Rxjava + Firebase

@Override
    public Observable<DataSnapshot> requestNotes() {
        return Observable.fromAsync(new Action1<AsyncEmitter<DataSnapshot>>() {
            @Override
            public void call(AsyncEmitter<DataSnapshot> listAsyncEmitter) {
                final ValueEventListener valueEventListener = ref.child(Constants.NOTES)
                        .addValueEventListener(
                        new ValueEventListener() {
                            @Override
                            public void onDataChange(DataSnapshot dataSnapshot) {
                                listAsyncEmitter.onNext(dataSnapshot);
                            }

                            @Override
                            public void onCancelled(final DatabaseError error) {
                                listAsyncEmitter.onError(new FirebaseException(error.getMessage()));
                            }
                        });

                listAsyncEmitter.setCancellation(() -> ref.child(Constants.NOTES)
                        .removeEventListener(valueEventListener));
            }
        }, AsyncEmitter.BackpressureMode.BUFFER);
    }