare a SimpleDateFormat object for the format of the date Button: private SimpleDateFormat dateFormat;
Declare a reference to the date Button: private Button date;
Add another SimpleDateFormat for the format of the time Button: private SimpleDateFormat timeFormat;
Next, declare a Button reference for the time Button object: private Button time;
At the end of the onCreate method, we need to initialize the Reservations tab. Start by assigning out the peopleLabel and fetching the peopleLabelFormat using the TextView.getText() method: peopleLabel = (TextView)findViewById(R.id.people_label); peopleLabelFormat = peopleLabel.getText().toString();
9.
Then fetch the date Button reference and its label format: date = (Button)findViewById(R.id.date); dateFormat = new SimpleDateFormat(date.getText().toString());
10.
Do the same for the time Button and its label format: time = (Button)findViewById(R.id.time); timeFormat = new SimpleDateFormat(time.getText().toString());
11.
Now we need to populate the Button objects with a default date and time, and for this we need a Calendar object:
Calendar calendar = Calendar.getInstance();
12.
If it's later than 4:00p.m., it's likely that the reservation should be made for the next day, so we add one day to the Calendar if this is the case: if(calendar.get(Calendar.HOUR_OF_DAY) >= 16) { calendar.add(Calendar.DATE, 1); }
13.
Now we set the default time of day for a reservation on the Calendar object: calendar.set(Calendar.HOUR_OF_DAY, 18); calendar.clear(Calendar.MINUTE); calendar.clear(Calendar.SECOND); calendar.clear(Calendar.MILLISECOND);
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14.
Set the label for the date and time button from the Calendar object:
Date reservationDate = calendar.getTime(); date.setText(dateFormat.format(reservationDate)); time.setText(timeFormat.format(reservationDate));
15.
Now we need the SeekBar so that we can fetch its default value (as declared in the layout application resource):
SeekBar people = (SeekBar)findViewById(R.id.people);
16.
Then we can use the label format, and the SeekBar value to populate the How Many People label: peopleLabel.setText(String.format( peopleLabelFormat, people.getProgress() + 1));
Now we have the various formats in which the labels need to be displayed on the user interface. This allows us to regenerate the labels when the user changes the reservation parameters.
What just happened?
The Reservations tab will now be populated with the default data for a reservation, and all the formatting in the labels has disappeared. You will probably have noticed the many calls to toString() in the previous code. Android View classes generally accept any CharSequence for labels. This allows for much more advanced memory management than the String class, as the CharSequence may be a StringBuilder, or may facade a SoftReference to the actual text data. However, most traditional Java APIs expect a String, not a CharSequence, so we use the toString() method to make sure we have a String object. If the underlying CharSequence is a String object, the toString() method is a simple return this; (which will act as a type cast). Again, to work around the fact that the SeekBar doesn't have a minimum value, we add 1 to its current value in the last line, when we populate the peopleLabel. While the date and time formats are stored as a SimpleDateFormat, we store the peopleLabelFormat as a String and will run it through String.format when we need to update the label.
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Time for action – listening to the SeekBar
The user interface is now populated with the default data. However, it's not interactive at all. If you drag the SeekBar the How Many People: label will remain at its default value of 2. We need an event listener to update the label when the SeekBar is used.
1. 2. 3.
Open the ReviewActivity Java source in your editor or IDE. Add SeekBar.OnSeekBarChangeListener to the interfaces that ReviewActivity implements. In onCreate, after fetching the SeekBar with findViewById, set its OnSeekBarChangeListener to this:
SeekBar people = (SeekBar)findViewById(R.id.people); people.setOnSeekBarChangeListener(this);
4.
Implement the onProgressChanged method to update peopleLabel: public void onProgressChanged( SeekBar bar, int progress, boolean fromUser) { peopleLabel.setText(String.format( peopleLabelFormat, progress + 1)); }
5. 6.
Implement an empty onStartTrackingTouch method: public void onStartTrackingTouch(SeekBar bar) {}
Implement an empty onStopTrackingTouch method: public void onStopTrackingTouch(SeekBar bar) {}
The String.format method is a common method of placing parameters in a localized string in Android. While this is rather different to the normal java.text.MessageFormat class, it's the preferred method in Android (although MessageFormat is still supported).
What just happened?
When you reinstall the application in the emulator, you'll now be able to use SeekBar to select the number of people that the reservation is to be made for. While we didn't implement the onStartTrackingTouch or onStopTrackingTouch methods, they can be extremely useful if you hide the actual status value by default. For example, you could use a Dialog containing icons of people to inform the user how many people the reservation is for. When they touch the SeekBar—display the Dialog, and then when they release the SeekBar—hide the Dialog again.
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Time for action – selecting date and time
We've made the SeekBar work as expected, but what about the date and time Button widgets? When the users touch them, they expect to be able to select a different date or time for their reservation. For this we'll need a good old OnClickListener, the DatePickerDialog and TimePickerDialog classes.
1. 2.
Open the ReviewActivity Java source in your editor or IDE again. Add View.OnClickListener, DatePickerDialog.OnDateSetListener, and TimePickerDialog.OnTimeSetListener to the interfaces that ReviewActivity implements. Your class declaration should now look something like this: public class ReviewActivity extends TabActivity implements ViewSwitcher.ViewFactory, Runnable, AdapterView.OnItemSelectedListener, SeekBar.OnSeekBarChangeListener, View.OnClickListener, DatePickerDialog.OnDateSetListener, TimePickerDialog.OnTimeSetListener {
3.
Implement a utility method to parse a CharSequence into a Calendar object with a specified SimpleDateFormat: private Calendar parseCalendar( CharSequence text, SimpleDateFormat format) {
4. 5. 6. 7. 8.
Open a try block to allow handling of parse errors if the CharSequence is not formatted according to the SimpleDateFormat. Parse the CharSequence into a Date object:
Date parsedDate = format.parse(text.toString());
Then create a new Calendar object:
Calendar calendar = Calendar.getInstance();
Set the time on the Calendar object to the time in the Date object: calendar.setTime(parsedDate); Return the parsed Calendar object: return calendar;
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9.
You'll need to catch(ParseException) in this method. I recommend wrapping it in a RuntimeException and re-throwing it: catch(ParseException pe) { throw new RuntimeException(pe); }
10.
In the onCreate method, after setting the labels of the date and time Button widgets, set their OnClickListener to this: date.setText(dateFormat.format(reservationDate)); time.setText(timeFormat.format(reservationDate)); date.setOnClickListener(this); time.setOnClickListener(this);
11.
Implement the onClick method to listen for when the user taps the date or time Button: public void onClick(View view) {
12. 13.
Use the View parameter to determine if the clicked View is the date Button: if(view == date) {
If so, use the parseCalendar method to parse the current value of the date Button widget's label:
Calendar calendar = parseCalendar(date.getText(), dateFormat);
14.
Create a DatePickerDialog and populate it with the date in the Calendar, then show() the DatePickerDialog: new DatePickerDialog( this, // pass ReviewActivity as the current Context this, // pass ReviewActivity as an OnDateSetListener calendar.get(Calendar.YEAR), calendar.get(Calendar.MONTH), calendar.get(Calendar.DAY_OF_MONTH)).show();
15. 16. 17.
Now check if the user has clicked on View Button instead of date: else if(view == time) {
If so, parse a Calendar using the time Button widget's label value:
Calendar calendar = parseCalendar(time.getText(), timeFormat);
Now create a TimePickerDialog with the selected time, then show() the new TimePickerDialog to the user: new TimePickerDialog( [ 94 ]
Chapter 3 this, // pass ReviewActivity as the current Context this, // pass ReviewActivity as an OnTimeSetListener calendar.get(Calendar.HOUR_OF_DAY), calendar.get(Calendar.MINUTE), false) // we want an AM / PM view; true = a 24hour view .show();
18.
Now implement the onDateSet method to listen for when the user accepts the DatePickerDialog with a new date selected: public void onDateSet( DatePicker picker, int year, int month, int day)
19. 20.
Create a new Calendar instance to populate the date into:
Calendar calendar = Calendar.getInstance();
Set the year, month, and day on the Calendar: calendar.set(Calendar.YEAR, year); calendar.set(Calendar.MONTH, month); calendar.set(Calendar.DAY_OF_MONTH, day);
21. 22.
Set the label of the date Button to the formatted Calendar: date.setText(dateFormat.format(calendar.getTime())); Implement the onTimeSet method to listen for when the user accepts the TimePickerDialog after selecting a new time: public void onTimeSet(TimePicker picker, int hour, int minute)
23. 24.
Create a new Calendar instance:
Calendar calendar = Calendar.getInstance();
Set the Calendar object's hour and minute fields according to the parameters given by the TimePickerDialog: calendar.set(Calendar.HOUR_OF_DAY, hour); calendar.set(Calendar.MINUTE, minute);
25.
Set the label of the time Button by formatting the Calendar object: time.setText(timeFormat.format(calendar.getTime())); Having stored the format for the date and time objects, we can now display the values selected by the user in the Button widgets. When the user has selected a new date or time we update the Button labels to reflect the new selections.
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What just happened
If you install and run the application in the emulator, you can now tap on either the date or time Button widgets, and you will be greeted by a modal Dialog allowing you to select a new value. Beware of overusing modal Dialog widgets, because they block access to the rest of your application. You should avoid using them for displaying status messages as they effectively render the rest of the application useless during that time. If you do display a modal Dialog, ensure that there is some way for the user to dismiss the Dialog without any other interaction (that is, a Cancel button or something similar). The first advantage to using a DatePickerDialog and TimePickerDialog comes from the fact that both include Set and Cancel buttons. This allows the user to manipulate the DatePicker or TimePicker, and then cancel the changes. If you used an inline DatePicker or TimePicker widget, you could provide a Reset button, but this would take up additional screen space, and generally would seem out-of-place (until it's actually needed). Another advantage of the DatePickerDialog over the DatePicker widget is that the DatePickerDialog displays a long-format of the selected date in it's title area. This long-format date generally includes the day of the week that the user has currently selected. The "day of the week" is a field that is noticeably missing from the DatePicker widget, which makes it surprisingly difficult to use. Most people think in terms of "next Thursday", instead of "the 2nd of August, 2010." Having the day of the week visible makes the DatePickerDialog a much better choice for date selection than an inline DatePicker.
Creating complex layouts with Include, Merge, and ViewStubs
In this chapter we've built a single layout resource with three different tabs in it. As a result of this, the main.xml file has become quite large and hence, more difficult to manage. Android provides several ways in which you can break up large layout files (such as this one) into smaller chunks.
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Using Include tags
The include tag is the simplest one to work with. It's a straight import of one layout XML file into another. For our previous example, we could separate each tab out into its own layout resource file, and then include each one in the main.xml. The include tag has only one mandatory attribute: layout. This attribute points to the layout resource to be included. This tag is not a static or compile-time tag, and so the included layout file will be selected through the standard resource selection process. This allows you to have a single main.xml file, but then add a special reviews.xml file (perhaps for Spanish). The layout attribute on the include tag is not prefixed with the android XML namespace. If you attempt to use the layout attribute as android:layout, you won't get any compiletime errors, but your application will strangely fail to run. The include element can also be used to assign or override several attributes of the root included element. These include the element android:id, and any of the android: layout attributes. This allows you to reuse the same layout file in several parts of your application, but with different layout attributes and a different ID. You can even include the same layout file several times on the same screen, but with a different ID for each instance. If we were to change our main.xml file to include each of the tabs from other layout resources, the file would look something more like this:
Merging layouts
The include element is very fine and well when you want to include a single View or ViewGroup into a larger layout structure. However, what if you want to include multiple elements into a larger layout structure, without implying the need for a root element in the included structure? In our example each tab needs a single root View in order that each tab carries a single and unique ID reference.
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However, having an additional ViewGroup just for the sake of an include can adversely affect the performance of large layout trees. In this case, the merge tag comes to the rescue. Instead of declaring the root element of a layout as a ViewGroup, you can declare it as . In this case, each of View objects in the included layout XML will become direct children of the ViewGroup that includes them. For example, if you had a layout resource file named main.xml, with a LinearLayout that included a user_editor.xml layout resource, then the code would look something like this:
The simple implementation of the user_editor.xml looks something like this:
However, when this is included into the main.xml file, we embed the user_editor. xml LinearLayout into the main.xml LinearLayout, resulting in two LinearLayout objects with identical layout attributes. Obviously it would be much better to simply put the TextView and EditView from user_editor.xml directly into the main.xml LinearLayout element. This is exactly what the tag is used for. If we now re-write the user_editor.xml file using the tag instead of a LinearLayout, it looks like this: [ 98 ]
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Note that we no longer have the LinearLayout element, instead the TextView and EditView will be added directly to the LinearLayout in the main.xml file. Beware of layouts that have too many nested ViewGroup objects, as they are almost certain to give trouble (more than about ten levels of nesting is likely to cause your application to crash!). Also be careful with layouts that have too many View objects. Again, more than 30 is very likely to cause problems or make your application crash.
Using the ViewStub class
When you load a layout resource that includes another layout, the resource loader will immediately load the included layout into the memory, in order to attach it to the layout you've requested. When main.xml is read in by the LayoutInflator, so are the reviews.xml, photos.xml, and reservations.xml files. In situations with very large layout structures, this can consume a huge amount of your application memory, and even cause your application to crash. The Android API includes a specialized View named ViewStub which allows lazy-loading of layout resources. A ViewStub is by default a zero-by-zero sized empty View, and when it's specialized, inflate() method is invoked. It loads the layout resource and replaces itself with the loaded View objects. This process allows the ViewStub to be garbage-collected as soon as its inflate() method has been called. If we were to make use of a ViewStub in the example, you would need to lazy-initialize the content of a tab when it is selected by the user. This also means that none of the View objects in a tab would exist until that tab has been selected. While using a ViewStub is a bit more work than a straight include, it can allow you to work with much larger and more complex layout structures than would otherwise be possible. Any layout attributes set on a ViewStub will be passed on to its inflated View object. You can also assign a separate ID to the inflated layout. If we wanted to include each of our tabs in a ViewStub, the main.xml file would look something like this:
Note that unlike the include tag, the ViewStub requires the android XML namespace for its layout attribute. After you inflate() one of the ViewStub objects, it will no longer be available by its original android:id reference. Instead, you will be able to access the inflated layout object using the android:inflatedId reference.
Have a go hero – separate the tabs
Extract each of the tabs into its own layout resource file, and use the include tag to load each of them. This shouldn't require any changes to the Java source code. For more of a challenge, try using ViewStub objects instead of the include tag. This will require you to break up the onCreate method and listen for when tabs are clicked. For this you'll need to use TabHost.OnTabChangeListener to know when to load a specific tab's content.
Summary
Tabs are a great way of breaking an Activity into different areas of work. With limited screen real estate, they are a great way to make an Activity more accessible to the user. They also have a performance impact since only one tab is rendered on the screen at a time. The RatingBar and SeekBar are two different methods of capturing, or displaying numeric data to the user. While they are closely related, and both function in the same way, each class is used to address different types of data. Keep in mind the limitations of both of these, before deciding whether and where to use them. The Gallery class is brilliant for allowing the user to view a large number of different objects. While in this example we used it to simply display thumbnails, it could be used as a replacement for tabs in a web browser by displaying a list of page thumbnails above the actual browser view. All you need to do to customize its function is to change the View objects returned from the Adapter implementation.
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When it comes to date and time capturing, try to stick to using the DatePickerDialog and TimePickerDialog instead of their inline counterparts (unless you have good reason). The use of these Dialog widgets helps you conserve screen space and improve the user experience. When they open a DatePickerDialog or TimePickerDialog, they have better access to the editor than you can generally provide as part of your user interface (especially on a device with a small screen). In the next chapter, we'll take a closer look at Intent objects, the activity stack, and the lifecycle of an Android application. We'll investigate how Intent objects and the activity stack can be used as a way to keep applications more usable. Also, we shall learn about improving the reuse of Activity classes.
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In many ways Android application management appears to be inspired by JavaScript and the web browser and rightly so! The web browser model has proved itself as a mechanism that users find easy to work with. Android as a system, Android has many things in common with a web browser, some of which are obvious, and others that you will need to look a little deeper for. The Activity Stack is much like a single-directional web browser history. When you launch an Activity using the startActivity method, you effectively hand control back to the Android system. When the user pushes the hardware "Back" button on their phone, the default action is to pop the top Activity off the stack, and display the one underneath (not always the one that started it).
4
In this chapter we'll explore a little of how Android runs an application and manages Activity instances. While not strictly necessary for user interface design, it's important to know how this works. Properly leveraging these concepts will help you ensure a consistent user interface experience. As you will also see, it will help you improve the performance of your application, and allow you to reuse more of your application components. It's also important to understand how an Activity is created (and when it is created), as well as how Android decides what Activity to create. We'll also look at some good practices to follow when building an Activity class, and how to behave nicely within the confines of an Android application.
Leveraging Activities and Intents
We've already encountered the "Activity Stack" in Chapter 1, Developing a Simple Activity and Chapter 2, Presenting Data for Views where we constructed Intent objects to launch specific Activity classes. When you used the hardware "Back" button, you were automatically taken to the previous Activity instance, no code needed (much like a web-browser). For this chapter we'll be looking at:
The life cycle of an Activity object Using the Bundle class to maintain application state Exploring the relationship between an Intent and an Activity Passing data into an Activity through an Intent
Exploring the Activity class
The life cycle of an Activity object is much more like a Java Applet than a normal application. It may be started, paused, resumed, paused again, killed, and then brought back to life in a seemingly random order. Most Android devices have very good performance specifications. However, most of them appear underpowered when compared to the top-ofthe-range devices. For those devices that do have good specifications, users tend to demand a lot more from them than the cheaper devices. On a phone, you're never going to get away from the fact that you have many applications and services sharing a very limited device. An Activity may be garbage-collected any time it is not visible to the user. This means it may be your application that is running, but because the user is looking at a different Activity, any non-visible or background Activity objects may be shut down or garbagecollected in order to save memory. By default, the Android APIs will handle these shut down/ start up cycles elegantly by storing their state before a shut down, and restoring it when they are re-created. A very simple diagram of the life cycle of an application with two Activity instances is shown in the following figure. When the "Main Activity" is paused, it becomes eligible for garbage-collection by the system. If this happens, it will first store its state in a temporary location, restoring the state when it is brought back to the foreground.
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Storage of user interface state If an Activity is stopped, all View objects that have an ID assigned will attempt to store their state before they are made available for garbage-collection. However, this state is only stored for the lifetime of the application. When the application is shut-down, this state is lost.
While it's possible to use the setContentView method over and over again to change the content on the screen (much the way you might build a wizard interface with an AWT CardLayout object), it's considered a very bad idea. You are effectively trying to take the control away from Android, which will always create problems for you. If for example, you developed an application with only one Activity class, and used multiple layout resources or your own custom ViewGroup objects to represent different screens, you would also have to take control of the hardware "Back" button on the device in order to allow the user to go backwards. Your application is released in the Android market, and a few months later a handset manufacturer decides to put a "Forward" button onto their new phone (in the same style as the "Forward" button on a web-browser). The Android system would be patched to handle this change to the device, but your application would not be. As a result, your users get frustrated with your application because "it doesn't work properly".
Using Bundle objects
In the onCreate method of the Activity class, we've been accepting a Bundle parameter named saveInstanceState, as you may have guessed. It's where state information is stored between stops and starts of an Activity. Despite what it looks like, a Bundle object is not a form of persistent storage. When the configuration of a device context changes (for example when the user selects a new language, or changes from "portrait" to "landscape" mode), the current Activity is "restarted". For this to happen, Android requests the Activity save its state in a Bundle object. It then shuts down and destroys the existing instance, and then creates a new instance of the Activity (with the new configuration parameters) with the Bundle that the state information was saved in. The Bundle class is effectively a Map containing any number of values. Since Bundle objects are used to store short term state (that is, the blog post a user was busy typing), they are mostly used to store the state of View objects. They have two major advantages over standard Java serialization in this regard:
You are forced to implement the storage of the object manually. This requires some thought as to how the object will be stored, and what parts of it need to be stored. For example, most of the time in a user interface, you don't need to store the layout information, since that can be recreated from the layout file. Being a key-value structure, a Bundle is more future-proof and flexible than a serialized object. You can leave out values that are set to their defaults, reducing the size of the Bundle.
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A Bundle object is also a type-safe structure. If you use the putString method, only then getString or getCharSequence will work to retrieve the object. I strongly advise that when using the get methods of Bundle, you should always provide a default value. Before an Activity is paused by the Android system, the system requests that it save any state information in a Bundle object. To do this, the onSaveInstanceState method will be invoked on the Activity. This happens before the onPause method. In order to restore the state of the Activity, the system will invoke the onCreate method with the saved state Bundle.
Handling Activity crashes If an Activity class throws an uncaught exception, the user will get the dreaded Force Close dialog box. Android will attempt to recover from these errors by terminating the Virtual Machine, and re-opening the root activity, providing a Bundle object with the last known state from onSaveInstanceState.
The View class also has an onSaveInstanceState method, as well as a corresponding onRestoreInstanceState method. As mentioned earlier, the Activity class' default functionality will attempt to save each View object with an ID within a Bundle. This is another good reason to stick to XML layouts instead of building your own. Having a reference to a View object is not enough for it to be saved and restored, and while you can assign IDs in Java code, it clutters your user interface code even more.
Time for action – building an example game: "guess my number"
We want to build a simple example that will save and restore its state from a Bundle object. For this example, we have a very simple "guess my number" game. The Activity object picks a number between one and ten and challenges the user to guess it. The basic user interface layout for this example will need a label telling the user what to do, an input area for them to input their guess, and a button to tell the application they wish to input a guess. The following diagram is a basic idea of how the user interface should be structured:
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If the user were to get an SMS while playing this game, there's a strong chance that we will lose the number he is trying to guess. For this reason we will store the number that he is trying to guess in a Bundle object when the system asks us to save our state. We'll also need to look for the stored number when starting up.
1.
From a command-prompt, create a new project named GuessMyNumber: android create project -n GuessMyNumber -p GuessMyNumber -k com. packtpub.guessmynumber -a GuessActivity -t 3
2. 3. 4.
Open the default res/layout/main.xml file in an editor or IDE. Remove the default content within the LinearLayout element. Add a new TextView to serve as a label, to tell the user what to do:
5.
Create a new EditText where the users will enter their guess. Use the android:numeric attribute of TextView to enforce only integer input:
6.
Add a Button that the users can click on to submit their guess:
7. 8.
Now open the GuessActivity.java file in your editor or IDE. Make the GuessActivity class implement OnClickListener: public class GuessActivity extends Activity implements OnClickListener {
9.
Create a field variable to store the number the user is supposed to guess: private int number;
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10.
Create a utility method to generate a random number between one and ten: private static int random() { return (int)(Math.random() * 9) + 1; }
11.
In the onCreate method, directly after the call to super.onCreate, check to make sure the Bundle passed in is not null: if(savedInstanceState != null) {
12. 13.
If the Bundle isn't null, then attempt to fetch the stored Number from it: number = savedInstanceState.getInt("Number", random());
If the Bundle is null, the Activity is running as a new instance—generate a random number: else { number = random(); }
14. 15. 16. 17. 18. 19. 20. 21.
Then setContentView to the main.xml layout resource: setContentView(R.layout.main); Find the Button object you declared in the main.xml layout resource:
Button button = (Button)findViewById(R.id.guess);
Set the Button object's OnClickListener to the GuessActivity object: button.setOnClickListener(this); Now override the onSaveInstanceState method: protected void onSaveInstanceState(Bundle outState) {
Be sure to first allow the default Activity behavior: super.onSaveInstanceState(outState); Then store the number variable in the Bundle: outState.putInt("Number", number);
We need to override the onClick method to handle the user's guess: public void onClick(View clicked) {
Find the EditText where the user enters the guessed number:
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22. 23.
Parse the current value of the EditText as an integer: int value = Integer.parseInt(input.getText().toString());
If the number they guessed is too low, use a Toast to tell them: if(value < number) { Toast.makeText(this, "Too low", Toast.LENGTH_SHORT).show(); }
24.
If the number they guessed is too high, again use a Toast to tell them: else if(value > number) { Toast.makeText(this, "Too high", Toast.LENGTH_SHORT).show(); }
25.
If they successfully guessed the correct number, then congratulate them: else { Toast.makeText( this, "You got it! Try guess another one!", Toast.LENGTH_SHORT).show();
26.
Then generate a new number for the user to guess: number = random(); }
The Toast class is used in the previous code to display the output messages for Too high, Too low, and You got it! The Toast class is the perfect mechanism for displaying short output messages, and they automatically disappear after a few seconds. However they're not suitable for long messages as the user has no control over them, and cannot leave the message open or close it on command as they are entirely non-interactive.
What just happened
In the previous example, we listened for a call to onSaveInstanceState in order to record the number that the user is supposed to guess. We also have the current guess which the user most recently made, in the form of an EditText. Since we assigned an ID value to EditText in the main.xml file, the call to super.onSaveInstanceState will handle the storage of the EditText widget's exact state (potentially including "selection" and "focus" state).
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In the onCreate method, the example first checks to make sure that the Bundle is not null. If Android is attempting to create a new instance of the GuessActivity object, it won't pass in any saved state. If however, we have a Bundle object, we invoke the Bundle. getInt method to attempt to fetch our previously stored number value. We also pass in a random() number as a second parameter. If the Bundle object (for whatever reason) doesn't have a stored Number, it will return this random number, eliminating the need for us to check such a condition. As a quick side-note, the example made use of the android:numeric attribute of the TextView class to enforce integer input on the EditText object. Switching to a numeric view stops the user from entering anything except "valid" characters. It also affects the softkeyboard. Instead of displaying the full keyboard, it will only display the numbers and symbols.
Creating and consuming intents
The Intent class is Android's primary method of "late binding". It's a form of very loose coupling which allows you to specify an action (along with some parameter data), while not specifying how the action should be carried out. For example, you may specify browse to http:// www.packtpub.com/ using an Intent, but you don't need to specify how Android should carry out this action. It may use the default "browser" application, or another web browser the user has installed, or it may even ask the user how exactly they want to get to http:// www.packtpub.com/. There are two primary types of Intent:
Explicit Intents Implicit Intents
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So far we've only made use of explicit Intent objects, where we specify the exact class we want to run. These are very important when switching from one Activity to another, as your application may depend on the exact implementation of an Activity. An implicit Intent is one where instead of specifying the exact class which we want to work with, we include an abstract name for the action we want carried out. Generally, an implicit Intent will have much more information content, due to the following reasons:
To allow the system to make a good selection of which component to interact with
Intent may point to a more generic structure than we would have built ourselves
and a more generic structure often requires more information about how it is expected to behave
Intent objects are what really make Android different from other (more traditional)
operating systems. They level the playing field between applications, and allow the user much more choice in how they want to run their phones. It's perfectly plausible for the user to not just install a new web browser, but also a new menu, desktop, or even dialler application. Each Activity instance holds onto the Intent object that started it. In Chapter 1, Developing a Simple Activity, we made use of the Activity.getIntent() method to fetch some parameters from the Intent object, which in turn told us which question to ask the user.
Defining Intent actions
The first thing looked at in an implicit Intent is its action. The action defines what the Intent "does", but not how it does it, or what it does it to. The Intent class defines a long series of constants which represent common actions. These common actions always have some form of backing logic, which is generally defined by the phone system. Thus they are always available to be used by an application. For example, you wanted to present the users with the dialler application, so they could dial a phone number and make a call, you would use an Intent with ACTION_DIAL: startIntent(new Intent(Intent.ACTION_DIAL));
The action value of an Intent is matched against one of the actions defined for an Activity. An Activity may have any number of actions that it may perform, and they're all specified as part of an application's AndroidManifest.xml file. For example, you wanted to define an askQuestion action and bind it to an Activity, your AndroidManifest.xml file would contain an Activity entry which would look something like this:
An Activity may have any number of elements, each defining a different type of match to perform on an Intent. The Activity with the closest match to any given Intent is chosen to perform the action requested by the Intent object.
Passing data in an Intent
Presenting the user with the dialler application in order to let them dial a phone number is very nice, but what if we actually need them to dial a phone number? The Intent class doesn't just work by using the action, it also provides a default space for us to tell it what we want the action to be performed on. It's not brilliantly useful being able to open a web browser without being able to tell the browser what URL to go to, is it? The default data provided by an Intent is provided as a Uri object. The Uri can be made to technically point to absolutely anything. For our earlier code snippet, we started the dialler to let the user dial a phone number. How would we then tell the dialler: "Dial 5551234"? Simple, just take a look at the following code: startActivity(new Intent( Intent.ACTION_DIAL, Uri.parse("tel://5551234")));
Adding extra data to an Intent
Sometimes a Uri doesn't allow enough data to be specified. For these cases, the Intent class provides you with a Map space of key-value pairs, called "extra" data. The methods for accessing the extra data correspond to the methods in the Bundle class. Back in Chapter 1, Developing a Simple Activity, we used the extra data to keep track of which question we were asking the user. When defining generic Activity classes (such as file viewers), it's a good idea to work on a three phase fall-back system when looking for operational data:
Any custom (non-standard) parameters can be passed in extra fields (and none of them should be mandatory) Inspect the data Uri to see what information you should be working with If no data Uri is specified, fall-back gracefully to a logical default, and provide some functionality to the user
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Have a go hero – generic questions and answers
Go back to the example question and answer application from Chapter 1, Developing a Simple Activity. Rework the QuestionActivity class to use the data Uri to specify the question ID (by name) instead of the extra parameters. Also, allow for the full question to be passed in using "extra" parameters—a parameter
Question for the question text to ask the user, and a parameter Answers, specifying
a string array of possible answers to the given question.
Using advanced Intent features
An Intent object is designed to indicate a single action as requested by the user. It's a selfcontained request, and in some ways it is quite similar to an HTTP request, containing both, the action to carry out, and the resource upon which the action should be carried out, and any additional information that may be required. In order to find the Activity (service or broadcast receiver) that will handle an Intent, the system makes use of intent-filters (as we discussed briefly earlier). Each intent-filter indicates a single type of action that could be carried out by the Activity. When two or more Activity implementations match an Intent, the system sends out an ACTION_PICK_ACTIVITY Intent to allow the user (or some automated system) to select which of the Activity implementations should be used to handle the Intent. The default behavior is to ask the users which of the Activity implementations they wish to use.
Getting data back from an Intent
An Intent is not always a one-way structure, some Intent actions will provide feedback. A great example of this is Intent.ACTION_PICK. The Intent.ACTION_PICK action is a way to ask the user to "pick" or select some form of data (a common use would be to ask the user to select a person or phone number from their contacts list). When you need information back from an Intent, you use the startActivityForResult method instead of the normal startActivity method. The startActivityForResult method accepts two parameters, the Intent object to execute, and a useful int value which will be passed back to you. As mentioned earlier, when another Activity is visible instead of yours, your Activity is paused, and may even be stopped and garbage-collected. For this reason, the startActivityForResult method returns immediately and you can generally assume your Activity will be paused directly after you return from your current event (passing control back to the system).
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In order to get information back out of the Intent you triggered, you will need to override the onActivityResult method. The onActivityResult method is invoked every time an Intent started with startActivityForResult returns some data to you. The first parameter passed back into the onActivityResult method is the same integer value that you passed into the startActivityForResult method (allowing you to pass simple parameters back).
Passing information to another Activity If you intend for an Activity implementation to pass information back to its caller, you can make use of the Activity.setResult method to pass both, a result-code and an Intent object with your response data.
Pop quiz
1. When does onCreate get passed a valid Bundle object? a. Every time the Activity is created b. When the application stored information in the Bundle in a previous execution c. When the Activity is being restarted due to configuration changes, or a crash 2. When is the onSaveInstanceState method invoked? a. After the onStop method b. Before the onPause method c. When the Activity is being restarted d. Before the onDestroy method 3. A Bundle object will be stored until: a. The application is closed b. The Activity is no longer visible c. The application is uninstalled
Time for action – viewing phone book contacts
In this example we will delve a little deeper into the workings of the Android system. We're going to override the default "view contact" option, providing our own Activity to view contacts from the phonebook on the device. When the user tries to open a contact to e-mail or call them, they will be presented with an option to view the contact using our Activity instead of the default one.
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1.
Start by creating a new project from the command line: android create project -n ContactViewer -p ContactViewer -k com. packtpub.contactviewer -a ViewContactActivity -t 3
2. 3. 4.
Open the res/layout/main.xml layout resource in an editor or IDE. Remove the default content within the LinearLayout element. Add a new TextView object to contain the contact's display name:
5.
Then add a Button which will be used to "dial" the default phone number of the displayed contact:
6. 7.
Open the ViewContactActivity.java source file in your editor or IDE. Make ViewContactActivity implement OnClickListener: public class ViewContactActivity extends Activity implements OnClickListener {
8.
After the setContentView(R.layout.main) in the onCreate method, find the TextView object you have created, to show the contact's name in:
TextView name = (TextView)findViewById(R.id.display_name);
9. 10.
Then find the Button widget to display the phone number in:
Button number = (Button)findViewById(R.id.phone_number);
Now use the Activity.managedQuery method to query the contact's database for the data Uri specified in our Intent:
Cursor c = managedQuery( getIntent().getData(), new String[]{ People.NAME, People.NUMBER }, null, [ 115 ]
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11.
In a try {} finally{} block, tell the Cursor to moveToNext() and make sure it does so (this works in exactly the same way as ResultSet.next()): if(c.moveToNext()) {
12. 13. 14.
Fetch and display the contact display name from the Cursor: name.setText(c.getString(0)); Fetch and display the contact default phone number from the Cursor: number.setText(c.getString(1)); In the finally{} block, close the Cursor: finally { c.close(); }
15. 16. 17.
Now set the OnClickListener of the number Button to this: number.setOnClickListener(this); Override the onClick method: public void onClick(View clicked) {
We know that the number Button is what was clicked (it's the only View with an event-listener at this point). Cast the View parameter to a Button so that we can use it:
Button btn = (Button)clicked;
18.
Create an Intent object to dial the selected phone number:
Intent intent = new Intent( Intent.ACTION_DIAL, Uri.parse("tel://" + btn.getText()));
19. 20. 21.
Use startActivity to open the dialler application: startActivity(intent); Now open the AndroidManifest.xml file in your editor or IDE. Before the declaration of the element, we need permission to read the contacts list: [ 116 ]
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22.
Change the label of the ViewContactActivity to View Contact:
23. 24. 25. 26.
Remove all of the default content inside the element. Declare an type of ACTION_VIEW for this :
Set the of this to CATAGORY_DEFAULT:
Add a element to filter person entries (this is a MIME type):
27.
Add another element to filter contact entries:
When installed on a device, the preceding code will become an option for opening "Contacts" in the user's address book. As you can see, replacing part of the standard Android framework is very simple, and allows more seamless integration of applications with the base system than is possible with a more conventional application architecture.
What just happened
If you install this application on the emulator, you'll notice that in the launcher, there's no icon to start it up. That's because this application doesn't have a main entry point like all of the others we've written thus far. Instead, if you open the "Contacts" application, and then click on one of the contacts in the address book, you'll be greeted by the following screen:
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If you select the second icon, your new ViewContactActivity will be started in order to view the selected contact. The user (as you can see) also has the ability to use your application in preference to the default (for as long as your application remains available on the device). Overriding a default behavior is a very important decision when developing a new application. Android makes it very easy to do, and as you can see, a third-party application can slot in almost seamlessly between two of the default applications. In a normal operating system environment, you would need to write an entire "contacts manager", while in Android you need only write the bits that interest you. This is a part of your user interface design since you can use it to extend the functionality of various default parts of the system. For example, if you wrote a chat application, such as a "Jabber" client, you could embed the client in the View contact Activity for each contact in the user's address book that was linked with a Jabber ID. This would allow users to chat with available contacts directly from their address book, instead of having to go to your application. You application becomes a way for them to check a contact's status, and possibly avoid a phone call entirely.
Summary
Implementing an Activity at the correct granularity is an important part of your user interface design process. Although it's not a graphical part directly, it defines how the system will interact with your application, and thus how the user will interact with it. It's a good idea to keep implicit intents in mind when structuring how your Activity will be started. Creating a generic Activity allows for other applications to integrate seamlessly with your own, effectively turning your new application into a platform for other developers to work with. An implicitly started Activity can be replaced or extended by another application, or it can be re-used in other application. In both cases, the user becomes free to customize your application in much the same way that they can customize the wallpaper image or theme. Always try and provide a single Activity implementation for each action the user might want to take, don't make an Activity do too many things in the same screen. A very good example of granularity is the "Contacts" application—there's a contact list, contact viewer, contact editor, and the dialler application. When working with tabbed interfaces (as we did in the previous chapter), it's possible to specify the tab content as an Intent, effectively embedding the Activity in your application. I would strongly urge you to consider doing exactly this when building a tabbed user interface, since it allows each tab to be re-used by your application far more easily, while also allowing third-party developers to create extensions to your interface, one tab at a time. So far we've only really worked with the LinearLayout class, and while it's a great base for simple user interfaces, it's almost never enough. In the next chapter, we'll be looking at the many other types of layouts that Android provides by default, exploring the way in which each layout works, and how they can be used.
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Non-linear layouts are normally a completely fundamental subject of user interface design. However, on a device with a small screen (as many Android devices are), it doesn't always make sense. That said, Android devices can be turned to landscape mode, where suddenly you have an abundance of horizontal space, and limited vertical space. In these situations (and as we'll see, in many other situations as well), you will want to work with a layout other than the plain old LinearLayout structure we've worked with so far. The real power of Android layouts comes from the same place as the power of the old Java AWT LayoutManagers—by combining the different layout classes with each other. For example, combining a FrameLayout with other ViewGroup implementations allows you to layer various parts of the user interface on top of each other.
5
It's important to consider how your layout will act on screens of different sizes. While Android does allow you to select different layouts based on the screen size of the device, this means that you will have to maintain multiple layouts for the different screen sizes and densities which your application will encounter in the wild. As far as possible you should make use of the tools that Android provides, and work with layouts that will scale according to the size of the various View objects. In this chapter, we'll look into the various other layout styles that Android provides us with by default, and investigate various alternative uses for each one of them. We'll also take a closer look at how you specify parameters for different layouts, and how they can help in usability, as opposed to simply putting your widgets in a particular order.
Developing Non-linear Layouts
Time for action – creating a layouts example project
Before we walk through each of the layouts, we need a common project inside which we will showcase each of them.
1.
From a command prompt, create a new project named Layouts: android create project -n Layouts -p Layouts -k com.packtpub. layouts -a LayoutSelectorActivity -t 3
2. 3. 4. 5.
Delete the standard res/layout/main.xml layout resource file. Open the res/values/strings.xml file in an editor or IDE. Add a new by the name of layouts to the file:
Add the following items to the new element:
Frame Layout Table Layout Custom Layout Relative Layout Sliding Drawer
6. 7. 8.
Open the LayoutSelectorActivity source file in your editor or IDE. Have the class inherit from ListActivity instead of Activity: public class LayoutSelectorActivity extends ListActivity {
In the onCreate method, set the contents of your ListActivity you declared in the strings.xml resource file to your layouts array: setListAdapter(new ArrayAdapter( this, android.R.layout.simple_list_item_1, Have the class inherit from" getResources().getStringArray(R.array.layouts)));
9.
Override the onListItemClick method: protected void onListItemClick( ListView l, View v, int position, long id) {
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10. 11.
Create a switch statement on the position parameter: switch(position) {
Add a default clause (the only one for now) to let yourself know that you haven't implemented an example for the selected item yet: default: Toast.makeText( this, "Example not yet implemented.", Toast.LENGTH_SHORT).show();
What just happened?
The new project will serve as a basis for each of the examples in this chapter. For each layout we work through, we'll build a new Activity that will become part of this application. Currently, the application consists of only a menu for accessing each of the layout examples. The idea as of now is to fill each one with something interesting. In this chapter, we will explore not just the basic layouts, but also how they can be made to interact with each other.
FrameLayout
The FrameLayout class anchors each of its widgets at the top-left corner of itself. This means each child widget is drawn on top of the previous one. This can be used to simulate a CardLayout from AWT by using View.setVisible to show one of the children while hiding all the others (this is effectively how TabHost works). As FrameLayout actually paints all of its visible children, it can be used to layer the child widgets on top of each other. It produces very strange effects in some cases, while in other cases it can be amazingly useful. For example, darkening out all of the widgets except one can be achieved by using a semi-transparent View object and a FrameLayout. The inactive widgets are the first layer in the FrameLayout, a semi-transparent View object is the second, and the active widgets are the third.
Common uses
The most common use of FrameLayout is probably in combination with TabHost—to hold the content View objects for each tab. You can also use it to simulate a more desktop feel, by layering widgets on top of each other. It can also be used very effectively in games, to display the in-game menu, or draw an animated background behind the game's main menu.
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By combining a FrameLayout object with widgets that also take up the entire screen, you can make use of the gravity attribute to place objects more precisely on top of other widgets. For this, you'll generally want each of the FrameLayout children to be a ViewGroup of some sort, since they generally don't paint in a background unless told to (leaving the lower layers visible). A FrameLayout is also capable of displaying a foreground. While all View objects have a background attribute, FrameLayout includes a foreground (which is also an optional Drawable). The foreground will be painted on top of all of the child widgets, allowing a "frame" to be displayed.
Time for action – developing a FrameLayout example
To really understand what a FrameLayout does, and how it can be used, it's best to kick it around a bit with an example. In this example, we'll use a FrameLayout to layer some Button widgets on top of an ImageView, and show-and-hide a TextView message when one of the buttons is clicked. For this example to work, you're going to need an image to serve as a background image. I'm going to use a photo of one of my friends. As always, place your image in the res/ drawable directory, and try to use a PNG file.
1. 2.
Create a new layout resource file named res/layout/frame_layout.xml. Declare the root element as a FrameLayout consuming all available space:
3.
Inside the FrameLayout, create an ImageView to serve as the background image. It should scale to fill all the available space:
4.
Now create a vertical LinearLayout where we will place two Button objects at the bottom of the screen:
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5.
Create a Button that we will use to toggle one of the child layers of our FrameLayout (creating a dialog-like effect):
6.
Create another Button to quit the demo and go back to the menu:
7.
After the , create a final TextView element that we will show and hide when the first button is clicked. By default it's hidden:
8.
Create a new FrameLayoutActivity Java class in the root package of your project, and open the source file in your editor or IDE. The new class needs to extend from Activity and implement the OnClickListener class (for events from those two Button widgets): public class FrameLayoutActivity extends Activity implements OnClickListener {
9. 10. 11.
Override the onCreate method: protected void onCreate(Bundle savedInstanceState) {
Invoke the super.onCreate method to get the Activity code working: super.onCreate(savedInstanceState); Set the content layout to the frame_layout resource you just created: setContentView(R.layout.frame_layout); [ 123 ]
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12.
Find the overlay Button widget you declared in the frame_layout resource file and create a reference to it:
Button overlay = (Button)findViewById(R.id.overlay_button);
13. 14. 15. 16.
Set its OnClickListener to the new FrameLayoutActivity object: overlay.setOnClickListener(this); Find the quit Button widget:
Button quit = (Button)findViewById(R.id.quit);
Then set it's OnClickListener to the FrameLayoutActivity object: quit.setOnClickListener(this); The OnClickListener interface requires us to implement an onClick method with the following signature: public void onClick(View view) {
17. 18.
Create a switch statement on the ID of the View parameter: switch(view.getId()) {
If the View clicked by the user widget is the overlay_button Button, then use the following: case R.id.overlay_button:
19. 20.
Fetch the overlay View object from the layout:
View display = findViewById(R.id.overlay);
Toggle its visibility according to its current state, then break from the switch: display.setVisibility( display.getVisibility() != View.VISIBLE ? View.VISIBLE : View.GONE); break;
21. 22.
If the View clicked by the user widget is the quit Button, then use the following: case R.id.quit:
Invoke the finish() method, and break from the switch statement: finish(); break;
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23. 24.
Open the LayoutSelectorActivity Java source in your editor or IDE. In the onListItemClick method, create a new case in the switch statement, for position value 0: case 0:
25.
Start the FrameLayoutActivity using an explicit Intent: startActivity(new Intent(this, FrameLayoutActivity.class)); break;
26. 27.
Open the AndroidManifest.xml file in an editor or IDE. Add the new FrameLayoutActivity to the manifest file:
What just happened?
The new FrameLayoutActivity makes use of a simple three layer FrameLayout. We use an ImageView object to draw a nice background image, on top of which we placed our two buttons. While the third layer (the TextView widget) is invisible until the top button is clicked, it's important to note that not only is the background of the top TextView transparent, it also delegates click events to widgets that are technically underneath it (the TextView has a widget and height that consumes the entire FrameLayout). This will continue to work, even if the background of the TextView is opaque. It has more to do with the fact that the TextView is not "clickable". If you would have added an OnClickListener to the overlay TextView object, the button underneath it would have stopped working. This means you need to be careful how you layer widgets in a FrameLayout (although so long as one widget doesn't take up the same space as another, this won't become a problem for you). In this example, we added a Quit button to the layout, and used the finish() method to close the Activity when the Button was clicked. You'll find that you generally don't use the finish() method directly since the user will mostly be moving forward through your application. If a user wants to go back, they will most often use the hardware "Back" button, or press the hardware "Home" button to exit your application entirely. A final note on the above example—in the frame_layout.xml file, we declare the overlay as a TextView widget. However, in the Java code we access it using the View class instead of TextView. This is a simple case of decoupling. Unless you're working in a performance-centric piece of code, it's a good idea to reference your layout widgets as high up the class-tree as possible. This will allow you to modify your user interface much more quickly later on. In this case, you could change the simple TextView to an entire LinearLayout without the need to change the Java code at all.
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Following are two screenshots of the FrameLayout example, with and without the overlay TextView enabled. This sort of layout is perfect for use in a game menu or a similar structure where you need to layer different widgets on top of each other.
Table Layout
The Table Layout arranges its children in a HTML-style grid. It's a bit like the AWT Grid Layout class, but with much more flexibility. Unlike most other layout classes in Android, Table Layout uses its own specialized direct-child View class, named Table Row. The Table Layout class also doesn't allow you to define the number of rows or columns (making it far more like an HTML element). Instead, the number of rows and columns is calculated by the number of widgets in the Table Layout and its Table Row children. A cell in a Table Layout may consume any number of rows and columns, although the default for a View placed inside a Table Row is to take up exactly a single table cell. However, if you place a View as a direct child of a Table Layout, it will consume an entire row.
Table Layout is also a relative layout structure, which is vitally important when working with
Android devices. Being able to align everything based on grid lines allows your user interface to scale from the lowest resolution on a tiny phone, to a high-density screen on a 7-inch tablet.
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The android:gravity attribute comes into play far more in a Table Layout than in many of the other layout classes. What looks great on a small screen may look completely different on a large screen, not due to the size of the screen, but instead due to the scaling of the fonts used. Be careful, especially with the vertical alignment of labels and widgets. The easiest way to start with this is to vertically center all of your table widgets, and work from there. Be sure to test any table based layout on a variety of screen resolutions and sizes.
Common uses
Most commonly you'll find yourself using a Table Layout to arrange input form. It's also useful for laying out complex information, especially when making some View objects span several rows and columns. The most important trait of Table Layout comes from the fact that it aligns its cells in a very strict manner, and the fact that its a relative-size layout. A Table Layout can also be used to achieve an effect similar to the AWT Border Layout class. Generally, when sizing a Table Layout to fit the entire screen, it becomes a very different tool to a simple grid, allowing you to fit a Scroll View in the middle of the control widgets. By using a Table Layout inside a FrameLayout, you can arrange a control View on top of a content View (think of the controls in Google Maps). Also, try to bear in mind that unlike an AWT GridLayout, the size of a View inside a TabelLayout is not attached to size of the table-cell in which it is placed. By making use of the gravity attribute (and possibly a layout margin), you can place a View object within the table-cell, leading to layouts that are far more user-friendly.
Using TableLayout for a memory game
To demonstrate the TableLayout, I thought it would be fun to write a simple memory-card game. You're presented with a grid (in the form of a TableLayout) of "cards" which you can touch to effectively turn over. You can then attempt to match all these cards, with the contents that are displayed on the cards (you're only allowed to turn over two at a time). For this example, you'll need some images to place on the cards (I've re-used the fruit icons from the delivery example). In this application, we'll also be creating a simple placeholder image, in the form of an XML file. To create the placeholder image, create a new XML resource in the res/drawable directory, named line.xml. This will be a "shape" resource. Shape resources are very useful for creating simple, scalable shapes. Also, shape resource files can make use of any color, texture, or gradient which you can provide from your code.
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Copy the following code into the line.xml file in order to create the simple placeholder image for our example:
Time for action – developing a simple memory game
Unlike almost all previous example, we'll generate the layout entirely in Java code in this game. The main reason for this is that the content is highly repetitive, each cell containing almost exactly the same widget. We use a TableLayout to create the grid, and display the "cards" in ImageButton widgets. To encapsulate the individual card behavior, we create a MemoryCard inner-class which holds a reference to the ImageButton it controls.
1. 2.
Create a new Java class in the root package of your project, and name it TableLayoutActivity. Make the new class extend Activity: public class TableLayoutActivity extends Activity { Declare and array of all the icon resources to use as card images, there must be eight images resources declared in this array:private static final int[] CARD_RESOURCES = new int[]{ R.drawable.apple, R.drawable.banana, R.drawable.blackberry, // … };
3. 4.
You'll need a timer in order to flip cards back over, so declare a Handler: private final Handler handler = new Handler();
Declare an array of MemoryCard objects: private MemoryCard[] cards;
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5.
We either have one or two cards turned over (that we want to keep track of). Declare a placeholder for the first: private MemoryCard visible = null;
6.
If there are two cards turned over, but they don't match, we disable touch with a simple boolean switch (our event listeners will check this): private boolean touchEnabled = true;
7.
Now declare an inner class named MemoryCard which implements the OnClickListener interface: private class MemoryCard implements OnClickListener {
8. 9.
The MemoryCard class holds a reference to an ImageButton: private ImageButton button;
The MemoryCard class also has a value, which is a reference to the image resource on its face: private int faceImage;
10.
Finally, a MemoryCard uses a boolean value to remember its state (whether the face image, or the placeholder image is visible): private boolean faceVisible = false;
11.
Declare a constructor for the MemoryCard class, it only needs to take the resource identifier for the face image:
MemoryCard(int faceImage) {
12. 13.
Store the faceImage resource identifier for later use: this.faceImage = faceImage;
Create a new ImageButton object, using the TableLayoutActivity object as its Context (which the ImageButton will use to load the images): this.button = new ImageButton(TableLayoutActivity.this);
14. 15.
Set the size of the ImageButton to a fixed 64 x 64 pixels: this.button.setLayoutParams(new TableRow.LayoutParams(64, 64));
Set the scale-type so that icons are made to fit into the ImageButton, then set the image to the placeholder resource: this.button.setScaleType(ScaleType.FIT_XY); this.button.setImageResource(R.drawable.line); [ 129 ]
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16.
Assign the MemoryCard object as the OnClickListener of the ImageButton object: this.button.setOnClickListener(this); 17.
For convenience later on, the MemoryCard needs a setFaceVisible method, which will toggle between showing the placeholder and the faceImage resource: void setFaceVisible(boolean faceVisible) { this.faceVisible = faceVisible; button.setImageResource(faceVisible ? faceImage : R.drawable.line); }
18. 19.
Implement the onClick method in the MemoryCard class: public void onClick(View view) {
First, make sure that the face isn't currently visible (so we're turned down), and that touch is enabled (and some other cards aren't about to be turned face-down again): if(!faceVisible && touchEnabled) {
20.
If these conditions are met, we tell the TableLayoutActivity that we've been touched and want to be turned face-up: onMemoryCardUncovered(this); 21.
After the MemoryCell inner class, create a simple utility method in the TableLayoutActivity to create an ordered array of MemoryCell objects with a specific size: private MemoryCard[] createMemoryCells(int count) {
22.
When we create each of the MemoryCell objects, we create them in pairs, and in the same sequence as was specified in our array of icon resources:
MemoryCard[] array = new MemoryCard[count]; for(int i = 0; i < count; i++) { array[i] = new MemoryCard(CARD_RESOURCES[i / 2]); }
23. 24.
When complete, return the new array of MemoryCell objects: return array;
Now override the onCreate method: protected void onCreate(Bundle savedInstanceState) {
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25. 26.
Invoke the Activity.onCreate method: super.onCreate(savedInstanceState); Now create a new TableLayout object, passing it the TableLayoutActivity as a Context for loading styles and resources:
TableLayout table = new TableLayout(this);
27.
By default, we create a four-by-four grid: int size = 4; cards = createMemoryCells(size * size);
28. 29.
Then we shuffle it to randomize the order:
Collections.shuffle(Arrays.asList(cards));
Create each of the required TableRow objects, and populate it with the ImageButtons, created by the MemoryCard objects in the grid: for(int y = 0; y < size; y++) { TableRow row = new TableRow(this); for(int x = 0; x < size; x++) { row.addView(cards[(y * size) + x].button); } table.addView(row); }
30. 31.
Set the Activity content view to the TableLayout object: setContentView(table); Now we write the onMemoryCardUncovered method, which is called by the MemoryCard.onClick implementation: private void onMemoryCardUncovered(final MemoryCard cell) {
32.
First, check to see if there is a currently visible MemoryCard, if not, the card touched by the user is turned face-up and we remember it: if(visible == null) { visible = cell; visible.setFaceVisible(true); }
33.
If there is already a face-up card, check to see if they have the same image. If the images are the same, disable the ImageButton widgets so we ignore the events: else if(visible.faceImage == cell.faceImage) { cell.setFaceVisible(true); cell.button.setEnabled(false); [ 131 ]
Finally, if the face images don't match, we turn the card touched by the user face-up and flip our touchEnabled switch so that the MemoryCard objects will ignore all other touch events for a second: else { cell.setFaceVisible(true); touchEnabled = false;
35.
Then we post a delayed message on our Handler, which will turn both cards face-up again and re-enable touch events: handler.postDelayed(new Runnable() { public void run() { cell.setFaceVisible(false); visible.setFaceVisible(false); visible = null; touchEnabled = true; } }, 1000); // one second before we flip back over again
What just happened
In the previous example, it should be obvious to see why we wrote the layout code manually, building it in an XML file would have been terribly repetitive. You'll notice that the code creates a TableRow object as the direct children of the TableLayout, just as we would have in an XML file. The onClick method of MemoryCard uses the touchEnabled switch to determine whether or not to call onMemoryCardUncovered. However, neither does this stop the user from pressing the ImageButton objects, nor does it stop the objects from responding to the user (although they won't turn over). For a more user-friendly experience, it would be better to use the setClickable method on each of the enabled ImageButton objects, to stop them completely from reacting to the user's touch. When we create the ImageButton objects, we pre-size them at 64 x 64 pixels. While this is fine for the big emulator screen, there are plenty of devices that wouldn't fit the 4 x 4 grid of buttons on the screen. I would recommend you use an XML resource to create the ImageButton objects.
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The previous code uses setLayoutParams(new TableRow.LayoutParams(64, 64)); to set the size of the ImageButton objects. It's important to note that because we are placing the ImageButton objects into a TableRow, their LayoutParams must be of the type TableRow.LayoutParams. If you try changing to a generic ViewGroup. LayoutParams, then the user interface won't layout (it'll just be blank). Following are two screenshots of the working application:
Have a go hero
The TableLayout example works great, but the location of the grid isn't great (on the top left of the screen), and having it against a black background is quite dull. Time to make it look great! Start by using a FrameLayout to add a background image to the game. This will enhance the overall appeal of the game by adding more color. You should also take this opportunity to center the grid on the screen. Having it in the top left makes it look lopsided somehow. You should also try removing the touchEnabled switch, instead using setClickable on each of the ImageButton objects. This will stop them from providing a visual "press and release" feedback when you're about to turn cards face-down.
AbsoluteLayout/Custom Layouts
Do not use AbsoluteLayout! AbsoluteLayout is Deprecated! That said, there are times when using the AbsoluteLayout class makes sense. So why shouldn't you use the AbsoluteLayout class, and where should you use it? The answer to the first question is very simple—all of the child widgets of an AbsoluteLayout have their locations specified exactly, they don't change size or location on different screens. It also makes your layout almost impossible to re-use (for example, importing it into another layout, or embedding it into another application).
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If you're going to work with an AbsoluteLayout, you should approach it in either one of the following two ways: 1. Carefully build a separate layout XML for each different screen size. 2. Write your layout data in Java code instead of XML. The first is impractical unless you specify that the application only runs on specific devices, and the layout cannot be used outside of your application. The second option however, opens up the "right" way—write a custom layout manager. Since AbsoluteLayout requires strict locations, and doesn't allow easy interaction with the measuring of child View objects, the best way to define layouts that don't fit well into any of the over layout classes is to define a custom layout in your own ViewGroup class.
Developing your own Layouts
Since AbsoluteLayout is deprecated, and yet many people seem to insist on using it, this example will be to demonstrate not just how easy it is to write your own ViewGroup class defining a new layout, but also how easy it is to then integrate that layout into a layout XML resource. This will thus prove that there is no compelling reason to use an AbsoluteLayout (unless it really makes sense).
Time for action – creating a custom layout
To really demonstrate the use of a custom layout, you need to try building something unusual. In the following example, you'll put together a ViewGroup that arranges its children in a nice circle. It's not a very brilliant layout, nor is it particularly useful, but circles are nice to look at, and it would provide useful negative space in the screen center (which could be filled using a FrameLayout).
1. 2. 3.
Create a new Java source file in the root package of the project named CircleLayout.java, and open it in your editor or IDE. Declare the CircleLayout as extending the ViewGroup class: public class CircleLayout extends ViewGroup
Declare the three ViewGroup constructors and have them delegate directly to the ViewGroup default constructors: public CircleLayout(Context context) { super(context); } // ...
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4.
We'll need to know the largest number of pixels taken up by a child View object's width, and the largest number of pixels taken up by a child View object's height. To avoid unnecessary overhead, we take this opportunity to measure the child View objects as well. Declare a utility method named measureChildrenSizes to perform these two operations: private int[] measureChildrenSizes(int sw, int sh) {
5.
Declare an int to hold the maximum width and height we find: int maxWidth = 0; int maxHeight = 0;
6.
Create a for loop to iterate over each of the child View objects in this CircleLayout object: for(int i = 0; i < getChildCount(); i++) {
7. 8.
Declare a reference to View at the current index:
View child = getChildAt(i);
As a layout widget, your class will be responsible for setting the display size for all of it's child widgets. In order to know a child widget's desired width and height, you need to use the measureChild method in the ViewGroup class: measureChild(child, sw, sh);
9.
Test the width and height of the child View object against the maximum width and height variables you created earlier: maxWidth = Math.max(maxWidth, child.getMeasuredWidth()); maxHeight = Math.max(maxHeight, child.getMeasuredHeight());
10.
At the end of the method, return an array containing the maximum width and height found during the procedure: return new int[]{maxWidth, maxHeight};
11.
Implement the onLayout method of ViewGroup: protected void onLayout(boolean changed, int l, int t, int r, int b) {
12.
Calculate the width and height of our available space: int w = r – l; int h = b - t;
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13. 14.
Declare a variable to hold the number of child View objects: int count = getChildCount();
Perform the measurement of all child View objects against the amount of available space: int[] max = measureChildrenSizes(w, h);
15.
Subtract the maximum width and height from the available space so that all the child View objects will fit on the screen: w -= max[0]; h -= max[1];
16.
Calculate the center point in the CircleLayout: int cx = w / 2; int cy = h / 2;
17. 18. 19.
Create a for loop to iterate over each of the child View objects again: for(int i = 0; i < count; i++) {
Declare a variable to hold the current child View object:
View child = getChildAt(i);
Calculate the x and y locations of the child View object: double v = 2 * Math.PI * i / count; int x = l + (cx + (int)(Math.cos(v) * cx)); int y = t + (cy + (int)(Math.sin(v) * cy));
20.
Invoke the layout method of the child View object with the calculated coordinates in the circle: child.layout( x, y, x + child.getMeasuredWidth(), y + child.getMeasuredHeight());
What just happened?
The CircleLayout class is a very simple implementation of a ViewGroup. Except for the requested width and height of its children, it has no special attributes that can be used in an XML resource. However, it will take notice of the sizing that you declare for its children, and so the layout_width and layout_height attributes will work normally. It's important to note that in order to make use of a custom View or ViewGroup from a layout XML resource, you need to have all three default constructors overridden.
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The LayoutInflater will make use of one of these constructors to create instances of your class. If the one it wants to use isn't in place, you will get the dreaded Force Close dialog when you try and inflate the layout XML file.
The CircleLayout has its own utility method to handle the measuring of its child View objects. Generally, a ViewGroup would use the ViewGroup.measureChildren utility method to see that all of its child View objects are measured before performing the actual layout. However, we need to iterate over the list of child View objects in order to find the largest used width and height, so instead of performing the iteration three times, we perform the measurements ourselves.
Using the CircleLayout
To make use of your custom ViewGroup implementation, it's good to know that Android has you covered as far as the XML layout resources are concerned. When you need to reference a custom View or ViewGroup class from an XML layout resource, you simply use the full class name instead of the simple class name. The following is a simple example of an XML layout that uses the CircleLayout:
11.
In the generated root package, declare a new Java source file named NewsFeedActivity.java.
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12.
Register the NewsFeedActivity class in your AndroidManifest.xml file:
13.
The new class should extend the Activity class and implement Runnable: public class NewsFeedActivity extends Activity implements Runnable {
14. 15. 16.
Declare a Handler to be used as a timing structure for changing the headlines: private final Handler handler = new Handler();
We need a reference to the TextSwitcher object: private TextSwitcher newsFeed;
Declare a string-array to hold the mock headlines you added to the strings.xml file: private String[] headlines;
17. 18. 19. 20. 21.
You'll also need to keep track of which headline is currently being displayed: private int headlineIndex;
Override the onCreate method: protected void onCreate(final Bundle savedInstanceState) {
Invoke the onCreate method of Activity: super.onCreate(savedInstanceState); Set the content view to the news layout resource: setContentView(R.layout.news); Store a reference to the headline string-array from the strings.xml application resource file: headlines = getResources().getStringArray(R.array.headlines);
22.
Find the TextSwitcher widget and assign it to the field declared earlier: newsFeed = (TextSwitcher)findViewById(R.id.news_feed);
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23.
Set the ViewFactory of the TextSwitcher to a new anonymous class that will create TextView objects when asked: newsFeed.setFactory(new ViewFactory() { public View makeView() { return new TextView(NewsFeedActivity.this); } });
24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.
Override the onStart method: protected void onStart() {
Invoke the onStart method of the Activity class: super.onStart(); Reset the headlineIndex so that we start from the first headline: headlineIndex = 0;
Post the NewsFeedActivity as a delayed action using the Handler: handler.postDelayed(this, 3000);
Override the onStop method: protected void onStop() {
Invoke the onStop method of the Activity class: super.onStop(); Remove any pending calls to the NewsFeedActivity: handler.removeCallbacks(this); Implement the run method which we'll use to swap to the next headline: public void run() {
Open a try block to swap the headline inside. Use the TextSwitcher.setText method to swap to the next headline: newsFeed.setText(headlines[headlineIndex++]); If the headlineIndex is past the total number of headlines, reset the headlineIndex to zero: if(headlineIndex >= headlines.length) { headlineIndex = 0; } [ 179 ]
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35.
Close the try block, and add a finally block. In the finally block, post the NewsFeedActivity back onto the Handler queue: finally { handler.postDelayed(this, 3000); }
36. 37.
Open the auto generated AnimationSelector Java source in an editor or IDE. The AnimationSelector class needs to implement OnClickListener: public class AnimationSelector extends Activity implements OnClickListener {
38.
In the onCreate method, ensure that the content view is set to the main layout resource created earlier: setContentView(R.layout.main); 39.
Find the declared Button and set its OnClickListener to this:
((Button)findViewById(R.id.news_feed)). setOnClickListener(this);
40. 41. 42. 43. 44.
Declare the onClick method: public void onClick(final View view) {
Use a switch to determine which View was clicked: switch(view.getId()) {
If it's the news feed Button, then use the following case: case R.id.news_feed:
Start the NewsFeedActivity using a new Intent: startActivity(new Intent(this, NewsFeedActivity.class));
Break from the switch statement, thus finishing the onClick method.
What just happened?
The TextSwitcher is an example of an animation utility View. In this case it's the perfect structure to swap between the news headlines, displaying one headline at a time and animating a transition between each of the texts. The TextSwitcher object creates two TextView objects (using the anonymous ViewFactory class). When you use the setText method, the TextSwitcher changes the text of the "off screen" TextView and animates a transition between the "on screen" TextView and the "off screen" TextView (with the new text content displayed).
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The TextSwitcher class requires that you specify two animation resources for it to work with, in order to create its transition effect:
Animate text onto the screen Animate text off the screen
In the previous case, we made use of the default slide_in_left and slide_out_right animations. Both of these are examples of translation-based animations due to the fact that they actually alter the "on screen" position of the TextView objects in order to create their effect.
Using flipper and switcher widgets
The first example of this chapter made use of the TextSwitcher class, an animating View class in the standard Android API. There are several other animation utility classes, some of which you may have encountered before (such as ImageSwitcher). Both TextSwitcher and ImageSwitcher are related classes, and both inherit from the more generic ViewSwitcher class. The ViewSwitcher class is a generic animation tool, and defines the ViewFactory interface that we implemented anonymously in the previous example. A ViewSwitcher is a ViewGroup with only two child View objects. One is displayed on the screen, while the other is hidden. The getNext utility method allows you to find out which is the "off screen" View object. While you generally use a ViewFactory to populate a ViewSwitcher, you have the option to populate it manually. You could have populated the TextSwitcher in the example by using the addView method that is inherited from ViewGroup.
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Using the ImageSwitcher and TextSwitcher implementations
The ImageSwitcher and TextSwitcher classes are specialized implementations of the ViewSwitcher that understand the type of View objects they contain. When you invoke the setText method of the TextSwitcher object, it's much like invoking the following code snippet on a ViewSwitcher containing two TextView children:
((TextView)switcher.getNext()).setText("Next text to display"); switcher.showNext();
TextSwitcher can be used to display content such as (as in the example) a news feed,
or as with the Android notification area, to display text content that doesn't fit into a single line. Displaying multiple lines in a TextSwitcher is particularly effective when the animation runs the text upwards, causing the text to appear to scroll upwards behind the TextSwitcher object. An ImageSwitcher is most commonly used in a gallery, slide show, or similar structure. You could also use an ImageSwitcher to allow the user to select from a small list of images, for example a short list of login avatars.
Have a go hero – populating the TextSwitcher
As an alternative to populating the TextSwitcher with a ViewFactory in the news feed example, try populating it in the XML layout resource. Remember that it requires exactly two TextView child widgets. If you get this right, try giving each of the two TextView objects different font colors and styles.
Animating layout widgets
Using the animation utility widgets such as TextSwitcher and ImageSwitcher can allow you to display much more information over time than you could fit on the screen at a time. ViewGroup objects can also be animated without any serious modification through the LayoutAnimationController class. However, in this case, animation needs to be added in your Java code. A LayoutAnimationController is best used to create "entry" or "exit" effects on a ViewGroup as it appears or just before it disappears off the screen. The controller simply starts a specified animation on each of the View children of a specified ViewGroup. However, it doesn't have to do it all at the same time, or in a sequential order. You can easily configure a
LayoutAnimationController to leave a slight delay between the starting of an animation
on each child widget, creating a staggered effect.
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If applied correctly to a LinearLayout, you could achieve a result similar to the one illustrated in the following diagram:
Time for action – animating a GridView
The GridView class has its own LayoutAnimationController specifically designed to animate it in terms of rows and columns, allowing more complex effects than can be achieved with a standard LayoutAnimationController. For this next part of the "animations" example we're going to build a lovely color selector out of a GridView. When the selector first appears on the screen, each color swatch will fade in, starting in the top left corner and ending on the bottom right corner.
1. 2.
Start by declaring a new Java source file in the root package of your project named ColorAdapter.java, which will generate the color swatches for the GridView. The ColorAdapter needs to extend BaseAdapter to take care of the boiler plate Adapter requirements: public class ColorAdapter extends BaseAdapter {
3.
The ColorAdapter will be created with a specified number of rows and columns, the same numbers which will be displayed on the GridView: private final int rows; private final int cols; public ColorAdapter(int rows, int cols) { this.rows = rows; this.cols = cols; }
4.
The number of items that the ColorAdapter will provide is the number of rows multiplied by the number of columns: public int getCount() return rows * cols; } [ 183 ]
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5.
The ID of a color is the position or index at which it's found: public long getItemId(int pos) { return pos; }
6.
We use a utility method to compose the color from an index in the "list." For this function we make use of the HSVtoRGB method in the Android Color class: private int getColor(int pos) { float h = (float)pos / (float)getCount(); return Color.HSVToColor(new float[]{h * 360f, 1f, 1f}); }
7.
The item at an index in the Adapter model is returned as it's color value: public Object getItem(int pos) { return getColor(pos); }
8.
To create the color swatch View objects, we implement the getView method of Adapter as usual: public View getView(int pos, View reuse, ViewGroup parent) {
9.
The View we return is going to be an ImageView object, so we either re-use the one given by the parent widget, or create a new one:
ImageView view = reuse instanceof ImageView ? (ImageView)reuse : new ImageView(parent.getContext());
10.
We make use of the ColorDrawable class to fill the ImageView with the color specified by our getColor utility method: view.setImageDrawable(new ColorDrawable(getColor(pos)));
11.
The ImageView needs to have its android.widget.AbsListView. LayoutParams set, and then it can be returned to the GridView for display: view.setLayoutParams(new LayoutParams(16, 16)); return view;
12. 13.
Create a new XML layout resource file named res/layout/colors.xml to hold the declaration of the GridView that will act as the color selector. The contents of the colors.xml layout file are just a single GridView widget:
14. 15. 16.
Define another new Java source file in the root package of your AnimationExamples project. Name this one ColorSelectorActivity.java. The new class declaration should extend Activity: public class ColorSelectorActivity extends Activity {
Override the onCreate method as normal, and set the content view to the colors XML layout resource you just wrote: protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.colors);
17.
Now you can load the default Android "fade in" animation using the handy AnimationUtils class from the android.view.animation package:
Animation animation = AnimationUtils.loadAnimation( this, android.R.anim.fade_in);
18.
In order to animate the GridView correctly, you need to instantiate a new GridLayoutAnimationController object, passing it the "fade in" animation:
GridLayoutAnimationController animationController = new GridLayoutAnimationController( animation, 0.2f, 0.2f);
19. 20.
Now look for the GridView which you have declared in the colors.xml file:
GridView view = (GridView)findViewById(R.id.colors);
Set the number of columns in the GridView to 10 (note that we didn't do this in the XML layout resource as you normally would): view.setNumColumns(10); 21.
When you set the adapter of the GridView to a ColorAdapter, you also need to know the number of columns, and the easiest way to do this is to keep both in Java: view.setAdapter(new ColorAdapter(10, 10));
22.
The view object is now ready for the GridLayoutAnimationController: view.setLayoutAnimation(animationController); [ 185 ]
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23.
In order to start the animation when the screen is displayed, we override the onStart method. It is in here that we look up the GridView again and start the animation: protected void onStart() { super.onStart(); ((GridView)findViewById(R.id.colors)). getLayoutAnimation().start(); }
24. 25.
In order to integrate this new example with the other animation examples, you'll need to open the res/layout/main.xml file in an editor or IDE. Add a new Button to the end of the LinearLayout, the one we'll use to start the color selection example:
26. 27.
Open the AnimationSelector source file in your editor or IDE. After setting the OnClickListener of the news_feed Button, find and set the OnClickListener of the new colors Button in the same way:
((Button)findViewById(R.id.colors)).setOnClickListener(this);
28.
In the onClick method, after the switch case for the news_feed Button, add another switch case for the new colors Button, and start the ColorSelectorActivity: case R.id.colors: startActivity(new Intent(this, ColorSelectorActivity.class)); break;
29. 30.
Open the AndroidManifest.xml file in your editor or IDE. At the bottom of the section, register the new ColorSelectorActivity:
What just happened?
The new example makes use of the GridLayoutAnimationController to start each "fade in" animation a fraction of a second after the previous one started. This creates a fluid animation effect of the color swatches appearing from the top-left to the bottom-right of the screen.
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When you instantiate a GridLayoutAnimationController, it requires you to provide the animation and two parameters which indicate the amount of time between starting animations for the next row, or the next column. The delay given is not specified in a "direct" time format, but instead by how long the given animation takes to complete. In our case, if the animation took one second to complete, the delay between each animation starting would be 200 milliseconds, since the delay is specified as 0.2. The fact that we animate the swatches just as this Activity becomes visible, effectively makes this a transition animation, introducing the user to this new screen. For these types of animations, it's imperative to take as little time as possible while still giving a pleasing introduction. When you run the new example you should get an animation similar to the ones illustrated in the following images:
Creating Custom Animations
So far we've explored using Android's stock animations with the normal widgets, but what about applying a custom animation to a widget that isn't built for animations? Android includes support for four basic animation types that can be applied to View objects:
Translate/Move Rotate Scale Alpha/Transparency
These different animation structures can be applied by themselves, or merged together in an animation set where any combination of the three can be run at the same time. By creating an animation with a delay time before it starts, you can create complex animations by having simple sets of animations follow each other.
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Like so many things in Android, the easiest way to create your own custom animations is to define it in a resource XML file. The elements in the animation format used by Android correspond directly to the classes in the android.animation.view package. An animation file can also reference animations in other animation resources, which makes it much easier to compose complex animations and re-use simple animations.
Time for action – writing a custom animation
Writing a custom animation is very simple, but not entirely intuitive. For this section you'll define a custom animation which will increase the size of its animated widget by a factor of five, while at the same time fading it until its entirely transparent.
1. 2. 3. 4. 5.
Create a new XML resource file named res/anim/vanish.xml and open it in an editor or IDE. The root element of the animation file will be an animation set element:
In the element, declare an element to define the scale up animation:
The duration of the scale up animation needs to be set as 300 milliseconds: android:duration="300" The animation starts to scale from the original size: android:fromXScale="1.0" android:fromYScale="1.0"
6.
The scale animation needs to increase the size by a factor of 5.0: android:toXScale="5.0" android:toYScale="5.0"
7.
We want the scale to expand from the center of the widget: android:pivotX="50%" android:pivotY="50%"
8.
The last part of the element defines the acceleration curve of the animation. Here we want the scale up to decelerate as it runs: android:interpolator="@android:anim/decelerate_interpolator" 9.
Next, define a new element to handle the fade out part of the animation: [ 188 ]
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10. 11. 12. 13.
The duration of the fade out animation is also 300 milliseconds: android:duration="300" We start with no transparency: android:fromAlpha="1.0" The fade out ends with a completely invisible widget: android:toAlpha="0.0" The fade out should accelerate as it runs, so we use an accelerate interpolator: android:interpolator="@android:anim/accelerate_interpolator" What just happened?
This is a relatively simple animation set, but its effect is visually pleasing. Keeping the animation at 300 milliseconds is quick enough to not interfere with the user's interaction, but just long enough to be seen in full by the user. When defining animations in an animation element, each non-set sub animation needs to have its duration defined. The element has no concept of its own duration. However, you can define a single interpolator for the entire set to share. The animation will by default, scale the widget using the top-left corner as the "pivot" point, causing the widget to grow to the right and downward, but not left and upward. This causes a lopsided animation which is not very appealing. In the preceding example, the scale animation runs with the pivot at the center of the animated widget.
Time for action – making a Button vanish
So how can we apply the nice shiny animation to a Button object? The Button object doesn't have an animation attribute, and so you can't just reference it from the layout resource file. What we want, is the animation to run when the Button widget is clicked.
1.
Create a new layout resource file named res/layout/vanish.xml and open in an editor or IDE.
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2.
At the root of the new layout, declare a RelativeLayout element:
3.
The Button needs to be nice and large, and centered on the screen. For this we give it some inner padding:
4. 5.
Create a new Java source file in the root package of the AnimationExamples project named VanishingButtonActivity.java. The new class needs to extend Activity and implement the OnClickListener interface: public class VanishingButtonActivity extends Activity implements OnClickListener {
6.
Override the onCreate method and invoke the Activity.onCreate method to perform the required Android setup: protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState);
7. 8.
Set the content view to the new vanish layout resource: setContentView(R.layout.vanish); Find the Button widget declared in the XML layout resource and set its OnClickListener:
Button button = (Button)findViewById(R.id.vanish); button.setOnClickListener(this);
9.
Implement the onClick method of OnClickListener: public void onClick(View clicked) {
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10.
Load the Animation from the resource file:
Animation vanish = AnimationUtils.loadAnimation( this, R.anim.vanish);
11. 12. 13.
Start the Animation on the Button object: findViewById(R.id.vanish).startAnimation(vanish); Open the AndroidManifest.xml file in an editor or IDE. Declare the VanishingButtonActivity at the end of the section with a display label:
14. 15.
Open the res/layout/main.xml layout resource in your editor or IDE. Add a new Button to the end of LinearLayout to activate the VanishingButtonActivity:
16. 17.
Open the AnimationSelector Java source file in your editor or IDE. At the end of the onCreate method, fetch the new vanish Button from the layout and set its OnClickListener:
((Button)findViewById(R.id.vanish)).setOnClickListener(this);
18.
In the onClick method, add a new switch case to start the VanishingButtonActivity: case R.id.vanish: startActivity(new Intent( this, VanishingButtonActivity.class)); break;
What just happened?
The addition of the preceding example will display a single Button in the middle of the screen. When clicked, the Button will be mutated by the vanish animation for 300 milliseconds. When it is complete, the animation won't have any effect on the Button anymore. This is an important characteristic of animations—when they are complete, the widget they have animated is returned to its original state.
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It's also important to note that it's not the widget itself that is modified by an animation, but rather the state of the Canvas it's painted on. This is much the same concept as modifying the state of Graphics or Graphics2D object in Java AWT, or Swing before a widget uses the Graphics object to paint itself. In the following images you can see the effect that animation has on the Button when it's clicked. The Button is actually re-painted for each frame in the animation, and remains entirely active during that time.
Summary
In this chapter, we explored the various methods by which you can apply animations to various parts of your user interface. We explored how some widgets are designed to animate themselves. Layouts can be animated for transitions in and out of an Activity. Several simple animations are available by default in the Android resource, but ultimately creating your own animations and applying them to your user interface manually creates, by far, the most visually appealing and user-friendly experience for your users. Many applications on a mobile device need to present a large amount of information on the screen, and present it in such a way that it can be easily absorbed. In the next chapter, we'll explore user interface design with regards to presenting information to the user in a friendly and useful manner. This allows users to access the information that they need as quickly as possible, in a swift and easy manner, while not limiting the information they have access to.
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Designing Content-centric Activities
When you have a lot of data to display to the user, and you need a content presentation Activity. Generally, such types of Activities turn out to be content-centric. The main purpose of a content-centric Activity is to give the user as much of the information as possible while not overwhelming them. This is a common requirement of applications that perform some sort of search, or present any type of specialized information.
8
Shopping and related e-commerce applications are an ideal example of a content-centric application. Much of the effort in the design is dedicated to displaying information about the products on sale. If the users can't find the information about a product they are looking for, they will look somewhere else. For this reason, the product display must not only be attractive and easy to use, but also provide as much information as possible, without being cryptic or cluttered. Another example of a content-centric layout is a user's profile page in a social-networking application. People generally have a lot to say about themselves, and if they don't, other people will often say a lot about them. These applications not only have a lot of information to present to the user, but the information varies widely in terms of quality and relevance. Just because one user thinks something is important, doesn't mean the next person will. In cases like these, it's also very important to have an interface that can be customized to the user's preferences (often just by re-organizing the order in which information is displayed), and is also able to draw the user's attention to new information or areas they may find interesting.
Designing Content-centric Activities
A great example of a good way to draw a user's attention is seen in a chat application. If the user has scrolled up, he/she is probably reading something that was said a few minutes ago. If a new message arrives, it's very rude to just scroll them to the new message, since they may well still be reading it. An audio tone to notify them of a new message is a common option, but will also draw others' attention to the user (this is a mobile device after all). The best option is a small animated icon at the bottom of the screen, possibly color-coded to tell the user the relevance of the message (if that is available). Such an icon could also be an interactive element, allowing the user to touch it in order to automatically scroll to the most recently posted message. This type of thinking is important when designing any application, but when building a content-centric Activity, putting some extra thought into your design is even more critical. In this chapter, we'll be exploring the different aspects to consider when displaying content to the user, as well as different ways in which content screens can be developed. Specifically, we'll be exploring:
Thought process when designing content displays on Android How users use and view content screens Using the WebView class to display content Building native layouts for displaying content Formatting and styling text in Android Drawing attention to specific areas of the screen
Considering design options when displaying content on an Android device
A content-centric Activity bears a strong resemblance to a web page, but has some key design considerations that people don't have in mind when creating a web page. For example, a touchscreen device generally doesn't have a software pointer, and so doesn't have any concept of a "roll over". However, many web pages are built using cursor roll over to drive everything from link highlighting to menus. When designing a content-centric Activity, you'll want to consider carefully the aesthetics of your design. The screen should avoid clutter since many elements may be interactive, presenting the user with additional information when touched. At the same time, you should attempt to minimize the need to scroll, especially horizontal scrolling. The need to keep information concise is often the motivator to make more of the elements interactive. As mentioned in previous chapters, it's a good idea to consider using icons instead of text where you can, and to organize the information in order of importance to the user.
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Also bear in mind that screen-sizes change. Some devices have a large number of pixels (such as the various Android Tablets), while others have tiny 3.5 inch screens. For this reason it's important to consider that while some people will be able to see all of the presented information on a single screen, others will be presented with three or four screens worth of content for the same amount of information. A web page is a great way to quickly and easily put together a content-centric layout when working on an Android application. It has the advantage of having great HTML and CSS support from WebKit, and easy integration with the rest of your application. It can also be handled by an existing web-designer, or even just display a web page if your application is connected to a web-based system. A web page is however constrained (to some degree) to the layout structures dictated in HTML and CSS. While these are extremely flexible at one level, HTML and CSS layout development can also be a tedious and frustrating process even when only targeting a single rendering engine (in Android's case: WebKit), if you are not used to building web-based systems. When it comes to animations and similar structures, you are further constrained by the performance of the HTML rendering engine, whether using JavaScript or CSS3 animations.
Considering user behavior
As with any type of user interface, it's important to understand your user behavior and how they will interact with the screens you provide them with. In the context of large amount of content information, it's important to understand both what information is important, and how users will read and absorb that information. While you may want to draw attention to a selected piece of information (such as price), running a looping animation to change the color of that element will distract the user from the other information on the screen. However, simply changing the font, placing the data in a box, or changing the text color can also have the desired effect. It's also important to consider how a user will interact with the screen. On a touchscreen device, users can and will touch almost every part of the screen. They'll also drag items that look movable, and use scroll gestures if the content appears to run over the screen length.
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Most people scan information in the same way. When a user is presented with a screen for the first time, or with lots of information on it, their minds approach reading the information in more-or-less the same way. The following are illustrations of the various movement patterns a user's eyes will follow when scanning for important information on the screen.
You'll generally want to make sure that important information is in the areas where one arrow meets another. The most important area is the corner in which your user normally starts reading. For most Western users, this is the top-left corner of the screen, while Asian and Arab users will often start at the top-right.
When designing a content screen, consider making the information in these areas stand out a little more than normal. This will create a "linger" time where the users' eyes will generally focus on that area a little longer than normal. This is why we normally put a logo on the top-left of a web page.
Drawing user attention
Almost always, some information is more important than other information. You want your user to be able to pick-out the important information as quickly as possible, and get on with what they are doing. Once a person is familiar with your application, they may well stop reading the fine print altogether. This is a good thing, you're helping your users make better use of your application by letting them get on with their lives. When you need to draw attention to specific information, such as a product's name or price, it's a good idea to make use of the extensive options provided by the TextView class. Simply changing an item's color can make it stand out for the user. If you need to go further, consider adding a shadow, or placing the content in a "highlight box". As we've already discussed in Chapter 7, Animating Widgets and Layouts, animations can also be used to draw attention to specific areas of the user interface. A simple "blink" animation (consisting of a fade-out followed by a fade-in animation) can be used to draw the users' attention to a change.
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A more specific example: money If you are selling something to your user, and allowing them to choose between different shipping methods and packaging options, the total price will change based on their selections. Make sure that the total amount stands out by rendering it in a bold font. When the price is updated, cycle through a series of "intermediate" prices so that the total is graphically "counted up" or "counted down" to its new value.
Think carefully about the widgets that you'll want to make use of in your user interface. Instead of using the normal TextView, you may well want to place a piece of what would normally be a single field in a TextSwitcher (or something similar) to allow you to animate a single word or value.
Displaying content with the WebView class
The WebView class (in the android.webkit package) is often a logical choice for contentcentric designs and holds some very serious advantages over building the user interface and a normal Android XML layout resource. The WebView class offers you a single point at which you can place all of the content for a screen, and it handles all of its own history and scrolling, making your code very simple to write. When displaying content that requires complex layout and/or lots of text content (which may require markup), the WebView class is a highly favorable option. Having built-in support for HTML and CSS mark up, it reduces the number of widgets that you'll need on the screen. Given that Android makes use of Web-Kit as a rendering engine, you also have many CSS3 structures available (such as CSS animations). Although the WebView is generally used for browser like networked applications where hyperlinks are very important, you can just as easily supply it with local content containing no links. You can also intercept link requests to allow navigation to other parts of your application. Generally when working with a WebView structure, you'll need some method by which you can generate the content that you will be displaying. In contrast to building the user interface in a layout resource, where you can simply ID the various View objects that you need to inject dynamic content into. That said, a full template engine is often much easier to work with than a hybrid of XML layout and Java code, although the ease of implementation is strongly dependant on both the skills available to you and the type of information you need to display on the screen.
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Using a WebView object
To work a bit with the WebView and give a more specific example on how it can be used to present large amounts of content, we'll be building an Activity to display a food recipe on the screen. For this example we'll be hard coding the actual recipe and the layout code to generate the HTML. In practice, you would want to make use of a template engine such as Velocity/FreeMarker or XSLT to generate the HTML code.
Time for action – creating a recipe viewer application
You'll notice that the following example doesn't use an XML layout resource, but rather creates the entire Activity in Java. In this example we use a Recipe object to generate HTML code into a StringBuilder for display. It's a simple but effective implementation. However, it requires that the Java code be modified if a change to the look and feel of the recipe is required.
1.
Create a new project to contain the recipe reader application: android create project -n RecipeViewer -p RecipeViewer -k com. packtpub.viewrecipe -a ViewRecipeActivity -t 3
2. 3.
Create a new Ingredient.java source file in the root package of the new application to hold information for a single required ingredient, and open this new file in your editor or IDE. Declare fields for the name, amount, and unit required for a recipe: private final String name; private final double amount; private final String unit;
4.
Create a constructor to take the parameters and assign them to the fields: public Ingredient( String name, double amount, String unit) { this.name = name; this.amount = amount; this.unit = unit; }
5.
Create a getter method for each of fields: public double getAmount() { return amount; } // . . . [ 198 ]
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6. 7. 8.
In the root package of the project, create a new source file named Recipe.java to contain a single recipe, and open it in your editor or IDE. Declare a field for the name of the Recipe object: private final String name;
Declare another field to contain the list of ingredients required for this Recipe. We store these as an array of Ingredient objects: private final Ingredient[] ingredients;
9.
Then declare an array of String objects that will contain the list of instructions that need to be followed for the Recipe: private final String[] instructions;
10.
Create a constructor to accept the field data and assign it for storage: public Recipe( String name, Ingredient[] ingredients, String[] instructions) { this.name = name; this.ingredients = ingredients; this.instructions = instructions; }
11.
Create a getter method for each of the three fields: public Ingredient[] getIngredients() { return ingredients; } // . . .
12.
In this example the Recipe class is responsible for generating the HTML. Declare a new method named toHtml: public String toHtml() {
13. 14. 15.
Create a DecimalFormat object to handle the formatting of the volumes:
DecimalFormat format = new DecimalFormat("0.##");
Create a new StringBuilder object to build the HTML into:
StringBuilder s = new StringBuilder();
Append the HTML headers:
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16. 17. 18. 19.
Append a first-level header element with the name of the recipe:
s.append("").append(getName()).append("");
Append a second-level header element to open the ingredients section:
s.append("You will need:");
Open an unordered list to list the ingredients required for the recipe:
s.append("");
For each Ingredient object open a list item for the new ingredient: for(Ingredient i : getIngredients()) { s.append("");
20.
Append the amount of the ingredient to the StringBuilder after formatting it with the DecimalFormat declared:
s.append(format.format(i.getAmount()));
21. 22.
Then append the measurement unit for the ingredient:
s.append(i.getUnit());
Now append the name of the ingredient to the StringBuilder, and close the ingredient list item:
s.append(" - ").append(i.getName()); s.append("");
23. 24. 25. 26.
After closing the for loop, close the unordered list:
s.append("");
Create a second-lever header opening the Instructions section of the recipe:
s.append("Instructions:");
Open another unordered list to render the recipe instructions into:
s.append("");
Use a for-each loop over the array of instructions to render them into the unordered list structure in the StringBuilder: for(String i : getInstructions()) { s.append("").append(i).append(""); }
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27.
Close the unordered list, and HTML headers, returning the String contents of the StringBuilder object:
s.append(""); s.append("").append(""); return s.toString();
28. 29.
Open the ViewRecipeActivity Java source code in your editor or IDE. In the onCreate method, directly after invoking super.onCreate, create a new WebView object passing this to it as its Context:
WebView view = new WebView(this);
30.
Set the WebView LayoutParams to take up all available screen space, since the WebView (much like a ListView) has built-in scrolling capabilities: view.setLayoutParams(new LayoutParams( LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT));
31.
Create a Recipe object to display in the WebView, the full recipe is at the end of this example section:
Recipe recipe = new Recipe( "Microwave Fudge", // . . .
32. 33.
Load the HTML content generated by the Recipe object into the WebView: view.loadData(recipe.toHtml(), "text/html", "UTF-8");
Set the content view of the Activity to the WebView object we created: setContentView(view); What just happened?
The recipe viewer example shows a simple structure which can be extended in many different ways to present large amounts of information to the user in an easy-to-use format. Thanks to the fact that WebView works with HTML, it makes presenting non-interactive lists of information more appealing than working with a ListView or similar structures. The loadData method used previously is limited in that it doesn't allow for your page to easily reference external structures such as style sheets or images. You can work around this limitation by using the loadDataWithBaseURL method which works in much the same way, but renders the page relative to a specified URL, which may be online or local on the device.
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The Recipe object is considered responsible for rendering its HTML, which works well in a pure Java situation. You could also pass the Recipe to a template engine, or use something like a visitor pattern to render the Recipe object as HTML code. The full code for the Recipe object in the previous example is as follows:
Recipe recipe = new Recipe( "Microwave Fudge", new Ingredient[]{ new Ingredient("Condensed Milk", 385, "grams"), new Ingredient("Sugar", 500, "grams"), new Ingredient("Margarine", 125, "grams"), new Ingredient("Vanilla Essence", 5, "ml") }, new String[]{ "Combine the condensed milk, sugar and margarine " + "in a large microwave-proof bowl", "Microwave for 2 minutes on full power", "Remove from microwave and stir well", "Microwave for additional 5 minutes on full power", "Add the Vanilla essence and stir", "Pour into a greased dish", "Allow to cool", "Cut into small squares" });
An unfortunate side effect of using the WebView object is that it doesn't conform to the look and feel of other widgets. It is for this reason it doesn't work well when you place it with other widgets on the same screen. The end effect of the previous example is effectively a non-interactive web page which looks as follows:
Have a go hero – improving the look of the recipe viewer
The previous example generates a very simple HTML page and doesn't include any styling. Including an inline CSS is a very simple operation, and could even be done by reading the styling content from an application resource. Create a CSS, include it inline in the HTML page, with rules such as:
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Color the background of first-level header and second-level header elements Change the font-color of the first and second-level headers to white Round the corners of the header elements by five pixels Change the list bullet to a square instead of a circle
Taking WebView further
The WebView class has significant functionality that can be very useful when dealing with content screens, for example, using hyperlinks to provide a show/hide disclosure section for less important content. This requires the use of JavaScript in the HTML page, at which point it's strongly advisable that your application use a template engine to produce the HTML pages instead of generating them in Java code (as the Java code will quickly become difficult to maintain). The WebView class also allows your application to interact with the JavaScript code on the page using a very simple mechanism by which you can expose Java objects to the JavaScript code. This is done with the addJavascriptInterface method. This allows the HTML page to invoke actions on a Java object that you provide, effectively allowing the page to take control of a part of your application. If your content screen needs to take a business action, such as Buy or Cancel, the required functionality can be exposed in a JavaScript interface object. When the Book HTML element is selected by the user, the JavaScript in the page can invoke the appInterface.buy(); method which you defined. Another important feature to consider with the WebView class is the "zoom" controls. When presenting your user with lots of information, it may be useful for the user to be able to zoom in or out in order to make some elements easier to read. To enable the built-in zoom controls of the WebView, you'll need to access the WebSettings object: webView.getWebSettings().setBuiltInZoomControls(true); The WebSettings object can be used to enable and disable a large number of other features that are available in the WebKit browser component, and it's well worth reading through the available documentation. The primary problem with the WebView class is its look and feel. Where an Android application with the default theme is light grey on a black background, the WebView class is black on a white background, which makes the screens driven by a WebView stand out to the user as though they are a separate application. The simplest way around the styling problem would appear to be to style the HTML pages to look just like the rest of the application. The problem is that some device manufacturers have their own Android application styling, so you can't really be sure what the rest of your application is going to look like. Changing the background and foreground of the HTML page to be in line with the standard Android theme could well make it stand out against the rest of your application when run on manufacturer-themed devices.
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Pop quiz
1. What is the best way to render large object graphs to HTML for rendering in a WebView? a. Convert them to XML and run them through XSLT b. Send them to an external web service to be rendered c. Hard code the HTML generation d. With a simple template engine 2. How can you access external CSS and images with a WebView? a. Use the loadDataWithBaseURL method b. Specify the full URL path in the HTML page c. Generate HTML code that includes the in line data 3. What rendering engine does Android use for WebView? a. Gecko b. MSIE/Trident c. KHTML d. WebKit
Creating relative layouts for content display
The WebView offers an easy way by which large amount of content can be displayed to the user in an easy-to-read format. It also has many built-in features designed specifically for viewing content. However, it doesn't always offer the easy way out and often doesn't allow for functionality that other widgets provide out-of-the-box. The RelativeLayout class provides much of the same layout functionality that the WebView class provides you with. As we just discussed, the WebView stands out almost as though it were a separate application. Working with a RelativeLayout, you'll be populating your screen with standard Android widgets, which in turn means that there will be no change in the look and feel from one screen to the next. While WebView requires some form of template engine (whether it be in an API or simply a StringBuilder as in the example), a RelativeLayout can be declared in an application resource as an XML file. Using a layout file also means that the screen layout will be selected through the resource selection process, allowing for sophisticated customizations that are difficult to achieve with the WebView class and HTML code.
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Using a RelativeLayout in a way provides a form of template engine. By only giving IDs to View objects that you will need to populate with data, you can populate the screen by injecting the relevant content into these exposed objects. When we built the HTML-based view, we needed to create header elements for the ingredients list and list of instructions, with a coded layout structure those headers would be loaded from within the layout file, or from a string bundle resource. When dealing with lists of information, which is a common requirement of a content layout, you can provide the data in several different ways. You could use a ListView object, or you could use an embedded LinearLayout to act as a list. When working with either of them, it's advisable to have a layout resource that can be reused for each of the items in the list. Making use of a ListView means you have an Adapter through which you can convert your data objects into View objects that can be displayed on the screen. However, ListView objects have various other constraints (such as the size of the contained items) and are best used when the items they display are interactive in some way. If you need a non-interactive list (or grid) of items, it's a good idea to follow the Adapter mechanism by creating a separate class that is responsible for creating View objects based on your data objects.
Taking full advantage of RelativeLayout
RelativeLayout structures have the major advantage that they offer direct integration with the rest of your application. They can also be more easily localized than an HTML page. The event structures provided by a direct ViewGroup structure are more versatile than those provided by a WebView object via its specialized event listeners and JavaScript.
The XML layout structures also provide much the same effect as a template engine, avoiding the need to import an external API such as an XSLT engine, a Java template engine, or hard coding the HTML generation. Standard Android Activity classes also have the builtin Android animation structures to work with. While the WebView class allows for CSS animations or could run JavaScript animations, this requires re-layout of the HTML structure for each frame in the animation. An Android Activity class implementing the entire content screen also has the advantage that it can load its external resources from the application resource structure. This not only allows you to do things such as localize your images more easily, but also means that all of the resources are run through the resource compiler and as such can be optimized by the Android tool chain. With a WebView you would need a base URL to load such resources from, or be able to encode them inline in the HTML page.
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Considering Android layout constraints
There are some drawbacks to developing the entire content view as an Android layout. From a skills point-of-view, only a developer can build and maintain the user interface. It also means that any styling done to individual widgets must be managed by a developer. With a WebView based layout, much of the creation work on the layout could be handled by a web developer and graphic designer.
Adding more widgets to your screen comes with another problem— performance. Not only can larger, more complex layouts lead to a very slow user experience, it can cause your Activity to crash entirely.
Keeping fewer widgets on the screen means that the interface will have less information for the user to absorb in one hit, and will be easier to work with. Layouts with either too much length, or too much depth will cause an application to crash. If you need to animate a single word in the middle of a sentence, you'll have to define two additional TextView widgets that will display the non-animated text on either side of the animated word. This increases the length of your layout. If you also needed a horizontal LinearLayout in which to place these three TextView objects, you would be increasing the depth of your layout structure. By factoring in both of these constraints, you can imagine how quickly you can run out of memory or processing power when it comes to layout rendering. Each of the widgets must be measured for layout before being rendered. Each measurement, layout step, or rendering step makes use of the language stack (by recursively invoking methods) in order to make sure all of the widgets are correctly rendered at the correct point on the screen (or not rendered if they are off screen). The software stack size in Android is limited, and each method call requires each of its parameters to be pushed onto the stack for the invocation. On top of that, all of the measurement information needs to be stored in the heap space, which is another seriously limited resource on the Android platform (by default the Dalvik VM only allocates 8 MB of heap space to begin with). The following diagram illustrates the difference between the length and depth of a layout structure. The left screen illustrates a long layout, while the right screen illustrates a deep layout:
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Styling TextView objects
At this point it's rather concerning to think about how you might make a single word in a sentence bold, or give it a shadow. In the WebView it's as easy as adding a element with some special styling on it, but in a native layout, wouldn't you need to add separate TextView objects for each section of the text? If this were so, you would be dramatically limited in the amount of text you would be able to display to the user, since you would be creating thousands of almost useless objects. Fortunately, Android makes it very easy to mark up the text in all of its default widgets. Any class that extends from TextView can handle text with style information or even images. Generally the classes available in the android.text.style package can be used to style sub-segments of the text strings you want to display. In order to use these different styling structures, you will need to use a SpannableString object. A SpannableString is a specialized type of Android string that keeps track of styling information in relation to a normal CharSequence of text that needs to be displayed. There are several other similar classes (such as SpannableStringBuilder) which handle easy modification of the text, and are thus suited to text that will be edited. For our current purposes, a SpannableString is perfect, and much simpler to work with. A SpannableString has a method that it's required to implement, based on the Spannable interface—setSpan. The setSpan method allows you to add markup structures to the SpannableString, which affect how a specific part of the text is rendered. If we simply wanted to write the text There is nothing to fear! on the screen, you would normally just use a TextView object with the specified string. What if we wanted to strike the nothing out of that string? The way forward now is to use a StrikethroughSpan object for characters 9 through 16. In this case, the string can't just be defined in the layout file anymore, a SpannableString needs to be created in the Java code. The following is a simple example of how this can be done, and what the resulting TextView would look like:
TextView fear = new TextView(this); SpannableString string = new SpannableString( "There is nothing to fear!"); string.setSpan(new StrikethroughSpan(), 9, 16, 0); fear.setText(string);
The result of this little snippet of Java code is a TextView widget displaying styled content instead of a plain String, as shown in the following screenshot:
As you can see, using this type of markup is brilliantly effective, and really quite easy to work with. This sample is also very quick to execute when compared to the WebView rendering, since it doesn't include any form of parsing.
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There are a few problems with the mechanism though. The most important one being the index handling. In order to know when to start or stop a Span of markup rendering, you need to specify the first and last character that needs to be rendered with the given Span. Not a problem unless you plan on changing your text, or even worse—internationalizing it. Fortunately, once again Android already has a built-in solution, although it comes at the expense of performance. You can convert almost any HTML text into a Spannable object which can in turn be passed directly to any TextView object for rendering. The class to use is the android.text.Html class, which includes utility methods for parsing HTML code into a Spannable object, and also for converting a Spannable object into a HTML code. If you need to internationalize strings that you plan on rendering with additional style attributes, the Html class is probably the only sensible way to do it. It also has the added advantage that the loading of images can be handled by your application (through use of the Html.ImageGetter interface). Additionally, the TextView will still look and feel like a normal Android widget, which enhances the users' experience. Most HTML tags are handled by the Html class, but not quite all of them. For one thing—CSS styles are ignored, so colors and borders are out of the question. However, great styling is still possible, and at least you don't need to record the character index values somewhere in an application resource so that all of the styling lines up. If you wanted to format some text in a Button label as bold, it's really easy to do with the Html class. It's much quicker to just pass the result of the fromHtml method directly to the TextView object. For example, the following code snippet would yield a Button object with the word Hello in italic script, while the word World would have a bold weight to it:
Button button = new Button(this); button.setText(Html.fromHtml("Hello World!"));
You can also specify HTML content in a layout resource XML file, and it will be parsed with the Html class before being passed into the TextView object's setText method. The above Java snippet creates a Button widget that would look as follows:
HTML tags can also be used to render mini-documents into a TextView object, and while they carry their own styling, they also adhere to the styling of the TextView object. This means that if you're looking for a solution that's quicker to work with than a WebView for carrying some static text (and no hyperlinks), then a TextView can actually serve as a good alternative. For example, consider the following code snippet:
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Chapter 8 TextView text = new TextView(this); text.setTextColor(0xff000000); text.setBackgroundColor(0xffffffff); text.setText(Html.fromHtml( "Cows Love to Eat Grass" + "Do not fear the Cow"));
This will render the TextView with a first-level header and a single-lined paragraph element. Both of which will include some padding in order to space them apart from the other elements on the screen. The resulting image should look quite familiar:
As you can see, a correctly styled TextView makes a great alternative to a WebView, especially if you are fitting it inline with a series of native widgets. However, the black-onwhite styling does bring back the inconsistency problem. So unless you entire application follows this model, it's a better idea to leave the styling as default. If you are planning on using a TextView for longer content, it's important to consider some additional factors:
Make sure that the user will be able to scroll if the text runs longer than the size of their screen. This is easily done by placing the TextView in a ScrollView object. If your text is very long, consider styling the content, either making the text brighter white or working with a black on white background. While it is very inconsistent with other Android applications, and other screens in your own application, it is much easier on the eyes and your users will thank you for that. Consider allowing the user to change the font size with a long touch or a menu. If their screen is low density, or they don't have perfect vision, you may be making their lives a little easier.
Pop quiz
1. If you need to display a non-interactive bullet point list, which of these is preferable? a. A WebView with an unordered list b. A specially styled ListView object c. A TextView object with HTML content
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2. With regards to hyperlinks, you might use a WebView instead of a TextView because: a. TextView cannot handle hyper links b. They look better in a WebView c. A WebView has built-in history management 3. A native interface works better for animation intensive applications because: a. You can use Android animation resource files b. The WebView class doesn't handle animations c. HTML animations are more expensive to run
Time for action – developing specialized content views
In many situations, you'll need a specific type of interactive logic that you would want to reuse in many parts of your application. On a content screen, some parts of the display will need to be updated, driven by changes that are made to other parts of the display. This is often because while some area of the screen is giving the user information, the other parts are capturing new data from them. Next, we'll build a simple widget responsible for displaying an amount of money to the user. Its main reason for its existence is the fact that it not only animates between changes, but also feeds back to the user whether the amount has gone up or down by changing its color.
1. 2.
Create a new Java source file named AmountBox.java for the new class, and open the new file in an editor or IDE. The new class should extend the TextSwitcher class and implement the ViewSwitcher.ViewFactory interface: public class AmountBox extends TextSwitcher implements ViewSwitcher.ViewFactory {
3.
Declare a field for the DecimalFormat to be used to render the amount: private DecimalFormat format = new DecimalFormat("0.##");
Also declare a field to store the current numeric value displayed: private double amount;
4.
Declare copies of the two constructors made available from the TextSwitcher class in order to allow the LayoutInflator class to instantiate the AmountBox class from resource files: public AmountBox(Context context, AttributeSet attrs) { super(context, attrs); [ 210 ]
Declare the init() method to take care of "common constructor" requirements: private void init() {
Set the "in" and "out" animations to the fade animations provided by Android: setOutAnimation(getContext(), android.R.anim.fade_out); setInAnimation(getContext(), android.R.anim.fade_in);
7. 8. 9.
Next, set the ViewFactory to the AmountBox: setFactory(this); Finally, invoke setAmount(0) to ensure the displayed amount is specified: setAmount(0); Declare a setter method to allow overriding of the default DecimalFormat: public void setFormat(DecimalFormat format) { this.format = format; }
10.
Declare a getter method to allow easy access to the current numeric value: public double getAmount() { return amount; }
11. 12. 13.
Override the makeView() method from ViewFactory: public View makeView() {
Create a new TextView object with the context given to this AmountBox:
TextView view = new TextView(getContext());
Specify a large text size since the amount will represent money, and then return the TextView object for display: view.setTextSize(18); return view;
14.
Now declare a setter method to allow the amount value to be changed: public void setAmount(double value) { [ 211 ]
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15.
This method will change the color of the text, so declare a variable for the new text color that will be displayed: int color;
16.
First check to see what color we should change the text to: if(value < amount) { color = 0xff00ff00; } else if(value > amount) { color = 0xffff0000; } else { return; }
17. 18. 19. 20. 21.
Fetch the off screen TextView object:
TextView offscreen = (TextView)getNextView();
Set the font color based on the change to the numeric value: offscreen.setTextColor(color); Render a shadow around the text in order to create a "halo" effect: offscreen.setShadowLayer(3, 0, 0, color);
Set the text of the TextView to the new value: offscreen.setText(format.format(value)); Display the off screen TextView and remember the new value: showNext(); amount = value;
What just happened?
The AmountBox class is a great example of a small unit of content that needs to be updated. This class provides information to the user, but also provides a form of feedback. When the user does something which affects the amount displayed, the AmountBox reacts by updating the font color to reflect the direction of the change—green for the amount going down, and red for an amount going up. The example makes use of the standard Android fade through animations as discussed in Chapter 7, Animating Widgets and Layouts. The speed of the animations provides a great cross fade effect between the two amounts. Notice that in the setAmount method, the updating of the text content and switching the View objects is handled manually.
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You could potentially replace the offscreen.setText and showNext method calls with a call to setText, but it's nice to see how it works under the hood. This method is also not subject to future implementation changes that may occur.
Developing an online music store
A great example of a content-centric layout is a music store built into a media player application. The ability to buy music directly from the media player is a massively userfriendly feature, and also fits nicely with the way Android applications behave as "connected" applications instead of purely offline systems. Android also makes it very easy to truly include the shop as part of the application instead of simply providing a link to an appropriate website. Generally, users are more inclined to feel a sense of trust if they pick the Buy Music button and are not suddenly whisked off to their web browser. Having both online and offline parts of your application properly integrated can also go a long way for your sales statistics. Buying music online is very different to purchasing it in a store. The availability of additional information about the songs, artists, or albums the user is looking at is part of the appeal. For this reason, an online music store for a mobile device must be carefully designed to provide as much information as possible without either cluttering the screen, or detracting from the fact that the user is there to purchase music. The feeling of integration with the application also helps build trust with the user, so the look and feel is very important. Another advantage of buying music online is you pay only for what you want to purchase. For this the user interface needs to allow the user to select which tracks from an album the users would like to purchase, and which they either don't want or plan to buy later. Also, how do they know which ones they like? They also need to be able to play a sample (whether it be time limited, or just a lower quality) of each track.
Designing the music store
To really illustrate how a content-centric design fits together, you need to build one. For this example we'll be working through the design process, and then the implementation of that design. Since the design and its implementation are the important parts here, we won't go into building a functional example. It'll really just be a pretty screen.
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To begin with, we need to have a basic user interface design. I find it best to start with a whiteboard or a piece of paper and a pen. While there are plenty of tools for drawing mock screens out there, none of them really approach the user interface of a paper and pen. To start off, we draw a high level wireframe of the overall screen design. This is simply a series of boxes that will tell us what type of information to show in what parts of the screen.
In the diagram, we've divided the user interface into three parts:
Album and Artist Information area: This area displays the name and cover art of the album the user is looking to buy Track List area: In this area, users can listen to samples and select which tracks they want to purchase Purchasing Area: This area displays the total amount users will be paying, and also a button to buy their selected tracks
In the previous diagram, I've stuck to the size of the screen, but depending on the screen size and number of tracks available, the user interface may need a scrollbar to be fully accessible. The next bit of work is to look at each of the sections of the user interface that we've defined and decide what widgets will go into each of them. Firstly we'll need to look at the album and artist information. The album information will be displayed as the album cover artwork and the album name. We'll include an image area for an artist logo, and also include a text block with the name of the recording label.
A simple block diagram like this lets you visually consider the various elements. It also allows you to start thinking about things such as font size, borders, and spacing. In the previous diagram, we want the three elements on the right to be roughly the same size as the cover art on the left. Unfortunately the Android RelativeLayout class doesn't currently allow us to directly stipulate this as a contract. The next element of the design we need to consider is the track listing box. For this, instead of drawing everything in the box, we'll focus on what a single line will look like and what information it will contain.
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The preceding structure is a very simple one line structure for displaying the details of a single track. The CheckBox on the left can be used to select those tracks that the user wants to purchase, while the button on the right side can be used to play a sample of the given track. The two button-like elements on either side create a kind of framing for the plain text elements in the middle of the line. Finally, we need to consider how we plan on asking the user to send their money. This is a very important part of the user interface, it needs to be obvious—the amount of money they will be expected to pay. We also need to make it really easy for the user to actually make the transaction, so a single Purchase or Buy Selected Tracks button is needed.
The final section of the user interface simply has two widgets in it, one on the left for purchasing, and the total amount the user is expected to pay on the right. For the left side button, we'll work with a simple Android Button widget, while on the right side we'll make use of the new AmountBox written in the previous part of this chapter.
Developing the music store
We'll start with the new example by building a new series of model classes, but first you'll need to create a new project for our conceptual media player. To do this, run the following command on a command line or console: android create project -n PacktTunes -p PacktTunes -k com.packtpub. packttunes -a ShopActivity -t 3
After creating the new project, copy the AmountBox source code into the root package of the new project. You then need to create a class to contain the data for a single track. This simply needs to have the name of the track and the duration of the track stored as the number of seconds. We'll also include utility methods to calculate the minutes:seconds values that we can use to display the duration data. public class Track { private final String name; private final int length; [ 215 ]
Designing Content-centric Activities public Track(final String name, final int length) { this.name = name; this.length = length; } public String getName() { return name; } public int getLength() { return length; } public int getMinutes() { return length / 60; } public int getSeconds() { return length % 60; } }
The Track class is a very simple structure which could easily be parsed from XML or deserialized from a binary stream. We also need another class to hold the information about a single artist. While the following class is really nothing more than a form of data store, it could easily be extended to store things like biography information if needed: public class Artist { private final Drawable logo; private final String description; public Artist( final Drawable logo, final String description) { this.logo = logo; this.description = description; } public String getDescription() { return description; } public Drawable getLogo() { return logo; } }
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Finally, on the data class front, we'll need a class for linking the two previous classes to a single album. This class will be used as a single point which can be handed to an Activity. Copy the following code into a new file named Album.java in the root package of your project: public class Album { private final Drawable cover; private final String name; private final Artist artist; private final String label; private final Track[] tracks; public Album( final final final final final Drawable cover, String name, Artist artist, String label, Track... tracks) {
this.cover = cover; this.name = name; this.artist = artist; this.label = label; this.tracks = tracks; } public Drawable getCover() { return cover; } public Artist getArtist() { return artist; } public String getLabel() { return label; } public String getName() { return name; } public Track[] getTracks() { return tracks; } }
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Time for action – building a track item
To get working on the new user interface, you'll need a few images. For this next section, you'll need an image for the play buttons. The play image should be a simple "play" arrow, the button we place it in will provide a background and border. The lines in the list structure will be placed into a TableLayout in order to align all of the sub-structures.
1. 2.
Create a new layout resource file in the res/layouts directory of the project, and name the new file track.xml. Declare the root element of the new file as a TableRow element consuming all of the available width and only the required height:
3.
As the first element of the TableRow, create a CheckBox the user can use to select and unselect the tracks they want to buy:
4.
Declare a TextView element to display the name of the track with a larger-thanusual font, and a pure-white font color:
5.
Follow the track name TextView with another right-aligned TextView object to be used to display the duration of the track:
6.
End the TableRow element with an ImageButton element, which can be used by the user to sample the track before buying it: [ 218 ]
Chapter 8
What just happened
The above layout resource file will handle the layout of the track list items for the second part of the user interface. We need to be able to create several of these structures to handle all of the tracks available in an album. We wrap them in a TableRow element which when placed in a TableLayout object, will automatically align each of its sub-elements with those in the other rows. Later, in the Java code we'll use the LayoutInflator to load this resource, populate it with the name and duration of a track, and then add it to a TableLayout object that we will declare as part of the main user interface. Once this new item has been populated with some data, it'll look something like the following screenshot:
Time for action – developing the main user interface layout
Having built the layout resource file that will become track items in a list later, we now need to define the remaining elements of this user interface. While this structure is relatively simple, it's also very easily extended and has a few minor details that keep it looking really good. It also needs some Java code in order to be correctly populated, but we'll get into that after we've finished with the resource file.
1. 2.
Create or open the res/layout/main.xml file in the new project. The root element of the main layout needs to be a ScrollView in order to handle the possibility that the interface runs longer than the available screen space. The ScrollView should take up all available screen space:
3.
As the only element to the ScrollView, declare a RelativeLayout which consumes the available width, but only the required height. The RelativeLayout should include some padding at the top and bottom to provide a little "breathing room" so that its contents don't look to cramped:
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Designing Content-centric Activities
4.
The first element of the RelativeLayout is the album art, a fixed size ImageView object that will fit the album cover art in the available space:
5.
The second element after the album art is the artist's logo image, also an ImageView. This element is required to center the logo in the available space:
6.
After the artist's logo, we need a plain TextView object with some font styling to hold the name of the album we're trying to sell. We'll place this below the artist's logo in the user interface as per the image we saw earlier:
7.
Below the TextView with the album name, we have a small non-styled TextView to hold the name of the record label under which the album is released: [ 220 ]
Chapter 8
8.
As promised, we follow these elements with a TableLayout which will hold the available track information. We layout the TableLayout element against the album art rather than the record label TextView:
9.
Underneath the track list, we start by placing the Buy Selected Tracks button element on the left side of the screen:
10.
Finally, on the right side of the screen, we add our custom AmountBox widget where we will tell the user how much they will be paying:
