Firebase / Android - Wait for all the thread for a Handle

I have the following 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 fixed I have two Handler, at the beginning I call a list of "rooms" from Firebase, and then for each one I will check the detail.

The problem I have is that the answer gives it to me empty, since it does not wait for all the for for the detail of the "rooms" to be executed, for which the variable roomList always returns empty.

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

Thank you very much! Greetings.

============== Edito ============

I added an accountant to know when to make the callback.

public synchronized void next(String key,final RoomListQueryResultHandler handler) {

        this.setLoading(true);
        roomList = new ArrayList<Room>();
        this.database.child("members").child(this.mUser.getUid()).child("rooms")
                .orderByKey()
                .addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        RoomListQuery.this.setLoading(false);
                        if (!dataSnapshot.hasChildren()) {
                            RoomListQuery.this.currentPage--;
                        }

                        // at the start we need to still load all children
                        final long[] pendingLoadCount = { dataSnapshot.getChildrenCount() };

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

                                    // we loaded a child, check if we're done
                                    pendingLoadCount[0] = pendingLoadCount[0] - 1;
                                    if (pendingLoadCount[0] == 0) {
                                        if (handler != null) {
                                            Friends.runOnUIThread(new Runnable() {
                                                public void run() {
                                                    handler.onResult(roomList, (customException) null);
                                                }
                                            });
                                        }
                                    }


                                }
                            });

                        }

                    }

                    @Override
                    public void onCancelled(final DatabaseError databaseError) {
                        if (handler != null) {
                            Friends.runOnUIThread(new Runnable() {
                                public void run() {
                                    handler.onResult((List) null, new customException(databaseError.toString()));
                                }
                            });
                        }
                    }
                });
    }