android_frameworks_base/docs/html/training/load-data-background/handle-results.jd

page.title=Handling the Results
trainingnavtop=true
startpage=true

@jd:body

<!-- This is the training bar -->
<div id="tb-wrapper">
  <div id="tb">
<h2>This lesson teaches you to</h2>
<ol>
  <li>
    <a href="#HandleResults">Handle Query Results</a>
  </li>
  <li>
    <a href="#HandleReset">Delete Old Cursor References</a></li>
</ol>

<h2>Try it out</h2>
<div class="download-box">
    <a href="{@docRoot}shareables/training/ThreadSample.zip" class="button">Download the sample</a>
    <p class="filename">ThreadSample.zip</p>
</div>

  </div>
</div>

<p>
    As shown in the previous lesson, you should begin loading your data with a
    {@link android.support.v4.content.CursorLoader} in your implementation of
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader
    onCreateLoader()}. The loader then provides the query results to your
    {@link android.app.Activity} or {@link android.support.v4.app.FragmentActivity} in your
    implementation of {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoadFinished
    LoaderCallbacks.onLoadFinished()}. One of the incoming arguments to this method is a
    {@link android.database.Cursor} containing the query results. You can use this object to
    update your data display or do further processing.
</p>
<p>
    Besides
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader onCreateLoader()} and
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoadFinished onLoadFinished()},
    you also have to implement
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoaderReset onLoaderReset()}.
    This method is invoked when {@link android.support.v4.content.CursorLoader} detects
    that data associated with the {@link android.database.Cursor} has changed. When the
    data changes, the framework also re-runs the current query.
</p>
<h2 id="HandleResults">Handle Query Results</h2>
<p>
    To display {@link android.database.Cursor} data returned by
    {@link android.support.v4.content.CursorLoader}, use a
    {@link android.view.View} class that implements {@link android.widget.AdapterView} and
    provide the view with an adapter that implements
    {@link android.support.v4.widget.CursorAdapter}. The system then automatically moves data from
    the {@link android.database.Cursor} to the view.
</p>
<p>
    You can set up the linkage between the view and adapter before you have any data to display,
    and then move a {@link android.database.Cursor} into the adapter in the
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoadFinished onLoadFinished()}
    method. As soon as you move the {@link android.database.Cursor} into the adapter, the
    system automatically updates the view. This also happens if you change the contents of the
    {@link android.database.Cursor}.
</p>
<p>
    For example:
</p>
<pre>
public String[] mFromColumns = {
    DataProviderContract.IMAGE_PICTURENAME_COLUMN
};
public int[] mToFields = {
    R.id.PictureName
};
// Gets a handle to a List View
ListView mListView = (ListView) findViewById(R.id.dataList);
/*
 * Defines a SimpleCursorAdapter for the ListView
 *
 */
SimpleCursorAdapter mAdapter =
    new SimpleCursorAdapter(
            this,                // Current context
            R.layout.list_item,  // Layout for a single row
            null,                // No Cursor yet
            mFromColumns,        // Cursor columns to use
            mToFields,           // Layout fields to use
            0                    // No flags
    );
// Sets the adapter for the view
mListView.setAdapter(mAdapter);
...
/*
 * Defines the callback that {@link android.support.v4.content.CursorLoader} calls
 * when it's finished its query
 */
&#64;Override
public void onLoadFinished(Loader&lt;Cursor&gt; loader, Cursor cursor) {
    ...
    /*
     * Moves the query results into the adapter, causing the
     * ListView fronting this adapter to re-display
     */
    mAdapter.changeCursor(cursor);
}
</pre>
<h2 id="HandleReset">Delete Old Cursor References</h2>
<p>
    The {@link android.support.v4.content.CursorLoader} is reset whenever its
    {@link android.database.Cursor} becomes invalid. This usually occurs because the data associated
    with the {@link android.database.Cursor} has changed. Before re-running the query,
    the framework calls your implementation of
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoaderReset onLoaderReset()}. In
    this callback, you should delete all references to the current {@link android.database.Cursor}
    in order to prevent memory leaks. Once
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoaderReset onLoaderReset()}
    finishes, {@link android.support.v4.content.CursorLoader} re-runs its query.
</p>
<p>
    For example:
</p>
<pre>
/*
 * Invoked when the CursorLoader is being reset. For example, this is
 * called if the data in the provider changes and the Cursor becomes stale.
 */
&#64;Override
public void onLoaderReset(Loader&lt;Cursor&gt; loader) {
    
    /*
     * Clears out the adapter's reference to the Cursor.
     * This prevents memory leaks.
     */
    mAdapter.changeCursor(null);
}
</pre>