firestore read is asynchronous and I want synchronous behaviour

Question

I want to store locally the data I am reading from the cloud. To achieve this I am using a global variable(quizzes) to hold all the data.

For this, whe

Answer 1

You can make your own callback. For this, make an interface

public interface FireStoreResults {
    public void onResultGet();
}

now send this call back when you get results

public void readData(final FireStoreResults){
    db.collection("users").document(user_id).collection("quizzes")
    .get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
            @Override
            public void onSuccess(QuerySnapshot queryDocumentSnapshots) {
                for (QueryDocumentSnapshot document : task.getResult()) {
                     Quiz quizDownloaded = getQuizFromCloud(document.getId());
                     quizzes.add(quizDownloaded);
                 }
                results.onResultGet();
            }
        }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {
                results.onResultGet();
            }
        });   
}

Now in your activity or fragment

new YourResultGetClass().readData(new FireStoreResults(){
             @Override
             public void onResultGet() {
                    new YourResultGetClass().getQuizzes(); //this is your list of quizzes
                    //do whatever you want with it
            }

Hope this makes sense!

Answer 2

I didn't understand if you tried this solution, but I think this is the better and the easier: add an onCompleteListener to the Task object returned from the get() method, the if the task is succesfull, you can do all your stuff, like this:

private void downloadQuizzesFromCloud(){


String user_id = FirebaseAuth.getInstance().getCurrentUser().getUid();
final FirebaseFirestore db = FirebaseFirestore.getInstance();
CollectionReference quizzesRefrence = db.collection("users").document(user_id).collection("quizzes");

quizzesRefrence.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            if (task.isSuccesful()) {
               for (QueryDocumentSnapshot document : task.getResult()) {
                    Quiz quizDownloaded = getQuizFromCloud(document.getId());
                    quizzes.add(quizDownloaded);
               }
             }
         });
}
}

In this way, you'll do all you have to do (here the for loop) as soon as the data is downloaded