All Activities and Services in an application run in a single process by
default. If needed, you can declare an android:process attribute
in your manifest file, to explicitly place a component (Activity/Service) in
another process.
By default, all of the application code in a single process runs in the main UI thread. This is the same thread that also handles UI events. The only exception is the code that handles IPC calls coming in from other processes. The system maintains a separate pool of transaction threads in each process to dispatch all incoming IPC calls. The developer should create separate threads for any long-running code, to avoid blocking the main UI thread.
It depends on the type of data that you want to share:
To share primitive data between Activities/Services in an application, use Intent.putExtras(). For passing primitive data that needs to persist use the Preferences storage mechanism.
For sharing complex non-persistent user-defined objects for short duration, the following approaches are recommended:
The android.app.Application is a base class for those who need to maintain global application state. It can be accessed via getApplication() from any Activity or Service. It has a couple of life-cycle methods and will be instantiated by Android automatically if your register it in AndroidManifest.xml.
An alternate way to make data accessible across Activities/Services is to use public static fields and/or methods. You can access these static fields from any other class in your application. To share an object, the activity which creates your object sets a static field to point to this object and any other activity that wants to use this object just accesses this static field.
You can also use a HashMap of WeakReferences to Objects with Long keys. When an activity wants to pass an object to another activity, it simply puts the object in the map and sends the key (which is a unique Long based on a counter or time stamp) to the recipient activity via intent extras. The recipient activity retrieves the object using this key.
There are advantages to using a static Singleton, such as you can refer to them without casting getApplication() to an application-specific class, or going to the trouble of hanging an interface on all your Application subclasses so that your various modules can refer to that interface instead.
But, the life cycle of a static is not well under your control; so to abide by the life-cycle model, the application class should initiate and tear down these static objects in the onCreate() and onTerminate() methods of the Application Class
Even while an application appears to continue running, the system may choose to kill its process and restart it later. If you have data that you need to persist from one activity invocation to the next, you need to represent that data as state that gets saved by an activity when it is informed that it might go away.
For sharing complex persistent user-defined objects, the following approaches are recommended:
If the shared data needs to be retained across points where the application process can be killed, then place that data in persistent storage like Application Preferences, SQLite DB, Files or ContentProviders. Please refer to the Data Storage for further details on how to use these components.
The general mechanism to start a new activity if its not running— or to bring the activity stack to the front if is already running in the background— is the to use the NEW_TASK_LAUNCH flag in the startActivity() call.
The remote service can define a callback interface and register it with the clients to callback into the clients. The RemoteCallbackList class provides methods to register and unregister clients with the service, and send and receive messages.
The sample code for remote service callbacks is given in ApiDemos/RemoteService
Please read the Designing for Responsiveness document.
Whenever a package is added, an intent with PACKAGE_ADDED action is broadcast by the system. Similarly when a package is removed, an intent with PACKAGE_REMOVED action is broadcast. To receive these intents, you should write something like this:
<receiver android:name ="com.android.samples.app.PackageReceiver">
<intent-filter>
<action android:name="android.intent.action.PACKAGE_ADDED"/>
<action android:name="android.intent.action.PACKAGE_REMOVED"/>
<data android:scheme="package" />
</intent-filter>
</receiver>