Firebase query inside Handler

Issue

I am trying to wrap a Service around a (Firebase query)x(Perform some action) functionality. Namely, I would need to retrieve data from Firebase and then, for each item retrieved, perform a certain download-a-file-like action (which is itself can be an AsyncTask or a Service). I have a handler inside my service:

        @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        FirebaseUser user= FirebaseAuth.getInstance().getCurrentUser();
        DatabaseReference ref= FirebaseDatabase.getInstance().getReference().child("users").child(user.getUid()).child("oj_goals");
        ref.orderByChild("ojTag").addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                OjGoal goal= dataSnapshot.getValue(OjGoal.class);
                Log.d("inside onChildAdded",goal.getMe());
                data.add(goal);
            }
            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
            }
            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
            }
            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {
            }
            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        });
        //FIXME: the below code continues to execute even if the Firebase-call has not yet completed
        //
        Intent intent= new Intent("com.example.sj.keymeasures.BAR_CHART_DATA_PREPARATION_FINISHED");
        /**
         * https://stackoverflow.com/questions/43800772/does-variable-order-matter-while-parcel-read-write-operation-in-parcelable
         */
        intent.putParcelableArrayListExtra("oj_goals",data);
        Log.d("[Handler]","sendingBroadcast with size "+data.size());
        sendBroadcast(intent);
        stopSelf(msg.arg1);
    }

Now as you can see in the FIXME in the code, what I receive on the other end -- inside the dedicated receiver -- is an empty list. By now I know this whole thing was a bad idea, but how would you design what I am trying to accomplish (see first paragraph), without weird stuff like nested async-tasks?

Solution

It's pretty straightforward: the code that needs to execute after the child node was loaded, should be inside the onChildAdded function:

public void handleMessage(Message msg) {
    super.handleMessage(msg);
    FirebaseUser user= FirebaseAuth.getInstance().getCurrentUser();
    DatabaseReference ref= FirebaseDatabase.getInstance().getReference().child("users").child(user.getUid()).child("oj_goals");
    ref.orderByChild("ojTag").addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            OjGoal goal= dataSnapshot.getValue(OjGoal.class);
            Log.d("inside onChildAdded",goal.getMe());
            data.add(goal);
            Intent intent= new Intent("com.example.sj.keymeasures.BAR_CHART_DATA_PREPARATION_FINISHED");
            intent.putParcelableArrayListExtra("oj_goals",data);
            Log.d("[Handler]","sendingBroadcast with size "+data.size());
            sendBroadcast(intent);
            stopSelf(msg.arg1);
        }
        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {
        }
        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
            throw databaseError.toException(); // don't ignore errors
        }
    });
}

Answered By - Frank van Puffelen