Android local binding

Question

Why do we use getService method here?

public class LocalBinder extends Binder {
    LocalService getService() {
        return LocalService.this;
    }
}

Answer 1

• The BoundService acts as a server in a client-server architecture.(You'd probably know this part).
• A server always exports some service to a client. That service may be a public method.(Like "public void reverse(String)").
• The getService() function is used to return an instance of LocalService so that Clients can access the public methods of that BoundService though that instance.

Below is a sample program from the official Android Documentation

public class LocalService extends Service {
    // Binder given to clients
    private final IBinder mBinder = new LocalBinder();
    // Random number generator
    private final Random mGenerator = new Random();

    /**
     * Class used for the client Binder.  Because we know this service always
     * runs in the same process as its clients, we don't need to deal with IPC.
     */
    public class LocalBinder extends Binder {
        LocalService getService() {
            // Return this instance of LocalService so clients can call public methods
            return LocalService.this;
        }
    }

    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }

    /** method for clients */
    public int getRandomNumber() {
      return mGenerator.nextInt(100);
    }
}

.

public class BindingActivity extends Activity {
    LocalService mService;
    boolean mBound = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }

    @Override
    protected void onStart() {
        super.onStart();
        // Bind to LocalService
        Intent intent = new Intent(this, LocalService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }

    /** Called when a button is clicked (the button in the layout file attaches to
      * this method with the android:onClick attribute) */
    public void onButtonClick(View v) {
        if (mBound) {
            // Call a method from the LocalService.
            // However, if this call were something that might hang, then this request should
            // occur in a separate thread to avoid slowing down the activity performance.
            int num = mService.getRandomNumber();
            Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show();
        }
    }

    /** Defines callbacks for service binding, passed to bindService() */
    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // We've bound to LocalService, cast the IBinder and get LocalService instance
            LocalBinder binder = (LocalBinder) service;
            mService = binder.getService();
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
}

Let's look more closer.

The server part.

public class LocalService is the service that exports(or gives) some functionality to client activities or services.The client imports(or uses) the funtcionality.

public int getRandomNumber() { return mGenerator.nextInt(100); }

Above code is the actual functionality or service exported by the server.

The client part.

LocalBinder binder = (LocalBinder) service; mService = binder.getService(); mBound = true;

Here the binder.getService() is used to get the running ( or current) instance (or object) of the LocalBinder class.

Then,

int num = mService.getRandomNumber(); Toast.makeText(this, "number: " +num,Toast.LENGTH_SHORT).show();

mService.getRandomNumber() is used to access the public method.