处理异步任务返回的数据 （Firebase）

Question

我试图弄清楚如何处理从异步任务返回的数据。

我有一个活动，我正在调用异步Firebase get操作（存在于其他类中的操作），并且我想使用检索到的ArrayList的大小更新活动中存在的TextView。以下是我在活动创建中获取的 Firebase 调用：

public void getAttendants() {

   ArrayList<AttendModel> attendees = new ArrayList<AttendModel>();
    FirebaseConnection.getInstance().getAllAttendeesFor(uuid, attendees);

    Log.d("attendees", String.valueOf(attendees.size()));
}

这是我的Firebase操作：

 public void getAllAttendeesFor(String UUID, final ArrayList<AttendModel> attendArray) {
    final DatabaseReference attendObjects = getDatabaseTableWith(Constants.tableAttendObject);

    Query queryRef = attendObjects.orderByChild(Constants.taAttendObjectUUID).equalTo(UUID);

    queryRef.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                HashMap<String, Object> attend = (HashMap<String, Object>) postSnapshot.getValue();
                String UUID = (String) attend.get(Constants.taAttendObjectUUID);
                String userUsername = (String) attend.get(Constants.AttendObjectUserUsername);

                AttendModel attendModel = new AttendModel(userUsername, UUID);
                attendArray.add(attendModel);

 //here would like to notify or somehow return the attendArray
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

}

在Android中，最好的方法是什么？是否有可能实现各种适配器或完成处理程序？

Answer 1

你正在寻找的是回调接口，至于你的需要，你需要一个简单的接口，实现单参数单方法，因为没有现成的Java 6 (Android Java版本)的约定，你可以很容易地自己创建它:

interface Callback {

void act(List<AttendModel> models);
}

将它作为参数传递给firebase包装方法，并用结果调用它。

public void getAllAttendeesFor(String UUID, final ArrayList<AttendModel> attendArray, final Callback callback) {
    final DatabaseReference attendObjects = getDatabaseTableWith(Constants.tableAttendObject);

    Query queryRef = attendObjects.orderByChild(Constants.taAttendObjectUUID).equalTo(UUID);

    queryRef.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                HashMap<String, Object> attend = (HashMap<String, Object>) postSnapshot.getValue();
                String UUID = (String) attend.get(Constants.AttendObjectUUID);
                String userUsername = (String) attend.get(Constants.AttendObjectUserUsername);

                AttendModel attendModel = new AttendModel(userUsername, UUID);
                attendArray.add(attendModel);

                callback.act(attendArray)
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

}

然后在你的上层方法中:

public void getAttendants() {

   ArrayList<AttendModel> attendees = new ArrayList<AttendModel>();
    FirebaseConnection.getInstance().getAllAttendeesFor(uuid, attendees, 
        new Callable<List<AttendModel) {
          void act(List<AttendModel> attendees){
            //here you can update the UI
            Log.d("attendees", String.valueOf(attendees.size()));
          }
        };


}

请注意，只有在主线程上调用firebase查询的onDataChange方法时，才可以更新UI。