Question

I’ve created a simple Android application for testing how to use a handler or handlers to pass data from a background Service/thread to some other activity other than the MainActivity that created the background service. I’ve got the Service, thread, and a handler working in the MainActivity. The last step is to get a handler to pass data to some other activity other than the MainActivity. I can get the Service to pass messages to the MainActivity’s handler, but I don’t know how to get it to pass data to some other activity.

Why would anyone want to do this? I thought this compared to a simple MP3 player, but what it actually compares to is a nice FM radio. The MainActivity uses a background Service that allows me to select an FM station. When I launch the Play activity it should bind to the same background Service so I can continue to listen while it (the nice part) displays a graphic equalizer or animation of the the audio. Basically, I don’t know how to bind to the background Service from more than one activity.

My code was originally based on a Service example page 304 in Pro Android 2 and was helped tremendously by a CommonsWare Sample Application.

Please have a look at my current code. It consists of three carefully commented files that describe what I am trying to do and the difficulties I am having passing data to some other activity in addition to the MainActivity:

/**************************************************************************************************
 * File: MainActivity.java
 * Application: BackgroundService
 * Description: This file contains the main activity that is run when the BackgroundService
 *     application is launched.
 **************************************************************************************************/

package com.marie.mainactivity;

import com.marie.mainactivity.BackgroundService;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.os.Messenger;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

/*
 * Class: MainActivity
 * Purpose: Using a button, the MainActivity class starts the backgroundService and
 *     the RcvMessages activity. Using another button MainActivity stops the backgroundService.
 *     NOTE: RcvMessages is only a stub that does nothing but display a simple message.
 * Handler: MainActivity defines and provides a reference to "handler" for the backgroundService.
 */
public class MainActivity extends Activity {
    private static final String TAG = "MainActivity";

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

        Log.d(TAG, "starting service");

        /*
         * The bind button: bindBtn
         * Clicking this button starts the background Service and launches the
         * RcvMessages activity. NOTE: RcvMessages is only a stub so far.
         */
        Button bindBtn = (Button)findViewById(R.id.bindBtn);
        bindBtn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {

                // Start the background Service for sending canned messages to the handler as a test.
                Intent backgroundService = new Intent(MainActivity.this, com.marie.mainactivity.BackgroundService.class);
                backgroundService.putExtra(BackgroundService.EXTRA_MESSENGER, new Messenger(handler));
                startService(backgroundService);

                // Start the RcvMessages activity to receive messages from the handler. But how???
                Intent messages = new Intent(MainActivity.this, com.marie.mainactivity.RcvMessages.class);
                startActivity(messages);
            }
        });

        /*
         * The unbind button: unbindBtn
         * Clicking this button stops the background Service.
         */
        Button unbindBtn = (Button)findViewById(R.id.unbindBtn);
        unbindBtn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {

                // Stop the background Service
                Intent backgroundService = new Intent(MainActivity.this, BackgroundService.class);
                stopService(backgroundService);
            }
        });
    }

    /*
     * This is the handler to be passed to the background Service via a Messenger.
     * NOTE: I want it to send messages to my RcvMessages activity.
     */
    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            // simple handler test (does not send messages to RcvMessages activity
            String obj = (String) msg.obj;
            Log.i("handleMessge", "obj: " + obj);  
        }
    };
}

/**************************************************************************************************
 * File: BackgroundService.java
 * Application: BackgroundService
 * Description: This file contains the background Service that is launched by the MainActivity's
 *     bind button.
 **************************************************************************************************/

package com.marie.mainactivity;

import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Intent;
import android.os.Bundle;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;

/*
 * Class: BackgroundService
 * Purpose: Using the onStart() method the BackgroundService gets the reference to the
 *     Messenger instance that was passed to BackgroundService. The messenger is then
 *     used by the ServiceWorker() thread to send messages to the handler that is defined
 *     in the MainActivity class.
 */
public class BackgroundService extends Service {
    private NotificationManager notificationMgr;

    public static final String EXTRA_MESSENGER = "com.marie.mainactivity.EXTRA_MESSENGER";
    private Messenger messenger;

    @Override
    public void onCreate() {
        super.onCreate();

        notificationMgr = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);

        displayNotificationMessage("starting Background Service");

        Thread thr = new Thread(null, new ServiceWorker(), "BackgroundService");
        thr.start();
    }   

    /*
     * This is the ServiceWorker thread that passes messages to the handler defined in
     * the MainActivity class.
     * NOTE: Instead of passing messages to a handler in MainActivity I would like
     * it to pass messages to a handler defined in the RcvMessages activity.
     */
    class ServiceWorker implements Runnable
    {
        public void run() {
            // do background processing here... something simple

            // send a message to the handler defined in the MainActivity class
            try {
                Message msg1 = Message.obtain();
                msg1.obj = "Hello 1";
                messenger.send(msg1);
            } catch (RemoteException e) {
                e.printStackTrace();
            }

            // stop the service when done...
            // BackgroundService.this.stopSelf();
            // Or use the unbindBtn in the MainActivity class.
        }
    }

    @Override
    public void onDestroy()
    {
        displayNotificationMessage("stopping Background Service");
        super.onDestroy();
    }

    /*
     * onStart is where we get our reference the Messenger that allows
     * us to send messages to the handler defined in the MainActivity class.
     */
    @Override
    public void onStart(Intent intent, int startId) {
        super.onStart(intent, startId);

        Bundle extras = intent.getExtras();
        messenger = (Messenger)extras.get(EXTRA_MESSENGER);
    }

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

    private void displayNotificationMessage(String message)
    {
        Notification notification = new Notification(R.drawable.note, message, System.currentTimeMillis());

        PendingIntent contentIntent = PendingIntent.getActivity(this, 0, new Intent(this, MainActivity.class), 0);

        notification.setLatestEventInfo(this, "Background Service", message, contentIntent);

        notificationMgr.notify(R.id.app_notification_id, notification);
    }
}

/**************************************************************************************************
 * File: RcvMessages.java
 * Application: BackgroundService
 * Description: This file contains stub code that displays a test message in an EditText.
 **************************************************************************************************/

package com.marie.mainactivity;

import android.app.Activity;
import android.os.Bundle;
import android.text.InputType;
import android.widget.EditText;

/*
 * Class: RcvMessages
 * Purpose: RcvMessages is stub code that I want to extend in some way to receive messages from
 *     the background Service.
 *     NOTE: I don't know who to do this.
 */
public class RcvMessages extends Activity {

    EditText myText;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.messages);

        myText = (EditText)findViewById(R.id.my_text);

        myText.setSingleLine();
        myText.setInputType(InputType.TYPE_NULL);

        // Display a simple test message for now.
        myText.setText("RcvMessages here");
    }
}

Any help using a background Service, thread, and handler(s) to pass data to some other activity in addition to the MainActivity that created the backgraound Service would be greatly appreciated.

Answer 1

The Handler is associated with specific thread, so for as long as you create it in UI thread all you need to do is to share the handler. I would put your handler in my Application class. And you can put the whole Messenger there, if you want. Then you can access handler in any activity via ((MyApplication)getApplication()).getHandler(). When activity starts or pauses, you can register it as a callback.

So something like this

public class MyApplication extends Application {
   Handler h = new Handler() {
      public void handleMessage(Message m) {
        if (realCallback!=null) {
           realCallback.handleMessage(m);
        }
      }
   };
   Handler.Callback realCallback=null;
   ......
   public Handler getHandler() {
      return h;
   }

   public void setCallback(Handler.Callback c) {
      realCallback = c;
   }
}

In any activity that needs to do receive requests via messenger

public class MyActivity extends Activity implements Handler.Callback

......

public void onStart() {
   ((MyApplication)getApplication()).setCallback(this);
}

public void onPause() {
   ((MyApplication)getApplication()).setCallback(null);
}

public void handleMessage(Message msg) {
  //.... Do stuff ....
}

}

This is just an idea. You may need to tune it to your needs.

And don’t forget to set MyApplication name in AndroidManifest.xml