为什么即使 Firebase 数据快照包含子项，我的 ArrayList 也是空的

Question

I am attempting to pull user data from firebase realtime database, the datasnapshot does contain children and data but when I attempt to put this into an array, the array is empty. I created a user class which contains the properties to match the ones in firebase, overloaded constructors, and some methods. It does not extend any class though as I know this would not work with retrieving data from Firebase.

private List<user> usersList = new ArrayList<>();
private DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference();
private DatabaseReference ref = databaseReference.child("Users");

public List<user> getUsers(){
    ref.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            if(dataSnapshot.exists()){
                Log.i("users", String.valueOf(dataSnapshot.getChildrenCount()));

                for(DataSnapshot child: dataSnapshot.getChildren()){
                    user userRetrieved = child.getValue(user.class);
                    Log.d("USER before array", userRetrieved.getUserName());
                    usersList.add(userRetrieved);
                    for(int i = 0;i < usersList.size(); i++){
                        Log.d("USER", usersList.get(i).getUserName());
                    }

                }
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            Log.i("ERROR", databaseError.getMessage());
        }
    });

    return usersList;
}

All feedback welcomed.

Answer 1

The Firebase APIs are asynchronous. addListenerForSingleValueEvent returns immediately, and the callback you pass to it is executed some time later, after query completes. There is no guarantee how quickly that will happen. Meanwile, your function is returning a list that's initially empty. You probably have code that's to read it with the assumption that the query has already completed, when it actually hasn't.

Bottom line is that you should only deal with the results of the query after the callback has been invoked, not before.

Answer 2

I figured it out, had to make sure my class was conforming to the object being returned from the DB. First I had to make my user class implement serializable and add @PropertyName() and also make sure all the property names matched exactly what was in the Firebase.

 public List<User> getUsers(){
    ref.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            try{
                for(DataSnapshot childSnapshot: dataSnapshot.getChildren()){
                    User User = childSnapshot.getValue(User.class);
                    usersList.add(User);
                        User[] UserArray = new User[usersList.size()];
                        UserArray = usersList.toArray(UserArray);
                    for(int i = 0;i < usersList.size(); i++){
                        Log.d("NAME", UserArray[i].getUserName());
                    }


                }
            }catch (Exception e){
                Log.d("Error in the try", e.getMessage());
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            Log.i("ERROR", databaseError.getMessage());
        }
    });

    return usersList;
}