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I am trying to download some Quiz objects from my database.
The following function is called from onCreate of a certain activity.

 private void downloadQuizzesFromCloud(){



        String user_id = FirebaseAuth.getInstance().getCurrentUser().getUid();

        final FirebaseFirestore db = FirebaseFirestore.getInstance();

        String user_quizzes_path = "users/".concat(user_id).concat("/quizzes");

        Query userQuizzes = db.collection(user_quizzes_path);



        userQuizzes.get()

                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {

                    @Override

                    public void onComplete(@NonNull Task<QuerySnapshot> task) {

                        if (task.isSuccessful()) {

                            quizzes.clear();

                            for (QueryDocumentSnapshot document : task.getResult()) {



                                Quiz quizDownloaded = getQuizFromCloud(document.getId());

                                quizzes.add(quizDownloaded);

                            }

                            Toast.makeText(QuizzesActivity.this,"downloaded to list ".concat(String.valueOf(quizzes.size()).concat("  quizzes")), Toast.LENGTH_SHORT).show();

                            //TODO put in recycle adapter

                        } else { }

                    }

                });



    }

(user_quizzes_path contains the correct path to a collection of Quiz objects stored on the cloud)

I debugged this functions and found out that after the command:

 userQuizzes.get()

            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>()

The function finishes execution, that is the onComplete cases aren't checked and executed and all this code is just skipped.

I tried to find this on the documentation of firebase but didn't find anything.
Why is this happening and how can I fix this?

Would appreciate some help here, thanks!

The onComplete is called when the read operation has completed from the Firestore servers. If it's not getting called, I can see two possible reasons:

1. You're not connected to the server. Unless you've read the data before (and it's in the local database that the Firestore client maintains), this means the read never completes locally.


2. You're not thinking asynchronously. Note that data is read from the server asynchronously, and there may be some time between when you call get() and when onComplete fires. To test if this is the case, put a breakpoint on if (task.isSuccessful()) { and run the app in the debugger. The breakpoint will hit when the data is read from the server.

Use a callback interface. Just like this below.

private void downloadQuizzesFromCloud(Consumer listener) {



    String user_id = FirebaseAuth.getInstance().getCurrentUser().getUid();



    final FirebaseFirestore db = FirebaseFirestore.getInstance();



    String user_quizzes_path = "users/".concat(user_id).concat("/quizzes");



    Query userQuizzes = db.collection(user_quizzes_path);



    userQuizzes.get()

            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {

                @Override

                public void onComplete(@NonNull Task<QuerySnapshot> task) {

                    if (task.isSuccessful()) {

                        List<Quiz> quizzes = new ArrayList<>();

                        for (QueryDocumentSnapshot document : task.getResult()) {

                            Quiz quizDownloaded = getQuizFromCloud(document.getId());

                            quizzes.add(quizDownloaded);

                        }

                        listener.onGet(quizzes);

                        Toast.makeText(QuizzesActivity.this,"downloaded to list ".concat(String.valueOf(quizzes.size()).concat("  quizzes")), Toast.LENGTH_SHORT).show();

                        //TODO put in recycle adapter

                    } else { }

                }

            });



 } 

 interface Consumer {

    void onGet(List<Quiz> quizzes);

 }