Wait for an async callback in a periodic task?

Question

I'd like to check the user's location every 4 hours or so, and I don't want to leave my app running to do this. It looks like using a GcmTaskService with a PeriodicTask will let my service get called (WakefulBroadcastReceiver has restrictions against starting tasks when the app is stopped in Android 6+), and it will be compatible back to Android 4.4 (unlike JobScheduler) - my lowest supported Android version.

The issue is that GcmTaskService's onRunTask method is synchronous, but I want to use that to ask for a location and process the result (LocationManager will call my LocationListener implementation asynchronously).

How should this be handled?

Answer 1

Use a simple wait/notify:

private interface RunnableLocationListener extends Runnable, LocationListener {}

@Override
public int onRunTask (TaskParams params) {
    final Object monitor = new Object();
    final AtomicBoolean located = new AtomicBoolean();
    new Thread(new RunnableLocationListener() {
        Context context = PeriodicCollector.this;
        public void run() {
            Criteria criteria = new Criteria();
            criteria.setHorizontalAccuracy(Criteria.ACCURACY_HIGH);
            LocationManager locationManager = locationManager = (LocationManager)PeriodicCollector.this.getSystemService(Context.LOCATION_SERVICE);
            locationManager.requestSingleUpdate(criteria, this, Looper.getMainLooper());
        }

        @Override
        public void onLocationChanged(Location location) {
            // handle location here

            synchronized (monitor) {
                located.set(true);
                monitor.notifyAll();
            }
        }

        @Override public void onProviderDisabled(String s) {}
        @Override public void onProviderEnabled(String s) {}
        @Override public void onStatusChanged(String s, int i, Bundle b) {}
    }).start();

    int status = GcmNetworkManager.RESULT_FAILURE;
    try {
        synchronized (monitor) {
            if (!located.get()) {
                monitor.wait();
            }
        }
    } catch (InterruptedException e) {
        status = GcmNetworkManager.RESULT_SUCCESS;
    }
    return status;
}