Firebase / Android - Wait for all thread finish for response

Question

I am have the follow code:

public synchronized void next(final RoomListQueryResultHandler handler) {
    this.setLoading(true);

    roomList = new ArrayList<Room>();
    this.database.child("members").child(this.mUser.getUid()).child("rooms")
            .limitToFirst(this.mLimit)
            .startAt(this.currentPage * this.mLimit)
            .addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    RoomListQuery.this.setLoading(false);
                    //mListAdapter.setLoading(false);

                    if (!dataSnapshot.hasChildren()) {
                        RoomListQuery.this.currentPage--;
                    }
                    for (DataSnapshot ds : dataSnapshot.getChildren()) {
                        Room room = ds.getValue(Room.class);
                        //roomList.add(Room.upsert(room));
                        Room.getRoom(room.getId(), new Room.RoomGetHandler() {
                            @Override
                            public void onResult(Room room, customException e) {
                                if (e != null) {
                                    // Error!
                                    e.printStackTrace();
                                    return;
                                }

                                roomList.add(room);
                            }
                        });
                        handler.onResult(roomList, (customException) null);
                    }

                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    handler.onResult((List) null, new customException(databaseError.toString()));
                }
            });
     }
}

If they are see, I have two Handlers, at first I call a list of "rooms" from Firebase, and then for each one I get the detail in other query.

The problem is that the response is a empty list, since the function not wait for all query details to be executed for the rooms, so the variable roomList always returns empty.

Any idea what I can implement, or what other methodology to use to solve it?

Thank you very much! Greetings.

Answer 1

Depending on how your application is structured, you might want to change the database design so that there is no need to perform an additional Firebase query for each room retrieved from the first query.

//mListAdapter.setLoading(false);

If you're creating a list view where each row is from the /members/<user_id>/rooms Firebase node, what are the minimum room attributes necessary to display that list? If it's just a few things like room name, photo url, owner, room_id, etc you might be better off duplicating those from the original source. Then clicking one of those rows can trigger the original additional Firebase query you had as part of Room.getRoom(room.getId(), new Room.RoomGetHandler() { ... });, to navigate to a new screen / display a modal with the full room details once retrieved.

Update

To address your comment about requiring the extra data, in that case, as part of the Room class I would include an extra boolean value _loadedDetails set initially to false. So that for rendering a room within the list, when _loadedDetails is currently false just display a loading spinner. That way you can still perform those additional queries and when completed, update the appropriate Room object within roomList based on the index. Something like this:

@Override
public void onDataChange(DataSnapshot dataSnapshot) {
    RoomListQuery.this.setLoading(false);
    //mListAdapter.setLoading(false);
    if (!dataSnapshot.hasChildren()) {
        RoomListQuery.this.currentPage--;
    }
    int i = 0;
    for (DataSnapshot ds : dataSnapshot.getChildren()) {
        Room room = ds.getValue(Room.class);
        roomList.add(room); // here instead
        updateRoom(room, i);
        i++;
    }
    handler.onResult(roomList, (customException) null);
}

...
// outside of the ValueEventListener
public void updateRoom(room, index) {
    Room.getRoom(room.getId(), new Room.RoomGetHandler() {
        @Override
        public void onResult(Room room, customException e) {
            if (e != null) {
                // Error!
                e.printStackTrace();
                return;
            }
            room._loadedDetails = true; // make that publicly accessible boolean, or include a setter method instead
            roomList.set(index, room);
        }
    });
}