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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">Clear Out Old Data</a></li>
</ol>
</div>
</div>
<p>
{@link android.support.v4.content.CursorLoader} returns its query results to your
implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoadFinished
LoaderCallbacks.onLoadFinished()}, in the form of a {@link android.database.Cursor}. In the
callback, you can update your data display, do further processing on the
{@link android.database.Cursor} data, and so forth.
</p>
<p>
When the loader framework detects changes to data associated with the query,
it resets the {@link android.support.v4.content.CursorLoader}, closes the current
{@link android.database.Cursor}, and then invokes your implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoaderReset onLoaderReset()}.
Use this callback to delete references to the current {@link android.database.Cursor}; when the
loader framework destroys the {@link android.database.Cursor}, you won't have outstanding
references that cause memory leaks.
</p>
<h2 id="HandleFinished">Handle Query Results</h2>
<p>
The following two snippets are an example of displaying the results of a query, using a
{@link android.widget.ListView} backed by a
{@link android.support.v4.widget.SimpleCursorAdapter}.
</p>
<p>
The first snippet shows the {@link android.widget.ListView} and
{@link android.support.v4.widget.SimpleCursorAdapter}:
</p>
<pre>
// Gets a handle to the Android built-in ListView widget
mListView = ((ListView) findViewById(android.R.id.list));
// Creates a CursorAdapter
mAdapter =
new SimpleCursorAdapter(
this, // Current context
R.layout.logitem, // View for each item in the list
null, // Don't provide the cursor yet
FROM_COLUMNS, // List of cursor columns to display
TO_FIELDS, // List of TextViews in each line
0 // flags
);
// Links the adapter to the ListView
mListView.setAdapter(mAdapter);
</pre>
<p>
The next snippet shows an implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoadFinished onLoadFinished()}
that moves the query results in the returned {@link android.database.Cursor} to the
{@link android.support.v4.widget.SimpleCursorAdapter}. Changing the
{@link android.database.Cursor} in the
{@link android.support.v4.widget.SimpleCursorAdapter} triggers a refresh of the
{@link android.widget.ListView} with the new data:
</p>
<pre>
public void onLoadFinished(Loader<Cursor> loader, Cursor cursor)
{
/*
* Move the results into the adapter. This
* triggers the ListView to re-display.
*/
mAdapter.swapCursor(cursor);
}
</pre>
<h2 id="HandleReset">Handle a Loader Reset</h2>
<p>
The loader framework resets the {@link android.support.v4.content.CursorLoader} whenever the
{@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, make sure to prevent memory leaks by deleting all references to the current
{@link android.database.Cursor}. Once you return from
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoaderReset onLoaderReset()},
the loader framework re-runs the query.
</p>
<p>
For example:
</p>
<pre>
public void onLoaderReset(Loader<Cursor> loader)
{
// Remove the reference to the current Cursor
mAdapter.swapCursor(null);
}
</pre>
|
