Fragment Layout中的ProgressBar

Question

我无法访问我在片段的Asynctask片段中onCreateView片段中创建的进度条。它由错误引起的是onPreExecute中的进度条给出了NullPointerException。 我在调用asynctask之前创建了progressBar。那可能是什么问题？

    public class HomeFragment extends Fragment {

    ProgressBar progressBar;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.home_fragment, container, false);

        progressBar = (ProgressBar) view.findViewById(R.id.progressbar);

        new getAnnouncements().execute();

        return view;
    }

    public static HomeFragment newInstance() {
        HomeFragment fragment = new HomeFragment();

        return fragment;
    }

    private class getAnnouncements extends AsyncTask<Void, Void, Void> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            progressBar.setVisibility(View.VISIBLE);

        }

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

        @Override
        protected void onPostExecute(Void args) {
            progressBar.setVisibility(View.GONE);

        }

    }
}

home_fragment.xml

 <ProgressBar
                    android:id="@+id/progressBar"
                    style="?android:attr/progressBarStyleLarge"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_centerInParent="true"
                    android:layout_marginTop="24dp"
                    android:visibility="gone" />

Answer 1

progressBar = (ProgressBar) view.findViewById(R.id.progressBar);