solution for AsyncTask.onPostExecute throws NullPointerException when fragment's view was destroyed in android

like title, there is the issue like example:

public class TestFragment extends Fragment {
    private boolean isDestroyed = false;
    private TextView mTextView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.simple_list_item_1, null);
        mTextView = (TextView) view.findViewById(R.id.text1);
        return view;
    }

    class FetchNewsTask extends AsyncTask {

        @Override
        protected void onPreExecute() {
            // show ProgressBar here
        }

        @Override
        protected String doInBackground(Void... params) {
            return null;
        }

        @Override
        protected void onPostExecute(String result) {
            mTextView.setText(result);
        }
    }
}

that is a very simple fragment in a program, but there have a bug if you do that.

AsyncTask well execute in another thread, but view is destroyed this thread maybe not executed, then it throws a nullpoint exception, so this is the issue.

I find a very simpleness solution for this issue, like this

public class TestFragment extends Fragment {
    private boolean isDestroyed = false;
    private TextView mTextView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.simple_list_item_1, null);
        mTextView = (TextView) view.findViewById(R.id.text1);
        return view;
    }

    class FetchNewsTask extends AsyncTask {

        @Override
        protected void onPreExecute() {
            // show ProgressBar here
        }

        @Override
        protected String doInBackground(Void... params) {
            return null;
        }

        @Override
        protected void onPostExecute(String result) {
            // when task executed the text view text well change, 
            // but when this task return after this fragment changed, it must throws a NullPointerException
            if (isDestroyed) return;
            mTextView.setText(result);
        }
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        // I set a flag here for this fragment's view was destroyed
        isDestroyed = true;
    }
}

now you app can not crash when you replace fragment that task has not return.