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Minggu, 25 Desember 2011

Android 4.0 Development Tutorial


Write : Lars Vogel

 

1. What is Android?

1.1. Android Operation System

Android is an operating system based on Linux with a Java programming interface. It provides tools, e.g. a compiler, debugger and a device emulator as well as its own Java Virtual machine (Dalvik Virtual Machine - DVM).
Android is officially guided by the Open Handset Alliance but in reality Google leads the project.
Android supports 2-D and 3-D graphics using the OpenGL libraries and supports data storage in a SQLite database.
Every Android applications runs in its own process and under its own user id which is generated automatically by the Android system during deployment. Therefore the application is isolated from other running applications and a misbehaving application cannot easily harm other Android applications.

1.2. Important Android components

An Android application consists out of the following parts:
  • Activity - represents the presentation layer of an Android application, e.g. a screen which the user sees. An Android application can have several activities and it can be switched between them during runtime of the application.
  • Views - the User interface of an Activities is built with widget classes which inherit from android.view.View. The layout of the views is managed by android.view.ViewGroups. Views often have attributes which can be used to change their appearance and behavior.
  • Services - perform background tasks without providing an UI. They can notify the user via the notification framework in Android.
  • ContentProvider - provides data to applications, via a content provider your application can share data with other applications. Android contains a SQLite DB which can serve as data provider
  • Intents - are asynchronous messages which allow the application to request functionality from other services or activities. An application can call directly a service or activity (explicit intent) or ask the Android system for registered services and applications for an intent (implicit intents). For example the application could ask via an intent for a contact application. Applications register themselves to an intent via an IntentFilter. Intents are a powerful concept as they allow the creation of loosely coupled applications.
  • BroadcastReceiver - receives system messages and implicit intents, can be used to react to changed conditions in the system. An application can register as a BroadcastReceiver for certain events and can be started if such an event occurs.
  • Widgets - interactive components primary used on the Android homescreen to display certain data and to allow the user to have quick access the the information

Other Android components are Live Folders and Android Live Wallpapers. Live Folders display data on the homescreen without launching the corresponding application.

1.3. Dalvik Virtual Machine

Android uses a special virtual machine, e.g. the Dalvik Virtual Machine. Dalvik uses special bytecode. Therefore you cannot run standard Java bytecode on Android. Android provides a tool dx which allows to convert Java Class files into dex (Dalvik Executable) files. Android applications are packed into an .apk (Android Package) file by the program aapt (Android Asset Packaging Tool) To simplify development Google provides the Android Development Tools (ADT) for Eclipse. The ADT performs automatically the conversion from class to dex files and creates the apk during deployment.

1.4. Security and permissions

Android defines certain permissions for certain tasks. For example if the application wants to access the Internet it must define in its configuration file that it would like to use the related permission. During the installation of an Android application the user receives a screen in which he needs to confirm the required permissions of the application.

2. Android Application Architecture

2.1. AndroidManifest.xml

An Android application is described in the file AndroidManifest.xml. This file must declare all Activities, Services, BroadcastReceivers and ContentProvider of the application. It must also contain the required permissions for the application. For example if the application requires network access it must be specified here. AndroidManifest.xml can be thought as the deployment descriptor for an Android application.

    


    
        
            
                
                
            
        

    
    

 
   

The package attribute defines the base package for the following Java elements. It also must be unique as the Android Marketplace only allows application for a specific package once. Therefore a good habit is to use your reverse domain name as a package to avoid collisions with other developers.
android:versionName and android:versionCode specify the version of your application. versionName is what the user sees and can be any string. versionCode must be an integer and the Android Market uses this to determine if you provided a newer version to trigger the update on devices which have your application installed. You typically start with "1" and increase this value by one if you roll-out a new version of your application.
The tag defines an Activity, in this example pointing to the class "de.vogella.android.temperature.Convert". An intent filter is registered for this class which defines that this Activity is started once the application starts (action android:name="android.intent.action.MAIN" ). The category definition category android:name="android.intent.category.LAUNCHER" defines that this application is added to the application directory on the Android device. The @string/app_name value refer to resource files which contain the actual values. This makes it easy to provide different resources, e.g. strings, colors, icons, for different devices and makes it easy to translate applications.
The "uses-sdk" part of the "AndroidManifest.xml" defines the minimal SDK version your application is valid for. This will prevent your application being installed on devices with older SDK versions.

2.2. R.java, Resources and Assets

The directory gen in an Android project contains generated values. R.java is a generated class which contains references to resources of the res folder in the project. These resources are defined in the res directory and can be values, menus, layouts, icons or pictures or animations. For example a resource can be an image or an XML file which defines strings.
If you create a new resource, the corresponding reference is automatically created in R.java. The references are static int values, the Android system provides methods to access the corresponding resource. For example to access a String with the reference id R.string.yourString use the method getString(R.string.yourString));. R.java is automatically maintained by the Eclipse development environment, manual changes are not necessary.
While the directory res contains structured values which are known to the Android platform the directory assets can be used to store any kind of data. In Java you can access this data via the AssetsManager and the method getAssets().

2.3. Reference to resources in XML files

In your XML files, e.g. your layout files you can refer to other resources via the @ sign. For example if you want to refer to a color you defined as resources you can refer to it via @color/your_id or if you have defined a "hello" string as resource you can access it via @string/hello .

2.4. Activities and Layouts

The user interface for Activities is defined via layouts. At runtime, layouts are instances of android.view.ViewGroups . The layout defines the UI elements, their properties and their arrangement.
UI elements are based on the class android.view.View . ViewGroup is a subclass of the class View and a layout can contain UI components ( Views ) or other layouts ( ViewGroups ). You should not nestle ViewGroups too deeply as this has a negative impact on performance.
A layout can be defined via Java code or via XML. You typically uses Java code to generate the layout if you don't know the content until runtime; for example if your layout depends on content which you read from the Internet.
XML based layouts are defined via a resource file in the folder /res/layout . This file specifies the ViewGroups , Views , their relationship and their attributes for a specific layout. If a UI element needs to be accessed via Java code you have to give the UI element an unique id via the android:id attribute. To assign a new id to an UI element use @+id/yourvalue . By conversion this will create and assign a new id yourvalue to the corresponding UI element. In your Java code you can later access these UI elements via the method findViewById(R.id.yourvalue) .
Defining layouts via XML is usually the preferred way as this separates the programming logic from the layout definition. It also allows the definition of different layouts for different devices. You can also mix both approaches.

2.5. Activities and Lifecycle

The operating system controls the life cycle of your application. At any time the Android system may stop or destroy your application, e.g. because of an incoming call. The Android system defines a life cycle for activities via pre-defined methods. The most important methods are:
  • onSaveInstanceState() - called if the activity is stopped. Used to save data so that the activity can restore its states if re-started
  • onPause() - always called if the Activity ends, can be used to release resource or save data
  • onResume() - called if the Activity is re-started, can be used to initialize fields

The activity will also be restarted if a so called "configuration change" happens. A configuration change for example happens if the user changes the orientation of the device (vertical or horizontal). The activity is in this case restarted to enable the Android platform to load different resources for these configuration, e.g. layouts for vertical or horizontal mode. In the emulator you can simulate the change of the orientation via CNTR+F11.
You can avoid a restart of your application for certain configuration changes via the configChanges attribute on your activity definition in your AndroidManifest.xml. The following activity will not be restarted in case of orientation changes or position of the physical keyboard (hidden / visible).

    


   

2.6. Context

The class android.content.Context provides the connections to the Android system. It is the interface to global information about the application environment. Context also provides access to Android Services, e.g. theLocation Service. As Activities and Services extend the class Context you can directly access the context via this.

3. Installation

The following assume that you have already Eclipse installed. For details please see Eclipse Tutorial .

3.1. Eclipse and automatic Android SDK

Use the Eclipse update manager to install all available components for the Android Development Tools (ADT) from the URL https://dl-ssl.google.com/android/eclipse/. If you are not familiar with the Eclipse update manager the usage is described in Eclipse update manager.
After the new Android development components are installed you will be prompted to install the Android SDK. You can do follow the following wizard or go to the next section to learn how to do it manually.

Wizard to install Android SDK - Part 1


Wizard to install Android SDK - Part 2


Wizard to install Android SDK - Part 3

3.2. Manually install Android SDK

The previous step downloads the Android SDK automatically for you. You can also download the Android SDK manuallz from the Android homepage under Android SDK download. The download contains a zip file which you can extract to any place in your file system, e.g. I placed it under "c:\android-sdk-windows". Avoid using spaces in the path name otherwise you may experience problems later.
You also have to define the location of the Android SDK in the Eclipse Preferences. In Eclipse open the Preferences dialog via WindowsPreferences. Select Android and enter the installation path of the Android SDK.

Setting up the Android SDK in the Eclipse Preferences

3.3. Install a specific Android version

The Android SDK Manager allows you to install specific versions of Android. Select WindowAndroid SDK Manager from the Eclipse menu.

Starting ADV Manager

The dialog allows you to install new package and also allow you to delete them. Select "Available packages" and open the "Third Party Add-ons". Select the Google API 14 (Android 4.0) version of the SDK and press "Install".

Install Android API

Press the "Install" button and confirm the license for all package. After the installation restart Eclipse.

3.4. Android Source Code

The following step is optional.
During Android development it is very useful to have the Android source code available as Android uses a lot of defaults.
Haris Peco maintains plugins which provides access to the Android Source code code. Use the Eclipse update manager to install the Android Source plugin from the following update site: "http://adt-addons.googlecode.com/svn/trunk/source/com.android.ide.eclipse.source.update".
More details can be found on the project website.

4. Using the Emulator

4.1. Create an Android Emulator Device

The Android tools include an emulator. This emulator behaves like a real Android device in most cases and allows you to test your application without having a real device. You can emulate one or several devices with different configurations. Each configuration is defined via an "Android Virtual Device" (AVD).
To define an AVD open the "AVD Manager" via WindowsAVD Manager and press "New".

Create a new AVD

Enter the following.

Settings for a new AVD

We can also select the box "Enabled" for Snapshots. This will make the second start of the virtual device much faster.
At the end press the button "Create AVD".This will create the device and display it under the "Virtual devices". To test if your setup is correct, select your device and press "Start".
After (a long time) your device should be started.

4.2. Emulator Shortcuts

Obviously you can use the emulator via the keyboard on the right side of the emulator. But there are also some nice shortcuts which are useful.
Alt+Enter maximizes the emulator. Nice for demos.
Ctrl+F11 changes the orientation of the emulator.
F8 turns network on / off.

4.3. Performance

Try to use a smaller resolution for your emulator as for example HVGA. The emulator gets slower the more pixels its needs to render as it is using software rendering.
Also if you have sufficient memory on your computer, add at least 1 GB of memory to your emulator. This is the value "Device ram size" during the creation of the AVD.
Also set the flag "Enabled" for Snapshots. This will save the state of the emulator and let it start much faster.

5. Error handling and typical problems

Things are not always working as they should. This section gives an overview over typical problems and how to solve them.

5.1. Clean Project

Several users report that get the following errors:
  1. Project ... is missing required source folder: 'gen'
  2. The project could not be built until build path errors are resolved.
  3. Unable to open class file R.java.

To solve any of these errors, go to the project menu and select Project -> Clean.

5.2. Problems with Android Debug Bridge (adb)

The communication with the emulator or your Android device might have problems. This communication is handle by the Android Debug Bridge (adb).
Eclipse allows to reset the adb in case this causes problems. Select therefore the DDMS perspective via WindowOpen PerspectiveOtherDDMS
To restart the adb, select the "Reset adb" in the Device View.

5.3. LogCat

The LogCat view shows you the log message of your Android device and help you analyzing problems. For example Java exceptions in your program would be shown here. To open this view, select "Window -> Show View -> Other -> Android -> LogCat" from the menu.

5.4. Emulator does not start

If your emulator does not start, make sure that the androd-sdk version is in a path without any spaces in the path name.

5.5. Error message for @override

The @override annotation was introduced in Java 1.6. If you receive an error message for @override change the Java compiler level to Java 1.6 via right-mouse click on the project -> Properties -> Java Compiler -> Compiler compliance level and set it to "1.6".

5.6. Missing Imports

Java requires that the classes which are not part of the standard Java Language are either fully qualified or declared via imports. In your editor use the click mouse click, select "Source-> Organize Imports" if you see error message with "XX cannot be resolved to a variable".

5.7. Eclipse Tips

To work more efficient with Eclipse, select Window -> Preferences -> Java -> Editor -> Save Actions and select that the source code should be formated and that the imports should be organized at every save.

6. Your first Android project

6.1. Create Project

This app is also available on the Android Marketplace. Search for "vogella" for find this example.
Select FileNewOtherAndroidAndroid Project and create the Android project "de.vogella.android.temperature". Enter the following.

New Android Project Wizard


New Android Project Wizard - Android Target


New Android Project Wizard - Package Definition

Press "Finish". This should create the following directory structure.

Android Project Structure

While "res" contains structured values which are known to the Android platform the directory "assets" can be used to store any kind of data. In Java you can access this data via the AssetsManager and the method getAssets().

6.2. Two faces of things

The Android SDK allows to define certain artifacts, e.g. strings and UI's, in two ways, via a rich editor and directly via XML. The following description tries to use the rich UI but for validation lists also the XML. You can switch between both things by clicking on the tab on the lower part of the screen. For example in the Package Explorer select "res/layout/main.xml".

ADT Resource Editor

6.3. Create attributes

Android allows you to create attributes for resources, e.g. for strings and / or colors. These attributes can be used in your UI definition via XML or in your Java source code.
Select the file "res/values/string.xml" and press "Add". Select "Color" and enter "myColor" as the name and "#3399CC" as the value.

Adding Android Attributes


Details for a String

Add also the following "String" attributes. String attributes allow to translate the application at a later point.

Table 1. String Attributes
NameValue
celsiusto Celsius
fahrenheitto Fahrenheit
calcCalculate


Switch to the XML representation and validate the values.

    


 Hello World, Convert!
 Temperature Converter
 #3399CC
 myClickHandler
 to Celsius
 to Fahrenheit
 Calculate

   

6.4. Add UI Elements

Select "res/layout/main.xml" and open the Android editor via a double-click. This editor allows you to create the UI via drag and drop or via the XML source code. You can switch between both representations via the tabs at the bottom of the editor. For changing the position and grouping elements you can use the outline view.
The following shows a screenshot of the Palette view from which you can drag and drop new UI elements into your layout. Please note that the "Palette" view changes frequently so your view might be a bit different.

Palette for the Android Layout Editor

Right-click on the text object “Hello World, Hello!” in the layout. Select Delete on the popup menu to remove the text object. Then, from the “Palette” view, select Text Fields and locate “Plain Text”. Drag this onto the layout to create a text input field. All object types in the section "Text Fields” derive from the class "EditText", they just specify via an additional attribute which text type can be used.
Now select the Palette section “Form Widgets” and drag a “RadioGroup” object onto the layout. The number of radio buttons added to the radio button group depends on your version of Eclipse. Make sure there are two radio buttons by deleting or adding radio buttons to the group.
From the Palette section Form Widgets, drag a Button object onto the layout.
The result should look like the following.

Current layout of main.xml

Switch to "main.xml" and verify that your XML looks like the following.

    


 
 
  
  
 
 


   

6.5. Edit UI properties

If you select a UI element you can change its properties via the properties view. Most of the properties can be changed via the right mouse menu. You can also edit properties of fields directy in XML. Typically you change properties directly in the XML file as this is much faster. But the right mouse functionality is nice if you are searching for a certain property.
Open your file "main.xml" We will delete the initial text for the EditText field in XML. Switch to the XML tab called "main.xml" and delete the android:text="EditText" property from the EditText part. Switch back to the "Graphical Layout" tab and check that the text is removed.
Use the right mouse click on the first radio button to assign the "celsius" string attribute to its "text" property. Assign the and "fahrenheit" string attribute to the second radio button.

Change the text property of the radio button


Selection of the right text from the pre-defined string values

From now on I assume you are able to use the properties menu on the UI elements. You can either edit the XML file or modify the properties via right mouse click.
Set the property "Checked" to true for the first RadioButton. Assign "calc" to the text property of your button and assign "myClickHandler" to the "onClick" property. Set the "Input type" property to "numberSigned" and "numberDecimal" on your EditText.
All your other UI controls are contained in a LinearLayout. We want to assign a background color to this LinearLayout. Right-click on an empty space in Graphical Layout mode, then select Other PropertiesAll by NameBackground. Select “Color” and then “myColor” in the list.

New look of the layout after the changes

Switch to the "main.xml" tab and verify that the XML is correctly maintained.

    


 
 
  
  
 
 
 
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