等到 firebase 检索数据 [英] wait until firebase retrieves data

问题描述

我想构建一个在 FireBase 中返回 child 值的方法.我试图做这样的事情:

I want to build a method that returns a child value in FireBase. I tried to do something like this:

public String getMessage(){

    root.child("MessagesOnLaunch").child("Message").addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            message = (String) dataSnapshot.getValue();
            Log.i("4r398", "work");
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            Log.e("error", firebaseError.getMessage());
        }
    });
    return message;
}

问题是该方法返回null，这可能是因为该方法没有等到侦听器完成并返回null，因为它的默认值消息.我怎样才能让这个方法等到监听器发生然后返回值.

The problem is that the method returns null that is probably because the method doesn't wait until the listener finishes and return null because its the default value of message. How can I make this method wait until the listener occurs and then return the value.

推荐答案

制作界面

 public interface OnGetDataListener {
    //this is for callbacks
    void onSuccess(DataSnapshot dataSnapshot);
    void onStart();
    void onFailure();
}

声明如下函数readData()

Declare the following function readData()

public void readData(Firebase ref, final OnGetDataListener listener) {
    listener.onStart();
    ref.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            listener.onSuccess(dataSnapshot);
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            listener.onFailure();
        }
    });

}

调用readData()函数如下

Call the readData() function as follows

readData(root.child("MessagesOnLaunch").child("Message"), new OnGetDataListener() {
                @Override
                public void onSuccess(DataSnapshot dataSnapshot) {

               //got data from database....now you can use the retrieved data


                }
                @Override
                public void onStart() {
                    //when starting
                    Log.d("ONSTART", "Started");
                }

                @Override
                public void onFailure() {
                    Log.d("onFailure", "Failed");
                }
            });

readData() 可以在你的 getMessage() 方法中调用，甚至可以在 onCreate() 中调用

readData() can be called inside your getMessage() method or even inside onCreate()

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