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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
*/
@Override
public void onLoadFinished(Loader<Cursor> 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.
*/
@Override
public void onLoaderReset(Loader<Cursor> loader) {
/*
* Clears out the adapter's reference to the Cursor.
* This prevents memory leaks.
*/
mAdapter.changeCursor(null);
}
</pre>
|
