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page.title=Defining and Launching the Query
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="#DefineLaunch">Define and Launch the Query</a>
</li>
</ol>
</div>
</div>
<p>
To perform a query, create the {@link android.support.v4.content.CursorLoader}, set up its
query, and pass it to the loader framework. From then on, the framework manages everything.
It runs the query on a background thread, returns the results to the foreground, and
watches for changes to the data associated with the query.
</p>
<p>
Pass a {@link android.support.v4.content.CursorLoader} to the loader framework in
your implementation of
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader onCreateLoader()}.
The loader framework calls this method when you <i>create</i> a loader by calling
{@link android.support.v4.app.LoaderManager#initLoader initLoader()}. You can create
a {@link android.support.v4.content.CursorLoader} anywhere,
but the preferred way is to create it in
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader onCreateLoader()},
because this defers creation until the object is actually needed.
</p>
<p>
Notice that {@link android.support.v4.app.LoaderManager#initLoader initLoader()} will only
{@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader onCreateLoader()}
if the {@link android.support.v4.content.CursorLoader} doesn't already exist; otherwise, it
re-uses the existing {@link android.support.v4.content.CursorLoader}. The loader framework
tracks {@link android.support.v4.content.CursorLoader} instance using the <code>id</code>
value passed to {@link android.support.v4.app.LoaderManager#initLoader initLoader()}.
</p>
<h2 id="DefineLaunch">Define and Launch the Query</h2>
<p>
To create a {@link android.support.v4.content.CursorLoader} and define its
query at the same time, call the constructor
{@link android.support.v4.content.CursorLoader#CursorLoader(Context, Uri, String[], String, String[], String)
CursorLoader(context, uri, projection, selection, selectionArgs, sortOrder)}. The
<code>context</code> and <code>uri</code> arguments are required, but the others are optional.
To use the default value for an optional argument, pass in <code>null</code>. The
{@link android.support.v4.content.CursorLoader} runs the query against the
{@link android.content.ContentProvider} identified by <code>uri</code>, just as if you had
called {@link android.content.ContentResolver#query ContentResolver.query()} with the same
arguments.
</p>
<p>
For example:
</p>
<pre>
public Loader<Cursor> onCreateLoader(int loaderID, Bundle bundle)
{
/*
* Takes action based on the ID of the Loader that's being created
*/
switch (loaderID) {
case URL_LOADER:
/*
* Return a new CursorLoader
*/
return new CursorLoader(
this, // Context
DataProviderContract.IMAGE_URI, // Provider's content URI
PROJECTION, // Columns to return
null, // Return all rows
null, // No search arguments
null); // Default search order
default:
// An invalid id was passed in
return null;
}
}
</pre>
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