How to have Android Service communicate with Activity

I'm writing my first Android application and trying to get my head around communication between services and activities. I have a Service that will run in the background and do some gps and time based logging. I will have an Activity that will be used to start and stop the Service.

So first, I need to be able to figure out if the Service is running when the Activity is started. There are some other questions here about that, so I think I can figure that out (but feel free to offer advice).

My real problem: if the Activity is running and the Service is started, I need a way for the Service to send messages to the Activity. Simple Strings and integers at this point - status messages mostly. The messages will not happen regularly, so I don't think polling the service is a good way to go if there is another way. I only want this communication when the Activity has been started by the user - I don't want to start the Activity from the Service. In other words, if you start the Activity and the Service is running, you will see some status messages in the Activity UI when something interesting happens. If you don't start the Activity, you will not see these messages (they're not that interesting).

It seems like I should be able to determine if the Service is running, and if so, add the Activity as a listener. Then remove the Activity as a listener when the Activity pauses or stops. Is that actually possible? The only way I can figure out to do it is to have the Activity implement Parcelable and build an AIDL file so I can pass it through the Service's remote interface. That seems like overkill though, and I have no idea how the Activity should implement writeToParcel() / readFromParcel().

Is there an easier or better way? Thanks for any help.

EDIT:

For anyone who's interested in this later on, there is sample code from Google for handling this via AIDL in the samples directory: /apis/app/RemoteService.java

The asker has probably long since moved past this, but in case someone else searches for this...

There's another way to handle this, which I think might be the simplest.

Add a BroadcastReceiver to your activity. Register it to receive some custom intent in onResume and unregister it in onPause. Then send out that intent from your service when you want to send out your status updates or what have you.

Make sure you wouldn't be unhappy if some other app listened for your Intent (could anyone do anything malicious?), but beyond that, you should be alright.

Code sample was requested:

In my service, I have this:

// Do stuff that alters the content of my local SQLite Database
sendBroadcast(new Intent(RefreshTask.REFRESH_DATA_INTENT));

(RefreshTask.REFRESH_DATA_INTENT is just a constant string.)

In my listening activity, I define my BroadcastReceiver:

private class DataUpdateReceiver extends BroadcastReceiver {
    @Override
	public void onReceive(Context context, Intent intent) {
	    if (intent.getAction().equals(RefreshTask.REFRESH_DATA_INTENT)) {
          // Do stuff - maybe update my view based on the changed DB contents
        }
    }
}

I declare my receiver at the top of the class:

private DataUpdateReceiver dataUpdateReceiver;

I override onResume to add this:

if (dataUpdateReceiver == null) dataUpdateReceiver = new DataUpdateReceiver();
IntentFilter intentFilter = new IntentFilter(RefreshTask.REFRESH_DATA_INTENT);
registerReceiver(dataUpdateReceiver, intentFilter);

And I override onPause to add:

if (dataUpdateReceiver != null) unregisterReceiver(dataUpdateReceiver);

Now my activity is listening for my service to say "Hey, go update yourself." I could pass data in the Intent instead of updating database tables and then going back to find the changes within my activity, but since I want the changes to persist anyway, it makes sense to pass the data via DB.

There are three obvious ways to communicate with services:

1. Using Intents
2. Using AIDL
3. Using the service object itself (as singleton)

In your case, I'd go with option 3. Make a static reference to the service it self and populate it in onCreate():

void onCreate(Intent i) {
  sInstance = this;
}

Make a static function MyService getInstance(), which returns the static sInstance.

Then in Activity.onCreate() you start the service, asynchronously wait until the service is actually started (you could have your service notify your app it's ready by sending an intent to the activity.) and get its instance. When you have the instance, register your service listener object to you service and you are set. NOTE: when editing Views inside the Activity you should modify them in the UI thread, the service will probably run its own Thread, so you need to call Activity.runOnUiThread().

The last thing you need to do is to remove the reference to you listener object in Activity.onPause(), otherwise an instance of your activity context will leak, not good.

NOTE: This method is only useful when your application/Activity/task is the only process that will access your service. If this is not the case you have to use option 1. or 2.

Use LocalBroadcastManager to register a receiver to listen for a broadcast sent from local service inside your app, reference goes here:

http://developer.android.com/reference/android/support/v4/content/LocalBroadcastManager.html

Using a Messenger is another simple way to communicate between a Service and an Activity.

In the Activity, create a Handler with a corresponding Messenger. This will handle messages from your Service.

class ResponseHandler extends Handler {
    @Override public void handleMessage(Message message) {
            Toast.makeText(this, "message from service",
                    Toast.LENGTH_SHORT).show();
    }
}
Messenger messenger = new Messenger(new ResponseHandler());

The Messenger can be passed to the service by attaching it to a Message:

Message message = Message.obtain(null, MyService.ADD_RESPONSE_HANDLER);
message.replyTo = messenger;
try {
    myService.send(message);
catch (RemoteException e) {
    e.printStackTrace();
}

A full example can be found in the API demos: MessengerService and MessengerServiceActivity. Refer to the full example for how MyService works.

I am surprised that no one has given reference to Otto event Bus library

http://square.github.io/otto/

I have been using this in my android apps and it works seamlessly.

You may also use LiveData that works like an EventBus.

class MyService : LifecycleService() {
    companion object {
        val BUS = MutableLiveData<Any>()
    }

    override fun onStartCommand(intent: Intent, flags: Int, startId: Int): Int {
        super.onStartCommand(intent, flags, startId)

        val testItem : Object

        // expose your data
        if (BUS.hasActiveObservers()) {
            BUS.postValue(testItem)
        }

        return START_NOT_STICKY
    }
}

Then add an observer from your Activity.

MyService.BUS.observe(this, Observer {
    it?.let {
        // Do what you need to do here
    }
})

You can read more from this blog.

The other method that's not mentioned in the other comments is to bind to the service from the activity using bindService() and get an instance of the service in the ServiceConnection callback. As described here http://developer.android.com/guide/components/bound-services.html

Binding is another way to communicate

> Create a callback

public interface MyCallBack{

   public void getResult(String result);

}

> Activity side:

1. Implement the interface in the Activity

2. Provide the implementation for the method

3. Bind the Activity to Service

4. Register and Unregister Callback when the Service gets bound and unbound with Activity.

   public class YourActivity extends AppCompatActivity implements MyCallBack{
   
         private Intent notifyMeIntent;
         private GPSService gpsService;
         private boolean bound = false;
   
         @Override
         public void onCreate(Bundle sis){
   
             // activity code ...
   
             startGPSService();
            
         }
   
         @Override
         public void getResult(String result){
          // show in textView textView.setText(result);
         }
   
         @Override
         protected void onStart()
         {
             super.onStart();
             bindService();
         }
   
         @Override
         protected void onStop() {
             super.onStop();
             unbindService();
         }
   
         private ServiceConnection serviceConnection = new ServiceConnection() {
   
               @Override
               public void onServiceConnected(ComponentName className, IBinder service) {
   
                     GPSService.GPSBinder binder = (GPSService.GPSBinder) service;
                     gpsService= binder.getService();
                     bound = true;
                     gpsService.registerCallBack(YourActivity.this); // register
                     
              }
   
              @Override
              public void onServiceDisconnected(ComponentName arg0) {
                     bound = false;
              }
         };
   
         private void bindService() {
              
              bindService(notifyMeIntent, serviceConnection, Context.BIND_AUTO_CREATE);
         }
   
         private void unbindService(){
              if (bound) {
                    gpsService.registerCallBack(null); // unregister            
                    unbindService(serviceConnection);
                    bound = false;
               }
         }
   
         // Call this method somewhere to start Your GPSService
         private void startGPSService(){
              notifyMeIntent = new Intent(this, GPSService.class);
              startService(myIntent );
         }
   
    }
   

> Service Side:

1. Initialize callback

2. Invoke the callback method whenever needed

    public class GPSService extends Service{
       
        private MyCallBack myCallback;
        private IBinder serviceBinder = new GPSBinder();
   
        public void registerCallBack(MyCallBack myCallback){
             this.myCallback= myCallback;
        }
   
        public class GPSBinder extends Binder{
   
            public GPSService getService(){
                 return GPSService.this;
            }
       }
   
       @Nullable
       @Override
       public IBinder onBind(Intent intent){
            return serviceBinder;
       }
    }
   

Another way could be using observers with a fake model class through the activity and the service itself, implementing an MVC pattern variation. I don't know if it's the best way to accomplish this, but it's the way that worked for me. If you need some example ask for it and i'll post something.

To follow up on @MrSnowflake answer with a code example. This is the XABBER now open source Application class. The Application class is centralising and coordinating Listeners and ManagerInterfaces and more. Managers of all sorts are dynamically loaded. Activity´s started in the Xabber will report in what type of Listener they are. And when a Service start it report in to the Application class as started. Now to send a message to an Activity all you have to do is make your Activity become a listener of what type you need. In the OnStart() OnPause() register/unreg. The Service can ask the Application class for just that listener it need to speak to and if it's there then the Activity is ready to receive.

Going through the Application class you'll see there's a loot more going on then this.

Besides LocalBroadcastManager , Event Bus and Messenger already answered in this question,we can use Pending Intent to communicate from service.

As mentioned here in my blog post

> Communication between service and Activity can be done using > PendingIntent.For that we can use > createPendingResult().createPendingResult() creates a new > PendingIntent object which you can hand to service to use and to send > result data back to your activity inside onActivityResult(int, int, > Intent) callback.Since a PendingIntent is Parcelable , and can > therefore be put into an Intent extra,your activity can pass this > PendingIntent to the service.The service, in turn, can call send() > method on the PendingIntent to notify the activity via > onActivityResult of an event. > > Activity > > public class PendingIntentActivity extends AppCompatActivity > { > @Override > protected void onCreate(@Nullable Bundle savedInstanceState) { > super.onCreate(savedInstanceState); > setContentView(R.layout.activity_main); >
> PendingIntent pendingResult = createPendingResult( > 100, new Intent(), 0); > Intent intent = new Intent(getApplicationContext(), PendingIntentService.class); > intent.putExtra("pendingIntent", pendingResult); > startService(intent); >
> } >
> @Override > protected void onActivityResult(int requestCode, int resultCode, Intent data) { > if (requestCode == 100 && resultCode==200) { > Toast.makeText(this,data.getStringExtra("name"),Toast.LENGTH_LONG).show(); > } > super.onActivityResult(requestCode, resultCode, data); > } > } > > Service > > public class PendingIntentService extends Service { >
> private static final String[] items= { "lorem", "ipsum", "dolor", > "sit", "amet", "consectetuer", "adipiscing", "elit", "morbi", > "vel", "ligula", "vitae", "arcu", "aliquet", "mollis", "etiam", > "vel", "erat", "placerat", "ante", "porttitor", "sodales", > "pellentesque", "augue", "purus" }; > private PendingIntent data; >
> @Override > public void onCreate() { > super.onCreate(); > } >
> @Override > public int onStartCommand(Intent intent, int flags, int startId) { >
> data = intent.getParcelableExtra("pendingIntent"); >
> new LoadWordsThread().start(); > return START_NOT_STICKY; > } >
> @Override > public IBinder onBind(Intent intent) { > return null; > } >
> @Override > public void onDestroy() { > super.onDestroy(); > } >
> class LoadWordsThread extends Thread { > @Override > public void run() { > for (String item : items) { > if (!isInterrupted()) { >
> Intent result = new Intent(); > result.putExtra("name", item); > try { > data.send(PendingIntentService.this,200,result); > } catch (PendingIntent.CanceledException e) { >
> e.printStackTrace(); > } > SystemClock.sleep(400); >
> } > } > } > } > }

My method:

Class to manage send and receive message from/to service/activity:

import android.os.Bundle;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.util.Log;

import java.util.ArrayList;
import java.util.List;

public class MessageManager {

    public interface IOnHandleMessage{
        // Messages
        int MSG_HANDSHAKE = 0x1;

        void onHandleMessage(Message msg);
    }

    private static final String LOGCAT = MessageManager.class.getSimpleName();

    private Messenger mMsgSender;
    private Messenger mMsgReceiver;
    private List<Message> mMessages;

    public MessageManager(IOnHandleMessage callback, IBinder target){
        mMsgReceiver = new Messenger(new MessageHandler(callback, MessageHandler.TYPE_ACTIVITY));
        mMsgSender = new Messenger(target);
        mMessages = new ArrayList<>();
    }

    public MessageManager(IOnHandleMessage callback){
        mMsgReceiver = new Messenger(new MessageHandler(callback, MessageHandler.TYPE_SERVICE));
        mMsgSender = null;
        mMessages = new ArrayList<>();
    }

    /* START Getter & Setter Methods */
    public Messenger getMsgSender() {
        return mMsgSender;
    }

    public void setMsgSender(Messenger sender) {
        this.mMsgSender = sender;
    }

    public Messenger getMsgReceiver() {
        return mMsgReceiver;
    }

    public void setMsgReceiver(Messenger receiver) {
        this.mMsgReceiver = receiver;
    }

    public List<Message> getLastMessages() {
        return mMessages;
    }

    public void addMessage(Message message) {
        this.mMessages.add(message);
    }
    /* END Getter & Setter Methods */

    /* START Public Methods */
    public void sendMessage(int what, int arg1, int arg2, Bundle msgData){
        if(mMsgSender != null && mMsgReceiver != null) {
            try {
                Message msg = Message.obtain(null, what, arg1, arg2);
                msg.replyTo = mMsgReceiver;
                if(msgData != null){
                    msg.setData(msgData);
                }
                mMsgSender.send(msg);
            } catch (RemoteException rE) {
                onException(rE);
            }
        }
    }

    public void sendHandshake(){
        if(mMsgSender != null && mMsgReceiver != null){
            sendMessage(IOnHandleMessage.MSG_HANDSHAKE, 0, 0, null);
        }
    }
    /* END Public Methods */

    /* START Private Methods */
    private void onException(Exception e){
        Log.e(LOGCAT, e.getMessage());
        e.printStackTrace();
    }
    /* END Private Methods */

    /** START Private Classes **/
    private class MessageHandler extends Handler {

        // Types
        final static int TYPE_SERVICE = 0x1;
        final static int TYPE_ACTIVITY = 0x2;

        private IOnHandleMessage mCallback;
        private int mType;

        public MessageHandler(IOnHandleMessage callback, int type){
            mCallback = callback;
            mType = type;
        }

        @Override
        public void handleMessage(Message msg){
            addMessage(msg);
            switch(msg.what){
                case IOnHandleMessage.MSG_HANDSHAKE:
                    switch(mType){
                        case TYPE_SERVICE:
                            setMsgSender(msg.replyTo);
                            sendHandshake();
                            break;
                        case TYPE_ACTIVITY:
                            Log.v(LOGCAT, "HERE");
                            break;
                    }
                    break;
                default:
                    if(mCallback != null){
                        mCallback.onHandleMessage(msg);
                    }
                    break;
            }
        }

    }
    /** END Private Classes **/

}

In Activity Example:

public class activity extends AppCompatActivity
      implements     ServiceConnection,
                     MessageManager.IOnHandleMessage { 

    [....]

    private MessageManager mMessenger;

    private void initMyMessenger(IBinder iBinder){
        mMessenger = new MessageManager(this, iBinder);
        mMessenger.sendHandshake();
    }

    private void bindToService(){
        Intent intent = new Intent(this, TagScanService.class);
        bindService(intent, mServiceConnection, Context.BIND_AUTO_CREATE);
        /* START THE SERVICE IF NEEDED */
    }

    private void unbindToService(){
    /* UNBIND when you want (onDestroy, after operation...)
        if(mBound) {
            unbindService(mServiceConnection);
            mBound = false;
        }
    }

    /* START Override MessageManager.IOnHandleMessage Methods */
    @Override
    public void onHandleMessage(Message msg) {
        switch(msg.what){
            case Constants.MSG_SYNC_PROGRESS:
                Bundle data = msg.getData();
                String text = data.getString(Constants.KEY_MSG_TEXT);
                setMessageProgress(text);
                break;
            case Constants.MSG_START_SYNC:
                onStartSync();
                break;
            case Constants.MSG_END_SYNC:
                onEndSync(msg.arg1 == Constants.ARG1_SUCCESS);
                mBound = false;
                break;
        }
    }
    /* END Override MessageManager.IOnHandleMessage Methods */

    /** START Override ServiceConnection Methods **/
    private class BLEScanServiceConnection implements ServiceConnection {

        @Override
        public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
            initMyMessenger(iBinder);
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {
            mMessenger = null;
            mBound = false;
        }
    }
    /** END Override ServiceConnection Methods **/

In Service Example:

public class Blablabla extends Service
    implements     MessageManager.IOnHandleMessage {

    [...]

    private MessageManager mMessenger;

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        super.onBind(intent);
        initMessageManager();
        return mMessenger.getMsgReceiver().getBinder();
    }

    private void initMessageManager(){
        mMessenger = new MessageManager(this);
    }

    /* START Override IOnHandleMessage Methods */
    @Override
    public void onHandleMessage(Message msg) {
    /* Do what you want when u get a message looking the "what" attribute */
    }
    /* END Override IOnHandleMessage Methods */

Send a message from Activity / Service:

mMessenger.sendMessage(what, arg1, arg2, dataBundle);

How this works:

on the activity you start or bind the service. The service "OnBind" methods return the Binder to his MessageManager, the in the Activity through the "Service Connection" interface methods implementation "OnServiceConnected" you get this IBinder and init you MessageManager using it. After the Activity has init his MessageManager the MessageHandler send and Handshake to the service so it can set his "MessageHandler" sender ( the "private Messenger mMsgSender;" in MessageManager ). Doing this the service know to who send his messages.

You can also implement this using a List/Queue of Messenger "sender" in the MessageManager so you can send multiple messages to different Activities/Services or you can use a List/Queue of Messenger "receiver" in the MessageManager so you can receive multiple message from different Activities/Services.

In the "MessageManager" instance you have a list of all messages received.

As you can see the connection between "Activity's Messenger" and "Service Messenger" using this "MessageManager" instance is automatic, it is done through the "OnServiceConnected" method and through the use of the "Handshake".

Hope this is helpful for you :) Thank you very much! Bye :D

As mentioned by Madhur, you can use a bus for communication.

In case of using a Bus you have some options:

Otto event Bus library (deprecated in favor of RxJava)

http://square.github.io/otto/

Green Robot’s EventBus

http://greenrobot.org/eventbus/

NYBus (RxBus, implemented using RxJava. very similar to the EventBus)

https://github.com/MindorksOpenSource/NYBus