A CursorLoader
runs an asynchronous query in the background against a ContentProvider
, and returns the results to the Activity
or FragmentActivity
from which it was called. This allows the Activity
or FragmentActivity
to continue to interact with the user while the query is ongoing.
Define an Activity That Uses CursorLoader
To use a CursorLoader
with an Activity
or FragmentActivity
, use the LoaderCallbacks<Cursor>
interface. A CursorLoader
invokes callbacks defined in this interface to communicate with the class; this lesson and the next one describe each callback in detail.
For example, this is how you should define a FragmentActivity
that uses the support library version of CursorLoader
. By extending FragmentActivity
, you get support for CursorLoader
as well as Fragment
:
public class PhotoThumbnailFragment extends FragmentActivity implements LoaderManager.LoaderCallbacks<Cursor> { ... }
Initialize the Query
To initialize a query, call 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 onCreate()
or onCreateView()
. For example:
// 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); ... }
Note: The method getLoaderManager()
is only available in the Fragment
class. To get a LoaderManager
in a FragmentActivity
, call getSupportLoaderManager()
.
Start the Query
As soon as the background framework is initialized, it calls your implementation of onCreateLoader()
. To start the query, return a CursorLoader
from this method. You can instantiate an empty CursorLoader
and then use its methods to define your query, or you can instantiate the object and define the query at the same time:
/* * 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. */ @Override 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: // 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; } }
Once the background framework has the object, it starts the query in the background. When the query is done, the background framework calls onLoadFinished()
, which is described in the next lesson.