java - 等到 firebase 查询中的 onChildAdded 完成，Android

Question

我是 Firebase 的新手，正在做一个简单的项目。我有一种方法可以从 Firebase 获取教师列表并将他们的电子邮件添加到 ArrayList 并返回。

public ArrayList<String> getTeacherList() {
    temp = new ArrayList();

    Firebase node = ref.child("teachers");
    Query query = node.orderByChild("subject").equalTo(selected_subject);

    query.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {

            Map<String,Object> value = (Map<String, Object>) dataSnapshot.getValue();
            Log.d("TEACHER"," "+value.toString());

            Iterator entries = value.entrySet().iterator();
            while (entries.hasNext()) {
                Map.Entry thisEntry = (Map.Entry) entries.next();
                String key = (String) thisEntry.getKey();
                Object data = thisEntry.getValue();

                if(key.equals("email")) {
                    temp.add(data.toString());
                    Log.d("ENTRY",": DATA "+data.toString());
                }
            }
            
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });


    Log.d("RETURN", temp.toString());
    return temp;
}

这个方法很好用。但问题是这会在完成添加子项之前返回 temp 变量。

例如我的 Android Monitor 就是这样的:

D/RETURN: []

D/TEACHER: {number=1, name=Teacher, email=teacher@new.lk, subject=Physics}

D/ENTRY: : DATA teacher@new.lk

有什么方法可以等到 onChildAdded 方法完成......？

============================================= ==========================

我也试过这个方法。但随后它在内部方法处停止了。

即使它没有转到 Log.d("TEACHER", ""+ value.toString()); 标签这里。

public  ArrayList getTeacherList() throws InterruptedException {
    temp = new ArrayList();

    Firebase node = ref.child("teachers");
    Query query = node.orderByChild("subject").equalTo(selected_subject);

    semaphore = new Semaphore(0);

    query.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {

            Map<String, Object> value = (Map<String, Object>) dataSnapshot.getValue();
            Log.d("TEACHER", " " + value.toString());

            Iterator entries = value.entrySet().iterator();
            while (entries.hasNext()) {
                Map.Entry thisEntry = (Map.Entry) entries.next();
                String key = (String) thisEntry.getKey();
                Object data = thisEntry.getValue();

                if (key.equals("email")) {
                    setArrayList(data.toString());
                    Log.d("ENTRY", ": DATA " + data.toString());
                }
            }
            semaphore.release();
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });
    semaphore.acquire();
    return temp;
}

Answer 1

public class MainActivity extends AppCompatActivity{
public static Bus bus;

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);         
    bus = new Bus(ThreadEnforcer.MAIN);
    bus.register(this);

    //CALL TO LOAD DATA FROM FIREBASE
    loadData();
}

private void loadData(){
    DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference();
    mDatabase.child("my-data").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            //CREATE YOUR OWN OBJECT USING RECEIVED DATA
            MyObject o = new MyObject();
            //CALL FOR METHOD nextMethod()
            bus.post(o);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.d("Error", databaseError.toString());
        }
    });
}

@Subscribe
private void nextMethod(MyObject o){

}
}

这是使用 otto - EventBus 的一个非常基本的示例。从 bus.post(o)，您可以调用相关的订阅方法。