notes/docs/lectures/mdp/05_services.md

Services

Uses of services

• MP3 Playback
  • Playing songs in the background
• Network Access
  • Long downloads, sending emails, polling an email server for new mail, etc.

Services

An application component that

• Has no UI
• Represents a desire to perform a longer-running operation

Activities are loaded and unloaded as users move around the app

• Services remain for as long as they are needed
• One service may be used by many other applications to avoid duplication of resources.

What are Services not?

Not a separate process - runs in the same process as the application in which it is declared

Not a thread - there's one thread per application

Services are simple

• It's a way of telling the system about a part of your app that is expected to run for a long time
  • i.e. longer than a few seconds

Tasks, Services and Activities

Email

• Checks for new mail occasionally -> Service
• Collects new mail and stores it -> Service
• Notifies user that there is new mail -> Service / Activity
  • New component - notifications
• User switches to inbox and retrieves/views new mail -> Activity

MP3 Playback task

• Play music while user does something else -> Service
• Create a playlist -> Activity
• Change the song playing & display current song progress -> Service / Activity

Creating a Service

• Services are designed to support communication with
  • Local activities (in the same process)
    • within VM
  • Remote activities
    • IPC (Inter-process communication)
  • Multiple components
    • providing a service for multiple clients

Services are components, similar to an activity

• Register the service in the manifest
• Create a subclass of android.app.Service
• Handle the relevant life cycle methods

Service Life Cycle

• By nature, services are singleton objects
  • A service might be used by many clients
• A service is started by something
  • via an intent
  • The service subclass object is instantiated if necessary
    • onCreate() is called
    • either onStartCommand or onBind will be called depending on how the service has been invoked
• onCreate()/onStart()/onBind() are called in the context of the main UI thread
  • A single thread for all components in an application
  • Must spawn a worker thread
• Something calls stopService()
  • Could be the service, one of the clients (the activity) or the OS
• onDestroy()

Code Examples

MainActivity class

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle ...) {
        ...
    }
    
    //if clicked twice - onStartCommand is called, onCreate is called ONCE
    public void onClickStartService(View v){
        this.startService(new Intent(MainActivity.this, SimpleService.class));
    }
    
    public void onClickStopService(View v){
        this.stopService(new Intent(MainActivity.this, SimpleService.class));
    }
    
    public void onClickClobberService(View v) {
        android.os.Process.killProcess(android.os.Process.myPid());
    }
}

SimpleService class

public class SimpleService extends Service {
    //called first
    public void onCreate() { 
        super.onCreate();
    }
    
    //called second
    public int onStartCommand(Intent i, int flags, int startId) {
        return Service.START_REDELIVER_INTENT;
    }
}

Android Manifest

<service
    android:name=".SimpleService"
    android:exported="false">
</service>