使用FragmentActivity中的事件侦听器在android应用程序类中实现侦听器

Just started programming and I'm coding my first app. I've been wrecking my brain over this for long enough. I picked this up from tutorial where a listener is implemented in a ListFragment and the event listener is in a FragmentActivity that hosts the ListFragment. Here is how it is in the example (showing only the relevant part):

刚开始编程,我正在编写我的第一个应用程序。我一直在破坏我的大脑足够长的时间。我从教程中选择了这个,其中一个监听器在ListFragment中实现,而事件监听器位于一个托管ListFragment的FragmentActivity中。以下是示例中的内容(仅显示相关部分):

MyList class:

public class MyList extends ListFragment{

   OnSomeItemSelectedListener mCallback;

   public interface OnSomeItemSelectedListener {
      public void onSomeItemSelectedListener(int position, long id);
    }

   @Override
   public void onAttach(Activity activity) {
      super.onAttach(activity);
      // This makes sure that the container activity has implemented
      // the callback interface. If not, it throws an exception.
      try {
         mCallback = (OnSomeItemSelectedListener) activity;
      } catch (ClassCastException e) {
         throw new ClassCastException(activity.toString()
         + " must implement OnSomeItemSelectedListener");
      }
   }

   @Override
   public void onListItemClick(ListView l, View v, int position, long id) {
      super.onListItemClick(l, v, position, id);
      mCallback.onSomeItemSelectedListener(position, id);
   }
}

MainActivity class:

public class MainActivity extends FragmentActivity implements
        MyList.OnSomeItemSelectedListener{

    @Override
    public void onSomeItemSelectedListener(int position, long id) {
       //some stuff done here when clicked on a item from the listfragment
    }
}

The above works flawlessly.

以上工作完美无瑕。

I'm trying to implement this in an application class to notify the event listener (a FragmentActivity) when an AsyncTask has finished. This is what I did in in the application class:

我正在尝试在应用程序类中实现它,以在AsyncTask完成时通知事件侦听器(FragmentActivity)。这就是我在应用程序类中所做的:

MyApplication class:

public class MyApplication extends Application {

    OnFetchCompleteListener mCallback;

    public interface OnFetchCompleteListener {
        public void onFetchCompleteListener();
    }

    public synchronized void fetchUpdates() {
        String[] mURLs = getURLs();
        new DownloadDataAsyncTask().execute(mURLs[0], mURLs[1]);
    }

    private class DownloadDataAsyncTask extends
            AsyncTask<String, Void, JSONObject[]> {

        @Override
        protected JSONObject[] doInBackground(String... params) {
            //Some stuff
            return mJSON;
        }

        @Override
        protected void onPostExecute(JSONObject[] result) {
            // Informing listeners on fetchData complete
            mCallback.onFetchCompleteListener();
        }
    }
}

How do I instantiate the mCallback in the application class? Without this,

如何在应用程序类中实例化mCallback?没有这个,

   mCallback = (OnSomeItemSelectedListener) activity;

I get a null pointer exception from the application class.

我从应用程序类中得到一个空指针异常。

Many thanks

1 个解决方案

#1

I think you can instantiate mCallback from the Activity instead of from the Application.

我认为您可以从Activity而不是Application中实例化mCallback。

In your activity's onCreate() or onResume() method, you can write:

在您的活动的onCreate()或onResume()方法中,您可以编写:

MyApplication myApplication = getApplication();
myApplicaton.mCallback = this;

but make sure your activity have implemented OnFetchCompleteListener (I don't why you typed OnSomeItemSelectedListener in the last piece of code.)

但请确保您的活动已实现OnFetchCompleteListener(我不知道您在最后一段代码中键入OnSomeItemSelectedListener的原因。)

#1