IntentService : How to enqueue correctly?

Question

In my code i'm using an IntentService to listen to location updates (either GPS or network updates) and this IntentService is triggered when an event is received, so it is started with startService() from any activity.

public class AddLocationService extends IntentService implements LocationListener {
    /*My code here*/
}

    @Override
protected void onHandleIntent(Intent intent) {
    if(getOldLoc() == null) 
    { 
        //Get a new location
        this.locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, TIME_INTERVAL_GPS, 0, this);
        this.locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, TIME_INTERVAL_GPS, 0, this);
        Log.d(AddLocationService.TAG, "Network listener started");

        this.time_start_listening = System.currentTimeMillis();
        mTimerThread mTimerRunnable = new mTimerThread();
        this.timerThread = new Thread(mTimerRunnable);
        this.timerThread.start();
    }
    else    
        /*REUSE OLD LOCATION*/
}

Now my problem is : When two events start this IntentService and the second starts it while the first one is still requesting for updates, I will like the second one to wait until first one is fully finished (location found OR timer thread finishes). However whenever the IntentService is executed a second time (first instance still running), it prints me the log and does as it was executing in parallel.

However I thought that the main goal of IntentService was that it is something sequential so a second intent would have to wait until first one is done...

Did I missunderstood something ?

Answer 1

It appears that your onHandleIntent method is not blocking the thread it is executing on, so it will return quickly and allow the second intent to be processed. Not only that, but any callbacks from the LocationManager to that thread are unlikely to be processed as the background thread is likely to be killed when onHandleIntent is finished.

If you really want to use IntentService to manage your intent queue then you will need to do your location handling on its own thread, and join the IntentService thread to the location thread whilst it is waiting for the location callback.

Heres a bit of code that demonstrates the idea:

public class TestService extends IntentService {
    private static final String TAG = "TestService";

    private Location mLocation = null;

    public TestService() {
       super(TAG);
    }

    @Override
    public void onHandleIntent(Intent intent) {
        Log.d(TAG, "onHandleIntent");

        if (mLocation == null) {
            Log.d(TAG, "launching location thread");
            LocationManager locationManager = (LocationManager)getSystemService(Context.LOCATION_SERVICE);

            LocationThread thread = new LocationThread(locationManager);
            thread.start();
            try {
                thread.join(10000);
            } catch (InterruptedException e) {
                Log.d(TAG, "timeout");
                return;
            }

            Log.d(TAG, "join finished, loc="+mLocation.toString());
        } else {
             Log.d(TAG, "using existing loc="+mLocation.toString());
        }
    }

    private class LocationThread extends Thread implements LocationListener  {
        private LocationManager locationManager = null;

        public LocationThread(LocationManager locationManager) {
            super("UploaderService-Uploader");
            this.locationManager = locationManager;
        }

        @Override
        public void run() {
            Log.d(TAG, "Thread.run");
            Looper.prepare();
           this.locationManager.requestLocationUpdates(LocationManager.NETWORK_PROVIDER, 0, 0, this);
            this.locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, this);

            Looper.loop();
        }

        @Override
        public void onLocationChanged(Location location) {
            // TODO Auto-generated method stub
            Log.d(TAG, "onLocationChanged("+location.toString()+")");
            mLocation = location;
            Looper.myLooper().quit();
         }

         @Override
         public void onProviderDisabled(String arg0) {
         }

         @Override
         public void onProviderEnabled(String arg0) {
         }

         @Override
         public void onStatusChanged(String arg0, int arg1, Bundle arg2) {
         }

   }
}

Of interest in there is the Looper that runs a message loop on the thread (to allow handling of the callbacks).

Given the effort required to do this with IntentService it might be worthwhile investigating deriving from Service instead and managing your own intent queue.

Answer 2

onHandleIntent is in it's own thread already. You don't (shouldn't) create on in there. It's all handled by IntentService for you.

Answer 3

Thanks a million, that is exactly what I needed to handle the location requests. Thank you for explanations and making it clear for me, I wasn't very familiar with all the looper concept, now I understand it better !

In case someone need the same kind of thing, don't forget to stop the thread looper if your location thread is not stopping naturally (end of time on join(millis)), by adding this in onHandleIntent() :

                if(thread.isAlive())
                {
                    thread.onThreadStop();
                    try{
                        thread.interrupt();
                    }catch (Exception e) {
                        Log.d(TAG, "Exception on interrupt: " + e.getMessage());
                    }
                }   

after thread.join(yourTime), so for example if you didn't find any location update you still stop the thread after a certain time. And on method onThreadStop() :

                /*We remove location updates here and stop the looper*/
                public void onThreadStop()
                {
                    this.locationManager1.removeUpdates(this);
                    handleLocationChange(AddLocationService.this.currentBestLocation);
                    Looper.myLooper().quit();
                }

However I thought I saw my two intents being treated the first time I ran this code, but now only the first one is treated when I have multiple intents while still requesting location updates. My method onHandleIntent() seems to execute correctly, stops the thread after the time specified and even displays the very last Log (last statement of the method) but the second intent is not executed... Would you have any idea why ?