Static method to get DataSnapshot returns null

Question

I'm building an Android app that uses Firebase database and I need DataSnapshot a lot in the app so I decided to make a helper method to get me the DataSnapshot and here how I managed to implement it inside a Helper class that I created:

static FirebaseDatabase database = FirebaseDatabase.getInstance();
static DatabaseReference playersReference =  database.getReference().child("players");
static DataSnapshot snapshot;

public static DataSnapshot getDataSnapshot(){

        ValueEventListener valueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                snapshot = dataSnapshot;
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
            }
        };

        playersReference.addListenerForSingleValueEvent(valueEventListener);

        return snapshot;

    }

But whenever I try to use the method it returns null, any help?

Why do you say so?, not that listeners for single value are like no other listeners it doesn't (listen) it executes only once. — Ahmed Nabil, Jan 09 '18 at 03:04
`addListenerForSingleValueEvent` doesn't block and wait for a response. Your return statement is executed probably before the network request to Firebase even starts — OneCricketeer, Jan 09 '18 at 03:07

score 1 · Answer 1 · edited Jan 09 '18 at 02:56

First, Create a separate helper class which breaks up the static calls to the database such as the following:

public class FirebaseDbRef {

    private static DatabaseReference ref; 

    private static DatabaseReference getFirebaseDatabase() {
        if (ref == null) 
            ref = FirebaseDatabase.getInstance().getReference();
        return ref;
    }

    public static DatabaseReference getPlayerRef() {
        return getFirebaseDatabase().child("player");
    }

}

Then in your activity/fragment, call the method and add a listener to retrieve the snapshot:

FirebaseDbRef.getPlayerRef().addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            //USE SNAPSHOT
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
            databaseError.getMessage();
        }
    });

Make sure your pathing is correct to access the proper snapshot. You can always Log to verify.

This is what I managed to implement it in the first place but I'm just trying to make my code more concise. — Ahmed Nabil, Jan 09 '18 at 02:33

score 0 · Answer 2 · answered Jan 09 '18 at 01:12

0

The getDataSnapshot() does three things:

Creates a listener
Registers the listener
Returns snapshot

Note that none of these steps sets the value of snapshot. You are setting it in one of the listener's methods which is not called until after getDataSnapshot() returns.

answered Jan 09 '18 at 01:12

Code-Apprentice

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So I have to take the body of the function inside an activity?. – Ahmed Nabil Jan 09 '18 at 02:31
@TheForgetten You need to pass the ValueEventListener as a parameter to your `getDataSnapshot()` – OneCricketeer Jan 09 '18 at 02:54

score 0 · Answer 3 · answered Jan 09 '18 at 08:30

You cannot return something that hasn't been loaded yet. onDataChange() method is called asynchronous which means that is called even before you are getting the DataSnapshot object from the database. For more details you can see my answer from this post.

If you want to use the DataSnapshot object outside onDataChange() method you need to create your own callback as explained at the end on that answer.

Static method to get DataSnapshot returns null

3 Answers3