android 程序中,在一个 fragment activity 中有一个 progressbar。当任何 fragment 调用接口时,这个 progress bar 就会设置为可见的。
public class MainScreen extends FragmentActivity {
public interface OnConnectingToInternet {
public void showProgressbar(boolean flag);
}
// rest of codes
.
.
.
// Implementing Interface
public void showProgressbar(boolean flag) {
if(flag){
myProgressbar.showProgressBar();
} else {
myProgressbar.hideProgressBar();
}
}
}
每一个fragment应该连接网络获取数据,但是在这之前要调用接口。其中一个fragment的代码如下:
public class TopRecipesFragment extends Fragment {
private OnConnectingToInternet onConnectingToInternet;
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
// onConnectingToInternet = (OnConnectingToInternet) activity;
Log.i(TAG, "Fragment attached to activity.");
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
New MyAsyncTask.execute();
}
public class MyAsyncTask extends AsyncTask<Void, Void, Boolean> {
@Override
protected void onPreExecute() {
Log.i(TAG, "myAsyncTask is about to start...");
onConnectingToInternet.showProgressbar(true);
}
@Override
protected Boolean doInBackground(Void... params) {
...
}
@Override
protected void onPostExecute(Boolean result) {
Log.i(TAG, "myAsyncTask is about to start...");
onConnectingToInternet.showProgressbar(false);
}
}
问题是我不知道如何实例化这个接口。如果我不加入这行:onConnectingToInternet = (OnConnectingToInternet) activity;
然后运行程序时,就会出错NullPointerException
。
但是,如果我这样实例化,然后程序就崩溃。我用通常的做法也不行:onConnectingToInternet = new MainScreen();
如何解决这个问题呢?