无法从Android上的Firebase获取列表大小 [英] Cannot get list size from Firebase on Android

问题描述

我的日志可以打印所有Exercise对象.但是当我打电话时listExercise为null.有什么问题吗?

My log can print all Exercise object. But listExercise is null when I call. What's the problem?

ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot snapshot) {
            for (DataSnapshot postSnapshot : snapshot.getChildren()) {
                Exercise exercise = postSnapshot.getValue(Exercise.class);
                String temp = exercise.toString();
                Log.d("exercise: ", temp + "\n"); // can log all
                listExercises.add(exercise);
            }
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            Log.d("The read failed: ", firebaseError.getMessage());
        }
    });
Log.d("LISTSIZE: ", String.valueOf(listExercises.size())); // return 0

[UPDATE]任何人需要的答案:

@Override
        public void onDataChange(DataSnapshot snapshot) {
            for (DataSnapshot postSnapshot : snapshot.getChildren()) {
                Exercise exercise = postSnapshot.getValue(Exercise.class);
                listExercises.add(exercise);
            }
            // action with data here
            Exercise ex = listExercises.get(0);
            tvQuestion.setText(ex.getQuestion);
        }

推荐答案

调用方法"paintListInScreen"时，您肯定知道列表中包含一些内容(或者可能没有，这取决于服务器，但是您正在与线程一致).

When the method "paintListInScreen" is called, you know for sure the list have some content (or maybe not, that depends on the server, but you are being consistent with threads).

ref.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        for (DataSnapshot postSnapshot : snapshot.getChildren()) {
            Exercise exercise = postSnapshot.getValue(Exercise.class);
            String temp = exercise.toString();
            Log.d("exercise: ", temp + "\n"); // can log all
            listExercises.add(exercise);
        }
         Log.d("LISTSIZE: ", String.valueOf(listExercises.size())); // return 0
        paintListInScreen(listExercises);
    }

    @Override
    public void onCancelled(FirebaseError firebaseError) {
        Log.d("The read failed: ", firebaseError.getMessage());
    }
});

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