Android Application Architecture
An Android application is described in the file
AndroidManifest.xml. This file must declare all
ContentProviderof 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.xmlcan be thought as the deployment descriptor for an Android application.
packageattribute 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:versionCodespecify the version of your application.
versionNameis what the user sees and can be any string.
versionCodemust 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.
Activity, in this example pointing to the class "com.convertor". An intent filter is registered for this class which defines that this
Activityis 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_namevalue 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 for which your application is valid. This will prevent your application being installed on devices with older SDK versions.
2. R.java, Resources and Assets
genin an Android project contains generated values.
R.javais a generated class which contains references to resources of the
resfolder in the project. These resources are defined in the
resdirectory 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.yourStringuse the method
getString(R.string.yourString));. R.java is automatically maintained by the Eclipse development environment, manual changes are not necessary.
While the directory
rescontains structured values which are known to the Android platform the directory
assetscan be used to store any kind of data. In Java you can access this data via the
AssetsManagerand the method
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_idor if you have defined a "hello" string as resource you can access it via
4. Activities and Layouts
The user interface for
Activitiesis 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
ViewGroupis a subclass of the class
Viewand a layout can contain UI components (
Views) or other layouts (
ViewGroups). You should not nestle
ViewGroupstoo 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
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:idattribute. To assign a new id to an UI element use
@+id/yourvalue. By conversion this will create and assign a new id
yourvalueto the corresponding UI element. In your Java code you can later access these UI elements via the method
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.
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.
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).
android.content.Contextprovides 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
Contextyou can directly access the context via