当从后端堆栈返回布局时，如何刷新ListFragment？

Question

First I should mention that I am using the ActionBarSherlock library for backwards compatibility.

I have an activity which adds a ListFragment when it is first started. I have a custom Loader which I implemented and follows the AsnycTaskLoader example very closely. My ListFragment implements the LoaderCallbacks<Cursor> interface. All the appropriate callback methods are called when the fragment is added ( onCreateLoader() , onLoaderFinished() ) and when it is replaced ( onLoaderReset() ).

My onActivityCreated(Bundle) method looks like this:

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    mAccountsDbAdapter = new AccountsDbAdapter(getActivity().getApplicationContext());

    setHasOptionsMenu(true);
    mCursorAdapter = new AccountsCursorAdapter(getActivity()
            .getApplicationContext(), R.layout.list_item_account, null,
            new String[] { DatabaseHelper.KEY_NAME },
            new int[] { R.id.account_name }, 0);

    setListAdapter(mCursorAdapter); 
    getLoaderManager().initLoader(0, null, this);
}

Later on, the ListFragment is replaced with another Fragment B. When the user presses the back button, Fragment B is removed and the ListFragment is added again. However, the list is empty and only the android:empty elements are displayed and none of the LoaderCallback methods are called. I can use the debugger to determine that getLoaderManager().initLoader(0, null, this); is actually called, but nothing else. When I change it to getLoaderManager().restartLoader(0, null, this); , the callbacks get called, but still my list remains empty (although there is data, the view is not refreshed).

How can I get my ListFragment to refresh itself when it is returned to the layout? Has anyone encountered this before, how did you fix it?

FYI, here are my callback methods

    @Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    return new AccountsCursorLoader(this.getActivity()
            .getApplicationContext());
}

@Override
public void onLoadFinished(Loader<Cursor> loaderCursor, Cursor cursor) {
    mCursorAdapter.swapCursor(cursor);
    mCursorAdapter.notifyDataSetChanged();
}

@Override
public void onLoaderReset(Loader<Cursor> arg0) {
    mCursorAdapter.swapCursor(null);
}

Some notes:

1. I cannot use the setListShown(true) methods in the example because I get an IllegalStateException that it cannot be used with a custom content view.
2. My AccountsCursorAdapter extends a SimpleCursorAdapter and modifies only the bindView() method.

Answer 1

Eureka!!! I found it (by accident, of course)

TL;DR;

In the AccountsListFragment , I now create my database adapter ( AccountsDatabaseAdapter ) in the onCreate() method and close it in the onDestroy() method and it now works. Previously I was creating my adapter in the onActivityCreated() and closing in onDestroyView() . Note that I am not referring to the ListAdapter , but rather to my database interfacing AccountsDbAdapter .

Attempt at long explanation:

Well, I was doing this because I thought that getting context through getActivity() will not be possible in the onCreate() method. It turns out you can getActivity() even before onActivityCreated() is called.

But I cannot really explain why this now works because the Loader has its own DatabaseAdapter object which it uses to retrieve the data. If I were to guess, I would say that the same database object is returned for both database adapters ( the database is cached ). Which would mean that when I close one in onDestroyView() , the other is also closed and the cursor data set becomes invalid which results in an empty list view. The loader does not reload because it thinks the data has not changed.

But even this explanation does not satisfy me completely, because some of the suggested solutions here which I tried like force restarting the loader each time did not work. (in the loadInBackground() method, a new DatabaseAdapter is created each time).

Anyway, if you happen to have a better understanding of what is going on, please hammer away in the comments.

Thanks everyone for the help with this!

Answer 2

Have you tried forcing the loader to restart in your activity, just after underlying data set is changed?

Try:

getLoaderManager().restartLoader(0, null, this);

Answer 3

尝试forceload（） ：

getLoaderManager().getLoader( 0 ).forceLoad();

Answer 4

What the Adapter sees:

The reason why you observe an empty ListView is because the position of the Cursor that was returned is STILL pointing at the last element . When you swap() the Cursor to an Adapter, the Adapter tries to iterate using a while(Cursor.moveToNext()) loop. Because that loop always evaluates FALSE, your ListView gives you the illusion of an empty Cursor.

Print the values of Cursor.getCount() and Cursor.getPosition() in onLoadFinished() . If I am correct, these two values should be equal. This clash of indexes creates the above illusion.

Why does the Adapter see this:

Loaders will re-use a Cursor whenever possible. If you request a Loader for a set of data that has not changed, the loader is smart and return the Cursor via onLoadFinished without doing any additional work, not even setting the position of the Cursor to -1.

ANS Call Cursor.moveToPosition(-1) in onLoadFinished() manually to work around this problem.

Answer 5

In my case I found out that the ListView simply was not drawn on the screen for some reason. If I went to the home screen and then switched back to the activity everything would appear as expected. So I searched for a method to force a redraw of the screen and I found the following:

Force a full screen Activity to remeasure/redraw on resume?

Based on the answer to that question I added the following code to the onActivityCreated() method of my ListFragment:

getActivity().getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 0);

With this code in place everything works as expected. But this is still a workaround, so if someone gives an answer to this question with a better solution or explanation, I will award them the bounty.

Answer 6

The problem could come from getLoaderManager() which would not send the callback to the correct place.

If you are using ActionBarSherlock then it means you are on Android 2.x right? Have you considered using Android support package v4? http://developer.android.com/sdk/compatibility-library.html

You would then use android.support.v4.app.ListFragment and get your LoaderManager by calling getActivity().getSupportLoaderManager()

Answer 7

Since you are using a custom loader, do you know if you have the same issues when using the native CursorLoader? If not, then perhaps, you can compare your Loader with the Android source and see if they are doing something differently?

https://github.com/android/platform_frameworks_base/blob/master/core/java/android/content/CursorLoader.java