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POAsyncTaskLoader onLoadFinished with a pending task and config change
 

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  1. POAsyncTaskLoader onLoadFinished with a pending task and config change
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    <p>I'm trying to use an <code>AsyncTaskLoader</code> to load data in the background to populate a detail view in response to a list item being chosen. I've gotten it mostly working but I'm still having one issue. If I choose a second item in the list and then rotate the device <em>before the load for the first selected item has completed</em>, then the <code>onLoadFinished()</code> call is reporting to the activity being stopped rather than the new activity. This works fine when choosing just a single item and then rotating.</p> <p>Here is the code I'm using. Activity:</p> <pre><code>public final class DemoActivity extends Activity implements NumberListFragment.RowTappedListener, LoaderManager.LoaderCallbacks&lt;String&gt; { private static final AtomicInteger activityCounter = new AtomicInteger(0); private int myActivityId; private ResultFragment resultFragment; private Integer selectedNumber; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); myActivityId = activityCounter.incrementAndGet(); Log.d("DemoActivity", "onCreate for " + myActivityId); setContentView(R.layout.demo); resultFragment = (ResultFragment) getFragmentManager().findFragmentById(R.id.result_fragment); getLoaderManager().initLoader(0, null, this); } @Override protected void onDestroy() { super.onDestroy(); Log.d("DemoActivity", "onDestroy for " + myActivityId); } @Override public void onRowTapped(Integer number) { selectedNumber = number; resultFragment.setResultText("Fetching details for item " + number + "..."); getLoaderManager().restartLoader(0, null, this); } @Override public Loader&lt;String&gt; onCreateLoader(int id, Bundle args) { return new ResultLoader(this, selectedNumber); } @Override public void onLoadFinished(Loader&lt;String&gt; loader, String data) { Log.d("DemoActivity", "onLoadFinished reporting to activity " + myActivityId); resultFragment.setResultText(data); } @Override public void onLoaderReset(Loader&lt;String&gt; loader) { } static final class ResultLoader extends AsyncTaskLoader&lt;String&gt; { private static final Random random = new Random(); private final Integer number; private String result; ResultLoader(Context context, Integer number) { super(context); this.number = number; } @Override public String loadInBackground() { // Simulate expensive Web call try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } return "Item " + number + " - Price: $" + random.nextInt(500) + ".00, Number in stock: " + random.nextInt(10000); } @Override public void deliverResult(String data) { if (isReset()) { // An async query came in while the loader is stopped return; } result = data; if (isStarted()) { super.deliverResult(data); } } @Override protected void onStartLoading() { if (result != null) { deliverResult(result); } // Only do a load if we have a source to load from if (number != null) { forceLoad(); } } @Override protected void onStopLoading() { // Attempt to cancel the current load task if possible. cancelLoad(); } @Override protected void onReset() { super.onReset(); // Ensure the loader is stopped onStopLoading(); result = null; } } } </code></pre> <p>List fragment:</p> <pre><code>public final class NumberListFragment extends ListFragment { interface RowTappedListener { void onRowTapped(Integer number); } private RowTappedListener rowTappedListener; @Override public void onAttach(Activity activity) { super.onAttach(activity); rowTappedListener = (RowTappedListener) activity; } @Override public void onActivityCreated(Bundle savedInstanceState) { super.onActivityCreated(savedInstanceState); ArrayAdapter&lt;Integer&gt; adapter = new ArrayAdapter&lt;Integer&gt;(getActivity(), R.layout.simple_list_item_1, Arrays.asList(1, 2, 3, 4, 5, 6)); setListAdapter(adapter); } @Override public void onListItemClick(ListView l, View v, int position, long id) { ArrayAdapter&lt;Integer&gt; adapter = (ArrayAdapter&lt;Integer&gt;) getListAdapter(); rowTappedListener.onRowTapped(adapter.getItem(position)); } } </code></pre> <p>Result fragment:</p> <pre><code>public final class ResultFragment extends Fragment { private TextView resultLabel; @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View root = inflater.inflate(R.layout.result_fragment, container, false); resultLabel = (TextView) root.findViewById(R.id.result_label); if (savedInstanceState != null) { resultLabel.setText(savedInstanceState.getString("labelText", "")); } return root; } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); outState.putString("labelText", resultLabel.getText().toString()); } void setResultText(String resultText) { resultLabel.setText(resultText); } } </code></pre> <p>I've been able to get this working using plain <code>AsyncTask</code>s but I'm trying to learn more about <code>Loader</code>s since they handle the configuration changes automatically.</p> <hr> <p><strong>EDIT</strong>: I think I may have tracked down the issue by looking at the source for <a href="http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/4.0.3_r1/android/app/LoaderManager.java?av=f#552">LoaderManager</a>. When <code>initLoader</code> is called after the configuration change, the <code>LoaderInfo</code> object has its <code>mCallbacks</code> field updated with the new activity as the implementation of <code>LoaderCallbacks</code>, as I would expect.</p> <pre><code>public &lt;D&gt; Loader&lt;D&gt; initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks&lt;D&gt; callback) { if (mCreatingLoader) { throw new IllegalStateException("Called while creating a loader"); } LoaderInfo info = mLoaders.get(id); if (DEBUG) Log.v(TAG, "initLoader in " + this + ": args=" + args); if (info == null) { // Loader doesn't already exist; create. info = createAndInstallLoader(id, args, (LoaderManager.LoaderCallbacks&lt;Object&gt;)callback); if (DEBUG) Log.v(TAG, " Created new loader " + info); } else { if (DEBUG) Log.v(TAG, " Re-using existing loader " + info); info.mCallbacks = (LoaderManager.LoaderCallbacks&lt;Object&gt;)callback; } if (info.mHaveData &amp;&amp; mStarted) { // If the loader has already generated its data, report it now. info.callOnLoadFinished(info.mLoader, info.mData); } return (Loader&lt;D&gt;)info.mLoader; } </code></pre> <p>However, when there is a pending loader, the main <code>LoaderInfo</code> object also has an <code>mPendingLoader</code> field with a reference to a <code>LoaderCallbacks</code> as well, and this object is never updated with the new activity in the <code>mCallbacks</code> field. I would expect to see the code look like this instead:</p> <pre><code>// This line was already there info.mCallbacks = (LoaderManager.LoaderCallbacks&lt;Object&gt;)callback; // This line is not currently there info.mPendingLoader.mCallbacks = (LoaderManager.LoaderCallbacks&lt;Object&gt;)callback; </code></pre> <p>It appears to be because of this that the pending loader calls <code>onLoadFinished</code> on the old activity instance. If I breakpoint in this method and make the call that I feel is missing using the debugger, everything works as I expect.</p> <p>The new question is: Have I found a bug, or is this the expected behavior?</p>
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    1. USPaul Blessing
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    1. COTake a look at the [`CursorLoader.java`](http://grepcode.com/file_/repository.grepcode.com/java/ext/com.google.android/android/4.0.1_r1/android/content/CursorLoader.java/?v=source) source code. Try implementing `onStartLoading`, `onStopLoading`, `onCanceled`, `onReset`, and `deliverResult` similar to how the source code does... the `LoaderManager` assumes that all of these methods are implemented correctly. This might be why your implementation is only partially working across configuration changes.
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      1. POAsyncTaskLoader onLoadFinished with a pending task and config change
      1. USAlex Lockwood
    2. COSo it turns out that `onLoadFinished()` *is* actually being called - it's just reporting to the old activity (the one from before the configuration change) rather than to the new one. Question has been edited and the code has been updated.
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      1. USPaul Blessing
    3. COHonestly, I think the problem here is the triviality of this example itself... in a real life situation the `Loader` wouldn't contain the actual data source (i.e. the `private final int` field is your actual data source, is it not?). `Loader`s are also supposed to monitor their data source and report back when changes are made. In most cases `onLoadFinished` won't be called on your new Activity after a configuration change since the Loader is smart enough to retain its old data... it will only re-load if it sees that changes to its backing data has been made.
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      1. USAlex Lockwood
 

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