Function returns null although data is fetched from firebase

Question

I am working on a project on Android Studio with libGDX and I have connected the game to firebase which is working perfectly.

When I am trying to fetch some data, the firebase returns proper expected values, but the function is returning a null value.

public String fetchData(String id)
{
    reference = FirebaseDatabase.
            getInstance().getReference("Users").
            child(FirebaseAuth.getInstance().getCurrentUser().getUid());
    reference.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot snapshot) {
            s = snapshot.child(id).getValue(String.class); //Here if we print s it gives the desired output.
        }

        @Override
        public void onCancelled(@NonNull DatabaseError error) {

        }
    });

    return s; // But here the function is returning null.
}


@Override
    public String Background_Active() { 
        return fetchData("Background Active"); //Here I am calling the above function 
    }

If someone is aware of the solution to this issue, please do help me out.

There is no way you can do that. Firebase API is asynchronous. So please check the duplicate to see how can you solve this using a callback. You might also be interested in reading this [resource](https://medium.com/firebase-tips-tricks/how-to-read-data-from-firebase-realtime-database-using-get-269ef3e179c5). — Alex Mamo, Aug 17 '22 at 09:52

Pradipto SenSarma · Answer 1 · 2022-08-18T08:54:09.697

The firebase onDataChange() is asynchronous you need to pass the value from inside onDataChange for example create a method like

onPostExecute(String s)

Pass the data to this method from inside onDataChange() so your code would somewhat look like

public void fetchData(String id)
{
reference = FirebaseDatabase.
        
getInstance().getReference("Users").
        child(FirebaseAuth.getInstance().getCurrentUser().getUid());
reference.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
        s = snapshot.child(id).getValue(String.class); //Here if we print s it gives the desired output.
 onPostExecute(s);
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {

    }
});

   // return s; // But here the function is returning null.
}

For returning the value to the caller you can define a abstract inner class like

abstract GetValueListener{
abstract void onValueReceived(String s);
abstract void onErrorReceived(Throwable error);
}

Now modify fetchData method as following

public void fetchData(String id, GetValueListener listener)
{
reference = FirebaseDatabase.

getInstance().getReference("Users").
        child(FirebaseAuth.getInstance().getCurrentUser().getUid());
reference.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
        s = snapshot.child(id).getValue(String.class); //Here if we 
print s it gives the desired output.
 listener.onValueReceived(s);
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
 listener.onErrorReceived(error.toException());
    }
});

   // return s; // But here the function is returning null.
}

now when you call fetchdata your code should look like

fetchData(id, new GetValueListener(){
void onValueReceived(String s)
{
//value will be returned here
}

void onErrorReceived(Throwable error)
{
//if any error is received it will be returned here
}
});

Hope it helps. If for Libgdx this does not work please refer to this link https://libgdx.com/wiki/third-party/firebase-in-libgdx as i am not sure what is the requirement for Libgdx

Could you please explain how should I implement onPostExecute(s) as I need to return the value s, as I am working on libGDX? — Ashish Jain, Aug 17 '22 at 06:23
If you just want to return the value to the caller then you can use a interface, i will edit the answer for it. I am not sure how libgdx works but i found this article on the web, https://libgdx.com/wiki/third-party/firebase-in-libgdx check if it helps. — Pradipto SenSarma, Aug 18 '22 at 08:17

Function returns null although data is fetched from firebase

1 Answers1