docs/html/training/load-data-background/setup-loader.jd

page.title=Running a Query with a CursorLoader
trainingnavtop=true
startpage=true

@jd:body

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<h2>This lesson teaches you to</h2>
<ol>
    <li>
        <a href="#Extend">Define an Activity That Uses CursorLoader</a>
    </li>
    <li>
        <a href="#InitializeLoader">Initialize the Query</a>
    </li>
    <li>
        <a href="#DefineLaunch">Start the Query</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>
    A {@link android.support.v4.content.CursorLoader} runs an asynchronous query in the background
    against a {@link android.content.ContentProvider}, and returns the results to the
    {@link android.app.Activity} or {@link android.support.v4.app.FragmentActivity} from which it
    was called. This allows the {@link android.app.Activity} or
    {@link android.support.v4.app.FragmentActivity} to continue to interact with the user while the
    query is ongoing.
</p>
<h2 id="Extend">Define an Activity That Uses CursorLoader</h2>
<p>
    To use a {@link android.support.v4.content.CursorLoader} with an
    {@link android.app.Activity} or {@link android.support.v4.app.FragmentActivity}, use the
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks LoaderCallbacks&lt;Cursor&gt;}
    interface. A {@link android.support.v4.content.CursorLoader} invokes callbacks defined
    in this interface to communicate with the class; this lesson and the next one
    describe each callback in detail.
</p>
<p>
    For example, this is how you should define a {@link android.support.v4.app.FragmentActivity}
    that uses the support library version of {@link android.support.v4.content.CursorLoader}. By
    extending {@link android.support.v4.app.FragmentActivity}, you get support for
    {@link android.support.v4.content.CursorLoader} as well as
    {@link android.support.v4.app.Fragment}:
</p>
<pre>
public class PhotoThumbnailFragment extends FragmentActivity implements
        LoaderManager.LoaderCallbacks&lt;Cursor&gt; {
...
}
</pre>
<h2 id="InitializeLoader">Initialize the Query</h2>
<p>
    To initialize a query, call
    {@link android.support.v4.app.LoaderManager#initLoader LoaderManager.initLoader()}. This
    initializes the background framework. You can do this after the user has entered data that's
    used in the query, or, if you don't need any user data, you can do it in
    {@link android.support.v4.app.FragmentActivity#onCreate onCreate()} or
    {@link android.support.v4.app.Fragment#onCreateView onCreateView()}. For example:
</p>
<pre>
    // Identifies a particular Loader being used in this component
    private static final int URL_LOADER = 0;
    ...
    /* When the system is ready for the Fragment to appear, this displays
     * the Fragment's View
     */
    public View onCreateView(
            LayoutInflater inflater,
            ViewGroup viewGroup,
            Bundle bundle) {
        ...
        /*
         * Initializes the CursorLoader. The URL_LOADER value is eventually passed
         * to onCreateLoader().
         */
        getLoaderManager().initLoader(URL_LOADER, null, this);
        ...
    }
</pre>
<p class="note">
    <strong>Note:</strong> The method {@link android.support.v4.app.Fragment#getLoaderManager
    getLoaderManager()} is only available in the {@link android.support.v4.app.Fragment} class. To
    get a {@link android.support.v4.app.LoaderManager} in a
    {@link android.support.v4.app.FragmentActivity}, call
    {@link android.support.v4.app.FragmentActivity#getSupportLoaderManager
    getSupportLoaderManager()}.
</p>
<h2 id="DefineLaunch">Start the Query</h2>
<p>
    As soon as the background framework is initialized, it calls your implementation of
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onCreateLoader onCreateLoader()}.
    To start the query, return a {@link android.support.v4.content.CursorLoader} from this method.
    You can instantiate an empty {@link android.support.v4.content.CursorLoader} and then use its
    methods to define your query, or you can instantiate the object and define the query at the
    same time:
</p>
<pre>
/*
* Callback that's invoked when the system has initialized the Loader and
* is ready to start the query. This usually happens when initLoader() is
* called. The loaderID argument contains the ID value passed to the
* initLoader() call.
*/
&#64;Override
public Loader&lt;Cursor&gt; onCreateLoader(int loaderID, Bundle bundle)
{
    /*
     * Takes action based on the ID of the Loader that's being created
     */
    switch (loaderID) {
        case URL_LOADER:
            // Returns a new CursorLoader
            return new CursorLoader(
                        getActivity(),   // Parent activity context
                        mDataUrl,        // Table to query
                        mProjection,     // Projection to return
                        null,            // No selection clause
                        null,            // No selection arguments
                        null             // Default sort order
        );
        default:
            // An invalid id was passed in
            return null;
    }
}
</pre>
<p>
    Once the background framework has the object, it starts the query in the background. When the
    query is done, the background framework calls
    {@link android.support.v4.app.LoaderManager.LoaderCallbacks#onLoadFinished onLoadFinished()},
    which is described in the next lesson.
</p>