How Google calendar or Instagram make their apps all the time up and running even after force stop

Question

Looks like force stop should prevent app from running and it\'s even disable all app\'s alarms. However I found that notification in Google Calendar still shown fine even after

Answer 1

Based on my limited knowledge, you need to use AlarmManager, WakefulBroadcastReceiver and create a Service or IntentService for all these background task. Read more about it here. I have a service that I used for my Firebase messaging app and it works fine even after user killed the app. In my case, I connect to Firebase at fixed period using the following service.

First, I have the following class to set the AlarmManager.

public class FirebaseHandler {

    private Context context;
    private static final long FIREBASE_ALARM_CYCLE_TIME = 300000;

    // constructors
    public FirebaseHandler(Context context) {
        this.context = context;
    }

    // set alarm
    public void setAlarm() {
        // create pending intent
        Intent intent = new Intent(context, AlarmReceiver.class);
        intent.addFlags(Intent.FLAG_RECEIVER_FOREGROUND);
        final PendingIntent pendingIntent = PendingIntent.getBroadcast(
            context,
            AlarmReceiver.ALARM_REQUEST_CODE,
            intent,
            PendingIntent.FLAG_UPDATE_CURRENT);

        // create alarm
        AlarmManager alarm = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
        alarm.setInexactRepeating(
            AlarmManager.ELAPSED_REALTIME_WAKEUP,
            SystemClock.elapsedRealtime() + FIREBASE_ALARM_CYCLE_TIME,
            FIREBASE_ALARM_CYCLE_TIME,
            pendingIntent);
    }
}

I just initiate this by calling new FirebaseHandler(getApplicationContext()).setAlarm() somewhere in the activity. The AlarmReceiver class is as below.

public class AlarmReceiver extends WakefulBroadcastReceiver {

    public static final int ALARM_REQUEST_CODE = 12345;

    @Override
    public void onReceive(Context context, Intent wakefulIntent) {
        Intent intent = new Intent(context, FirebaseAlarmService.class);
        startWakefulService(context, intent);
    }
}

The FirebaseAlarmService class is as below.

public class FirebaseAlarmService extends Service {

    private static final String CLASSNAME = FirebaseAlarmService.class.getSimpleName();
    private HandlerThread handlerThread;

    // onStartCommand
    @Override
    public int onStartCommand(final Intent intent, int flags, final int startId) {
        // start a new thread
        // this depends on your need. I need to do a continuous operation for some time
        // you can use IntentService too
        handlerThread = new HandlerThread(CLASSNAME);
        handlerThread.start();
        Handler handler = new Handler(handlerThread.getLooper());
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
            // all your codes here for the background task...

            // remember to release the wake lock and stop service 
            AlarmReceiver.completeWakefulIntent(wakefulIntent);
            stopSelf();
        } 
        return START_NOT_STICKY;
    }

    // this is the part that does the trick
    @Override
    public void onTaskRemoved(Intent rootIntent) {
        super.onTaskRemoved(rootIntent);
        // when the service is killed, start it again
        new FirebaseHandler(getApplicationContext()).setAlarm();
    }

    // onDestroy
    @Override
    public void onDestroy() {
        super.onDestroy();
        // make sure to quit the thread
        handlerThread.quit();
    }
}

In summary, FirebaseHandler sets an AlarmManager, which will call WakefulBroadcastReceiver periodically, which will starts a Service periodically. When the service is killed, the service itself will start the AlarmManager again onTaskRemoved.

In the AndroidManifest, you'll need to add in the following permission for wake lock.

Also remember to add the receiver.

BONUS: You might want to start the service once the phone is rebooted or the app is updated via PlayStore. Create another WakefulBroadcastReceiver.

public class BootBroadcastReceiver extends WakefulBroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent wakefulIntent) {
        // check if user is login, then set the alarm
        if (isLogin) {
            new FirebaseHandler(context).setAlarm();
        }

        // remove wake lock
        WakefulBroadcastReceiver.completeWakefulIntent(wakefulIntent);
    }
}

In the AndroidManifest, add the required permission.

Add the intent filter.

TIPS: Sometimes you'll find that the above method will not work. This is mostly not your fault. It is the phone's security settings that caused this, especially Mi phones. User will need to "trust" the app and enable "auto-start" in the phone settings, then everything should work just fine.