在 firebase 响应之前执行的返回语句

Question

I am using FirebaseFirestore to nosql database, I have a dedicated java class to execute all the firebase Read, Write and Delete queries.

The problem I am facing is while fetching my Users data from Firestore, and storing it in an array list. After all the users are update in the arraylist, then i am returning that list to update the UI. but my funciton below is returning the empty list and then the firebase query is updating the data into the list.

public List<Users> getAllUsers() {
    return loadAllUserData();
}
private List<Users> loadAllUserData() {
    List<Users> usersList  = new ArrayList<>();

    mFirebaseFirestore.collection(USER_DATA).get()
            .addOnSuccessListener(queryDocumentSnapshots -> {
                List<DocumentSnapshot> docSnap = 
                queryDocumentSnapshots.getDocuments();
                
                for (DocumentSnapshot docs : docSnap) {
                    Users user = docs.toObject(Users.class);
                    usersList.add(user);
                    Log.d(TAG, "onSuccess: loaded Users : " + usersList.size());
                }
            });
    Log.d(TAG, "loadAllUserData: Returning users : " + usersList.size());

    return usersList;
}

logs

2022-09-01 23:13:38.786 /com.app D/Repository: Repository: Called

2022-09-01 23:13:38.787 /com.app D/Repository: Repository: Called

2022-09-01 23:13:38.787 /com.app D/Repository: loadAllUserData: Returning users: 0

2022-09-01 23:13:38.962 /com.app D/Repository: onSuccess: loaded Users: 1

2022-09-01 23:13:38.962 /com.app D/Repository: onSuccess: loaded Users: 2

2022-09-01 23:13:38.962 /com.app D/Repository: onSuccess: loaded Users: 3

2022-09-01 23:13:38.963 /com.app D/Repository: onSuccess: loaded Users: 4

Answer 1

The get() operation is an asynchronous operation. Because of that, the return will be executed before the addOnSuccessListener.