Ways to Communicate Between Activity and Service

by Ankit Agarwal

The main purpose of this post is to provide the idea of different android constructs which can be used for communication between activity and service. So to keep it small and simple I won’t be adding much code here but I will provide links to the code snippets for help.

You can create bound services which acts as a server in client-server interface. There are three ways to create a bound service:

• If application and service are running in same process which is mostly the case then create your own interface by extending Binder class and return an instance of it in onBind().

The following code snippet is taken from here. This implementation of Binder defines a function to return the instance of currently running service. This instance then can be used to call public functions of the service.

public class LocalService extends Service {
    
    ...    // This is the object that receives interactions from clients.
    private final IBinder mBinder = new LocalBinder();    public class LocalBinder extends Binder {
        LocalService getService() {
            return LocalService.this;
        }
    }
    @Override
    public void onCreate() {
        ...
    }
    @Override
    public int onStartCommand(Intent intent, int flags, int startId
    {
        ...
    }
    @Override
    public void onDestroy() {
        ...
    }
    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }
    ...

Now put the following code in activity to bind to the service and get the instance of the service.

private LocalService mBoundService;
private ServiceConnection mConnection = new ServiceConnection() {
    public void onServiceConnected(ComponentName className, IBinder service) {
        // This is called when the connection with the service has
        // been established, giving us the service object we can use
        // to interact with the service.  Because we have bound to a
        // explicit service that we know is running in our own
        // process, we can cast its IBinder to a concrete class and
        // directly access it.
        mBoundService = ((LocalService.LocalBinder)service).getService();
        // Tell the user about this for our demo.
        Toast.makeText(Binding.this,
                       R.string.local_service_connected,
                       Toast.LENGTH_SHORT).show();
    }
    public void onServiceDisconnected(ComponentName className) {
        // This is called when the connection with the service has
        // been unexpectedly disconnected -- that is, its process
        // crashed. Because it is running in our same process, we
        // should never see this happen.
        mBoundService = null;
        Toast.makeText(Binding.this,
                       R.string.local_service_disconnected,
                       Toast.LENGTH_SHORT).show();
    }
};
void doBindService() {
    // Establish a connection with the service.  We use an explicit
    // class name because we want a specific service implementation
    // that we know will be running in our own process (and thus
    // won't be supporting component replacement by other
    // applications).    bindService(new Intent(Binding.this, LocalService.class),
                mConnection,
                Context.BIND_AUTO_CREATE);
    mIsBound = true;
}
void doUnbindService() {
    if (mIsBound) {
        // Detach our existing connection.
        unbindService(mConnection);
        mIsBound = false;
    }
}
@Override
protected void onDestroy() {
    super.onDestroy();
    doUnbindService();
}

• If service is running in separate process then you can create an AIDL interface. Most applications should not use this as it may require a lot of multi-threading capabilities and may make code more complex.
• If you need an interface to communicate across processes you can create a Messenger. It handles communication on single thread. See Using Messenger. Client can also define a messenger and pass it to service using which service can send data back to client. Here is a sample demo.

Following is the code snippet taken from here.

public class MessengerService extends Service {
    /** Command to the service to display a message */
    static final int MSG_SAY_HELLO = 1;
    /**
     * Handler of incoming messages from clients.
     */
    class IncomingHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_SAY_HELLO:
                    Toast.makeText(getApplicationContext(),
                                   "hello!",
                                   Toast.LENGTH_SHORT).show();
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }
    /**
     * Target we publish for clients to send messages to
     * IncomingHandler.
     */
    final Messenger mMessenger = new Messenger(
                                     new IncomingHandler()
                                 );
    /**
     * When binding to the service, we return an interface to our
     * messenger for sending messages to the service.
     */
    @Override
    public IBinder onBind(Intent intent) {
        Toast.makeText(getApplicationContext(),
                       "binding",
                       Toast.LENGTH_SHORT).show();
        return mMessenger.getBinder();
    }
}

You can use Broadcast Receivers

1. If you want to send broadcasts to your application components only then use LocalBroadcastManager.
2. If you want to send broadcasts across applications then use BroadcastReceiver.

You can use Bus architecture

You can use EventBus from greenrobot or otto from square.

You can use Result Receiver

It can be used to send data across processes. Here is SO Question for implementation.

I have used only these so far. If there are some other better suggestions please comment.