CursorLoader解析

在应用程序中使用CursorLoader所需的元素

• An Activity or Fragment.(一个Acttivity或者Fragment)
• An instance of the LoaderManager.(LoaderManager的实例)
• A CursorLoader to load data backed by a ContentProvider. Alternatively, you can implement your own subclass of Loader or AsyncTaskLoader to load data from some other source.
• An implementation for LoaderManager.LoaderCallbacks. This is where you create new loaders and manage your references to existing loaders.(实现LoaderManager.LoaderCallbacks，点击这里查看接口详解)
• A way of displaying the loader's data, such as a SimpleCursorAdapter.
• A data source, such as a ContentProvider, when using a CursorLoader.(数据源，例如ContentProvider)

实例化一个Loader：

getLoaderManager().initLoader(0, null, this);

第一个参数：用来区分loader实例的唯一ID

第二个参数：可选参数

第三个参数：一个实现了LoaderManager.LoaderCallbacks接口的实例

 

LoaderManager.LoaderCallbacks包含三个方法：

onCreateLoader() 初始化并且返回一个给定ID的Loader

onLoadFinished()  在之前创建的loader完成之后调用

onLoaderReset() 在之前创建的loader reset时调用

 

onCreateLoader(int id,Bundle args)

在下面这个例子中，我们用到了CursorLoader，在构造函数中有以下参数：

• uri — The URI for the content to retrieve.
• projection — A list of which columns to return. Passing null will return all columns, which is inefficient.
• selection — A filter declaring which rows to return, formatted as an SQL WHERE clause (excluding the WHERE itself). Passing null will return all rows for the given URI.
• selectionArgs — You may include ?s in the selection, which will be replaced by the values from selectionArgs, in the order that they appear in the selection. The values will be bound as Strings.
• sortOrder — How to order the rows, formatted as an SQL ORDER BY clause (excluding the ORDER BY itself). Passing null will use the default sort order, which may be unordered.

 // If non-null, this is the current filter the user has provided.
String mCurFilter;
...
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    // This is called when a new Loader needs to be created.  This
    // sample only has one Loader, so we don't care about the ID.
    // First, pick the base URI to use depending on whether we are
    // currently filtering.
    Uri baseUri;
    if (mCurFilter != null) {
        baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,
                  Uri.encode(mCurFilter));
    } else {
        baseUri = Contacts.CONTENT_URI;
    }

    // Now create and return a CursorLoader that will take care of
    // creating a Cursor for the data being displayed.
    String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
            + Contacts.HAS_PHONE_NUMBER + "=1) AND ("
            + Contacts.DISPLAY_NAME + " != '' ))";
    return new CursorLoader(getActivity(), baseUri,
            CONTACTS_SUMMARY_PROJECTION, select, null,
            Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
}

 

onLoadFinish(Loader<Cursor> loader,Cursor data)

这个方法会确保在释放此loader提供的最后的数据之前调用（原文：This method is guaranteed to be called prior to the release of the last data that was supplied for this loader.）

loader会自动释放应用不再使用到的data，例如，如果这个data是一个来自CursorLoader的cursor，不应该使用close()来释放它，如果这个cursor被放到了一个CursorAdapter，应该使用swapCursor方法来确保旧的Cursor不会被关闭，例如：

// This is the Adapter being used to display the list's data.
SimpleCursorAdapter mAdapter;
...

public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
    // Swap the new cursor in.  (The framework will take care of closing the
    // old cursor once we return.)
    mAdapter.swapCursor(data);
}

 

onLoaderReset(Loader<Cursor> loader)

这个方法在已创建的loader reset的时候调用，会使得loader的数据不可用

下面的实现调用swapCursor(null):

 

// This is the Adapter being used to display the list's data.
SimpleCursorAdapter mAdapter;
...

public void onLoaderReset(Loader<Cursor> loader) {
    // This is called when the last Cursor provided to onLoadFinished()
    // above is about to be closed.  We need to make sure we are no
    // longer using it.
    mAdapter.swapCursor(null);
}

 

下面是一个完整的例子，它使用一个Fragment读取联系人，必须在manifest文件中添加READ_CONTACTS权限

public static class CursorLoaderListFragment extends ListFragment
        implements OnQueryTextListener, LoaderManager.LoaderCallbacks<Cursor> {

    // This is the Adapter being used to display the list's data.
    SimpleCursorAdapter mAdapter;

    // If non-null, this is the current filter the user has provided.
    String mCurFilter;

    @Override public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        // Give some text to display if there is no data.  In a real
        // application this would come from a resource.
        setEmptyText("No phone numbers");

        // We have a menu item to show in action bar.
        setHasOptionsMenu(true);

        // Create an empty adapter we will use to display the loaded data.
        mAdapter = new SimpleCursorAdapter(getActivity(),
                android.R.layout.simple_list_item_2, null,
                new String[] { Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },
                new int[] { android.R.id.text1, android.R.id.text2 }, 0);
        setListAdapter(mAdapter);

        // Prepare the loader.  Either re-connect with an existing one,
        // or start a new one.
        getLoaderManager().initLoader(0, null, this);
    }

    @Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // Place an action bar item for searching.
        MenuItem item = menu.add("Search");
        item.setIcon(android.R.drawable.ic_menu_search);
        item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);
        SearchView sv = new SearchView(getActivity());
        sv.setOnQueryTextListener(this);
        item.setActionView(sv);
    }

    public boolean onQueryTextChange(String newText) {
        // Called when the action bar search text has changed.  Update
        // the search filter, and restart the loader to do a new query
        // with this filter.
        mCurFilter = !TextUtils.isEmpty(newText) ? newText : null;
        getLoaderManager().restartLoader(0, null, this);
        return true;
    }

    @Override public boolean onQueryTextSubmit(String query) {
        // Don't care about this.
        return true;
    }

    @Override public void onListItemClick(ListView l, View v, int position, long id) {
        // Insert desired behavior here.
        Log.i("FragmentComplexList", "Item clicked: " + id);
    }

    // These are the Contacts rows that we will retrieve.
    static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {
        Contacts._ID,
        Contacts.DISPLAY_NAME,
        Contacts.CONTACT_STATUS,
        Contacts.CONTACT_PRESENCE,
        Contacts.PHOTO_ID,
        Contacts.LOOKUP_KEY,
    };
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        // This is called when a new Loader needs to be created.  This
        // sample only has one Loader, so we don't care about the ID.
        // First, pick the base URI to use depending on whether we are
        // currently filtering.
        Uri baseUri;
        if (mCurFilter != null) {
            baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,
                    Uri.encode(mCurFilter));
        } else {
            baseUri = Contacts.CONTENT_URI;
        }

        // Now create and return a CursorLoader that will take care of
        // creating a Cursor for the data being displayed.
        String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
                + Contacts.HAS_PHONE_NUMBER + "=1) AND ("
                + Contacts.DISPLAY_NAME + " != '' ))";
        return new CursorLoader(getActivity(), baseUri,
                CONTACTS_SUMMARY_PROJECTION, select, null,
                Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
    }

    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        // Swap the new cursor in.  (The framework will take care of closing the
        // old cursor once we return.)
        mAdapter.swapCursor(data);
    }

    public void onLoaderReset(Loader<Cursor> loader) {
        // This is called when the last Cursor provided to onLoadFinished()
        // above is about to be closed.  We need to make sure we are no
        // longer using it.
        mAdapter.swapCursor(null);
    }
}