As shown in the previous lesson, you should begin loading your data with a
CursorLoader
in your implementation of
onCreateLoader()
. The loader then provides the query results to your
Activity
or FragmentActivity
in your
implementation of LoaderCallbacks.onLoadFinished()
. One of the incoming arguments to this method is a
Cursor
containing the query results. You can use this object to
update your data display or do further processing.
Besides
onCreateLoader()
and
onLoadFinished()
,
you also have to implement
onLoaderReset()
.
This method is invoked when CursorLoader
detects
that data associated with the Cursor
has changed. When the
data changes, the framework also re-runs the current query.
Handle Query Results
To display Cursor
data returned by
CursorLoader
, use a
View
class that implements AdapterView
and
provide the view with an adapter that implements
CursorAdapter
. The system then automatically moves data from
the Cursor
to the view.
You can set up the linkage between the view and adapter before you have any data to display,
and then move a Cursor
into the adapter in the
onLoadFinished()
method. As soon as you move the Cursor
into the adapter, the
system automatically updates the view. This also happens if you change the contents of the
Cursor
.
For example:
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 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);
}
Delete Old Cursor References
The CursorLoader
is reset whenever its
Cursor
becomes invalid. This usually occurs because the data associated
with the Cursor
has changed. Before re-running the query,
the framework calls your implementation of
onLoaderReset()
. In
this callback, you should delete all references to the current Cursor
in order to prevent memory leaks. Once
onLoaderReset()
finishes, CursorLoader
re-runs its query.
For example:
/* * 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); }