Implementing LocationListener on a service thread

Question

I'm trying to get the location updates running in a background service. The service is running a workerthread of its own doing a lot of other stuff already, like socket communication. I'd like it to also handle location updates but this seems to only work on an activity. As far as I can read this is due to the message loop missing on a workerthread. I think I need to use Looper.prepare() somewhere, but maybe I need another thread just to handle locations requests? I can't seem to get the emulator to respond to any geo fix events, so I must be doing something wrong.

Below is the service code, stripped for all the non-relevant parts.

public class MyService extends Service implements LocationListener {
    private Thread runner;
    private volatile boolean keepRunning;
    private LocationManager locationManager = null;


    @Override
    public void onCreate() {
        keepRunning = true;

        runner = new Thread(null, new Runnable() {
            public void run() { workerLoop(); }
        });
        runner.start();
        startGps();
    }

    @Override
    public void onDestroy() {
        keepRunning = false;
        runner.interrupt();
    }

    private void workerLoop() { 
        //Looper.myLooper(); How does this work??
        //Looper.prepare();

        // Main worker loop for the service
        while (keepRunning) {
            try {
                if (commandQueue.notEmpty()) {
                    executeJob();
                } else {
                    Thread.sleep(1000);
                }
            } catch (Exception e) {
            }
        }   
        stopGps();
    }

    public void onLocationChanged(Location location) {
        // Doing something with the position...
    }       
    public void onProviderDisabled(String provider) {}
    public void onProviderEnabled(String provider) {}
    public void onStatusChanged(String provider, int status, Bundle extras) {}

    private void startGps() {
        if (locationManager == null)
            locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
        if (locationManager != null) {
            Criteria criteria = new Criteria();
            criteria.setAccuracy(Criteria.ACCURACY_FINE);
            criteria.setAltitudeRequired(false);
            criteria.setBearingRequired(true);
            criteria.setCostAllowed(false);
            criteria.setSpeedRequired(true);
            String provider = locationManager.getBestProvider(criteria, true);
            if (provider != null)
                locationManager.requestLocationUpdates(provider, 10, 5, (LocationListener) this);           
        }       
    }

    private void stopGps() {
        if (locationManager != null)
            locationManager.removeUpdates((LocationListener) this);
        locationManager = null;
    }

}

Answer 1

The LocationListener doesn't care whether it is in the context of an activity or a service. You want to use the location uodates in your workerLoop(), right? The location updates (the call to the LocationListener) and the workerLoop() are both acting independently from each other. To get the location update to the workerLoop() you need to join the two threads. One method to do this is to use the blackboard pattern: The LocationListener writes the new location to a field of your class (this is easy since both are in the context of the same class):

private Location blackboard = null;
public void onLocationChanged(final Location location) {
    if( location != null )
        this.blackboard = location;
}

private void workerLoop() {
    ...
    if( this.blackboard != null ) {
        final Location locationUpdate = this.blackboard;
        this.blackboard = null;
        // .. do something with the location
    }
    ...
}

With the above code you may get race conditions when the LocationListener writes to the blackboard while the workerLoop() is reading or erasing the location. This can be solved by surrounding the access to the blackboard by a synchronized block like this:

synchronized(this) {
        this.blackboard = location
}

and likewise in the workerLoop(), and we must declare the blackboard volatile:

private volatile Location blackboard = null;

Alternatively you may use a Lock, confer to the docs for more details: http://docs.oracle.com/javase/tutorial/essential/concurrency/newlocks.html