AsycTask Throwing IllegalStateException - Fragment

I have the following AsyncTask in my Android application. This AsyncTask is contained with within the OnCreate() method of a class that extends PreferenceFragment.

public class NotificationsPreferenceFragment extends PreferenceFragment {

private static Context context;

public NotificationsPreferenceFragment() {

}

public NotificationsPreferenceFragment(Context context) {
    this.context = context;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    addPreferencesFromResource(R.xml.pref_notifications);

    getPreferenceManager().findPreference(getString(R.string.send_all_notifications))
            .setOnPreferenceClickListener(new Preference.OnPreferenceClickListener() {
                @Override
                public boolean onPreferenceClick(Preference preference) {

                        class NotificationSendTask extends DialogAsyncTask {

                            public static final String TAG = "NotificationFragment";

                            public NotificationSendTask(Activity activity, String dialogMsg) {
                                super(activity, dialogMsg);
                            }

                            @Override
                            protected String doInBackground(String... params) {
                                String url = PreferenceManager.getDefaultSharedPreferences(getActivity()).getString(getString(R.string.notification_web_service_url), getString(R.string.default_notification_web_service_url));

                                if (NetworkingHelper.isNetworkAvailable(getActivity())) {
                                    NotificationDao notificationDao = new NotificationDaoImpl(DatabaseManager.getInstance(getActivity().getApplicationContext()), getActivity().getApplicationContext());

                                    List<Notification> unsentNotificationList = notificationDao.findAllNotSent();
                                    if (unsentNotificationList.size() != 0) {
                                        NotificationSenderTask ns = new NotificationSenderTask(url, context);
                                        try {
                                            if (ns.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, (unsentNotificationList)).get()) {
                                                return getString(R.string.success);
                                            }
                                        } catch (InterruptedException e) {
                                            Log.e(TAG, e.getMessage());
                                        } catch (ExecutionException e) {
                                            Log.e(TAG, e.getMessage());
                                        }

                                        return getString(R.string.failed_to_send_notifications);
                                    } else {
                                        return getString(R.string.no_notifications_to_send);
                                    }
                                } else {
                                    return getString(R.string.no_connection_notifications);
                                }
                            }

                            public void onPostExecute(String result) {
                                super.onPostExecute(result);
                                if (dialog != null && dialog.isShowing()) {
                                    dialog.hide();
                                }
                                Toast.makeText(activity, result, Toast.LENGTH_SHORT).show();
                            }
                        }
                        NotificationSendTask notificationSendTask = new NotificationSendTask(getActivity(), "Sending unsent notifications...");
                        notificationSendTask.execute();
                        return true;

                }
            });

    getPreferenceManager().findPreference(getString(R.string.export_notifications)).setOnPreferenceClickListener(new Preference.OnPreferenceClickListener() {
        @Override
        public boolean onPreferenceClick(Preference preference) {
            NotificationExportTask notificationExportTask = new NotificationExportTask(NotificationsPreferenceFragment.this.getActivity(), 1);
            notificationExportTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
            return true;
        }
    });
}
}

I am getting the following exception:

java.lang.IllegalStateException: Fragment NotificationsPreferenceFragment{416092f8} not attached to Activity
at android.app.Fragment.getResources(Fragment.java:741)
at android.app.Fragment.getString(Fragment.java:763)

Can someone please explain to me why this is happening and suggest ways to fix this issue?

UPDATE:

Here is the code for the Activity:

public class SettingsActivity extends PreferenceActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Override
protected void onDestroy() {
    super.onDestroy();
}

@Override
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
public void onBuildHeaders(List<Header> target) {
    loadHeadersFromResource(R.xml.pref_headers, target);
}
}

回答1:

Since you are performing background task in your app. there is no guarantee that user will stay on same screen until task finishes so if user navigates to other screen or presses home button before task is completed; your fragment is detached from activity. So always make sure that you have fragment attached with the Activity. try checking with

if (isAdded) { //Do your UI stuff here }

add above check wherever you get callback

回答2:

Move your code from onCreate to onActivityCreated instead of trying to getActivity @ onCreate.

That's because the fragment can be created when the activity is not yet ready, that's when you are trying to use it.

That is of course if you are adding the fragment to an activity like:

FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(android.R.id.content, new PreferenceFragment()).commit();