android - 从 valueEventListener java 返回值

Question

我试图返回一个 bool 值，但返回的值始终为 false，即在本例中为 0。

检查变量是一个实例变量，按下按钮时将调用以下函数。

private boolean checkAnswerSubmission() {

    DatabaseReference answerDatabase = FirebaseDatabase.getInstance().getReference().child("userPuzzleHistory").child(uid);
    answerDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (dataSnapshot.hasChild(String.valueOf(imagename))) {
                check = 1;
            } else {
                check = 0;
            }
     //here the value check is working fine but the value cannot be returned from here.
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    //here the value returned is always 0
    if (check == 1)
        return true;
    else
        return false;

}

Answer 1

与所有异步操作一样，我经常以回调方式进行。

您的解决方案可以像这样工作:

1. 创建一个简单的回调接口(interface)(我在几乎每个应用程序中使用的库中都有我的回调接口(interface))

   public interface SimpleCallback {
       void callback(Object data);
   }
   
   // You could do it as well generic, that's what I do in my lib:
   public interface SimpleCallback<T> {
       void callback(T data);
   }
   

然后像这样重新设计方法的签名:

private void checkAnswerSubmission(@NonNull SimpleCallback<boolean> finishedCallback) {

    DatabaseReference answerDatabase = FirebaseDatabase.getInstance().getReference().child("userPuzzleHistory").child(uid);
    answerDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // This will simple call your callback containing a boolean true/false
            finishedCallback.callback(dataSnapshot.hasChild(String.valueOf(imagename)));
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}

你调用它:

checkAnswerSubmission(new SimpleCallback<boolean>() {
   @Override
   void callback(boolean data) {
       if (data) {
          // true was returned
       } else {
          // false was returned
       }
   }
});