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POAndroid: CursorLoader crash on non-topmost Fragment
 

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  1. POAndroid: CursorLoader crash on non-topmost Fragment
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    <p>I have a few ListFragments that use a CursorLoader to retrieve their content. As the user drills down through the content, one Fragment replaces another (the Activity remains the same). But if the content changes on a non-topmost Fragment, the app crashes:</p> <pre><code>E/AndroidRuntime(18830): FATAL EXCEPTION: main E/AndroidRuntime(18830): java.lang.RuntimeException: Unable to resume activity {com.example.ExampleApp/com.example.ExampleApp.ExampleActivity}: java.lang.IllegalStateException: Content view not yet created E/AndroidRuntime(18830): at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2120) E/AndroidRuntime(18830): at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2135) E/AndroidRuntime(18830): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:957) E/AndroidRuntime(18830): at android.os.Handler.dispatchMessage(Handler.java:99) E/AndroidRuntime(18830): at android.os.Looper.loop(Looper.java:130) E/AndroidRuntime(18830): at android.app.ActivityThread.main(ActivityThread.java:3683) E/AndroidRuntime(18830): at java.lang.reflect.Method.invokeNative(Native Method) E/AndroidRuntime(18830): at java.lang.reflect.Method.invoke(Method.java:507) E/AndroidRuntime(18830): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) E/AndroidRuntime(18830): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597) E/AndroidRuntime(18830): at dalvik.system.NativeStart.main(Native Method) E/AndroidRuntime(18830): Caused by: java.lang.IllegalStateException: Content view not yet created E/AndroidRuntime(18830): at android.support.v4.app.ListFragment.ensureList(ListFragment.java:328) E/AndroidRuntime(18830): at android.support.v4.app.ListFragment.setListShown(ListFragment.java:280) E/AndroidRuntime(18830): at android.support.v4.app.ListFragment.setListShownNoAnimation(ListFragment.java:266) E/AndroidRuntime(18830): at com.example.ExampleApp.FirstListFragment.onLoadFinished(FirstListFragment.java:102) E/AndroidRuntime(18830): at com.example.ExampleApp.FristListFragment.onLoadFinished(FirstListFragment.java:20) E/AndroidRuntime(18830): at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:414) E/AndroidRuntime(18830): at android.support.v4.app.LoaderManagerImpl$LoaderInfo.reportStart(LoaderManager.java:298) E/AndroidRuntime(18830): at android.support.v4.app.LoaderManagerImpl.doReportStart(LoaderManager.java:751) E/AndroidRuntime(18830): at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:512) E/AndroidRuntime(18830): at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1129) E/AndroidRuntime(18830): at android.app.Activity.performStart(Activity.java:3791) E/AndroidRuntime(18830): at android.app.Activity.performRestart(Activity.java:3821) E/AndroidRuntime(18830): at android.app.Activity.performResume(Activity.java:3826) E/AndroidRuntime(18830): at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2110) E/AndroidRuntime(18830): ... 10 more </code></pre> <p>This can happen when a background update changes the content, or when the user returns from the home screen and another Fragment is at the top of the back stack. The <code>onLoadFinished</code> handler tries to update the UI, which no longer exists. But why is the fragment still getting cursor updates anyway?</p> <p>I've worked around it by aborting the update if the fragment is not visible, but this seems like the wrong thing to do.</p> <p>Here's what my Fragment looks like, edited a little for brevity:</p> <pre><code>public class FirstListFragment extends ListFragment implements LoaderManager.LoaderCallbacks&lt;Cursor&gt; { @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); // Initialize empty cursor adapter. mAdapter = new ExampleCursorAdapter(getActivity(), null, 0); setListAdapter(mAdapter); // Start with a progress indicator setListShown(false); // Prepare the loader. Either re-connect with an existing one, or start a new one. getLoaderManager().initLoader(EXAMPLE_LOADER_ID, null, this); } @Override public Loader&lt;Cursor&gt; onCreateLoader(int id, Bundle args) { return new CursorLoader(getActivity(), ExampleProvider.CONTENT_URI, PROJECTION, null, null, null); } @Override public void onLoadFinished(Loader&lt;Cursor&gt; loader, Cursor data) { // Do nothing if we're not visible. if(!isVisible()) { return; } // Swap the new cursor in. (The framework will take care of closing the // old cursor once we return.) mAdapter.swapCursor(data); // The list should now be shown. if(isResumed()) { setListShown(true); } else { setListShownNoAnimation(true); } } @Override public void onLoaderReset(Loader&lt;Cursor&gt; loader) { // This is called when the last Cursor provided to onLoadFinished() above is about to be // closed. We need to make sure we are no longer using it. mAdapter.swapCursor(null); } } </code></pre> <p>The <code>isVisible()</code> check prevents the crash, but it's not in any of the example code I've seen. What's wrong here?</p> <hr> <p><strong>Edit:</strong> Sure enough, I wound up with a StaleDataException with an old view trying to use a closed cursor. So now I'm destroying the LoaderManager when the view is destroyed. Still not sure if this is the right thing to do, and I can't reproduce the StaleDataException.</p> <p>I removed the <code>isVisible()</code> hack from above and added this:</p> <pre><code>@Override public void onDestroyView() { super.onDestroyView(); // The CursorLoader example doesn't do this, but if we get an update while the UI is // destroyed, it will crash. Why is this necessary? getLoaderManager().destroyLoader(EXAMPLE_LOADER_ID); } </code></pre>
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