如何在代码中的确切位置将数据从 firebase 提取到 android studio？

Question

Is there any way to pull data from firebase where the code is? I currently have valueEventListeners, but they all run after the code below them, thus invalidating the following code. I want to be able to pull a value exactly where the code is, not later.

As of yet, I have not found anything online about this.

A good example of my problems in the code:

public void onItemClick(AdapterView<?> l, View v, final int position, long id) {

        FirebaseAuth mAuth = FirebaseAuth.getInstance();
        FirebaseUser user = mAuth.getCurrentUser();
        final String uid = user.getUid();

        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                cCESnapshot = dataSnapshot.child(uid).child("currChallenges").child(challengeList.get(position));
            }

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

            }
        });

        Intent intent = new Intent();
        intent.setClass(this, ChallengeView.class);
        intent.putExtra("snapshot", cCESnapshot.toString());
        intent.putExtra("name", challengeList.get(position));
        startActivity(intent);
    }

cCESnapshot is null because the intent runs before the valueEventListener.

Answer 1

The onDataChange() is asynchronous, so the only way to use the retrieved data is inside onDataChange() , example:

databaseReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            cCESnapshot = dataSnapshot.child(uid).child("currChallenges").child(challengeList.get(position));

    Intent intent = new Intent();
    intent.setClass(this, ChallengeView.class);
    intent.putExtra("snapshot", cCESnapshot.toString());
    intent.putExtra("name", challengeList.get(position));
    startActivity(intent);
        }

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

        }
    });

Answer 2

This is an asynchronous operation, when the data will arrive, the onDataChange callback will be triggered. Your startActivty code will be executed sequentially and this is why you get no value in cCESnapshot .

Move the startActivity code inside the listener.

But be careful , because each time the onItemClick click listener will be called, you'll add a value event listener. That way, you'll have multiple calls to onDataChange in each click and so multiple startActivities .

Instead, i recommend using addListenerForSingleValueEvent which will be triggered only after a single change in data.