wait until firebase retrieves data

Question

I want to build a method that returns a child value in FireBase. I tried to do something like this:

public String getMessage(){

    root.child("MessagesOnLaunch").child("Message").addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            message = (String) dataSnapshot.getValue();
            Log.i("4r398", "work");
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            Log.e("error", firebaseError.getMessage());
        }
    });
    return message;
}

The problem is that the method returns null that is probably because the method doesn't wait until the listener finishes and return null because its the default value of message. How can I make this method wait until the listener occurs and then return the value.

Answer 1

Make an interface

 public interface OnGetDataListener {
    //this is for callbacks
    void onSuccess(DataSnapshot dataSnapshot);
    void onStart();
    void onFailure();
}

Declare the following function readData()

public void readData(Firebase ref, final OnGetDataListener listener) {
    listener.onStart();
    ref.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            listener.onSuccess(dataSnapshot);
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            listener.onFailure();
        }
    });

}

Call the readData() function as follows

readData(root.child("MessagesOnLaunch").child("Message"), new OnGetDataListener() {
                @Override
                public void onSuccess(DataSnapshot dataSnapshot) {

               //got data from database....now you can use the retrieved data


                }
                @Override
                public void onStart() {
                    //when starting
                    Log.d("ONSTART", "Started");
                }

                @Override
                public void onFailure() {
                    Log.d("onFailure", "Failed");
                }
            });

readData() can be called inside your getMessage() method or even inside onCreate()