1.加载器特征

• 用于每个 Activity 和 Fragment
• 支持异步加载数据。
• 监控其数据源并在内容变化时传递新结果。

2. Loader API

3. 在应用中使用Loader

主要步骤

1. Activity 或 Fragment
2. LoaderManager 的实例
3. 一个 CursorLoader，用于加载由 ContentProvider 支持的数据。您也可以实现自己的 Loader 或 AsyncTaskLoader 子类，从其他源中加载数据。
4. 一个 LoaderManager.LoaderCallbacks 实现。您可以使用它来创建新加载器，并管理对现有加载器的引用。
5. 一种显示加载器数据的方法，如 SimpleCursorAdapter。
6. 使用 CursorLoader 时的数据源，如 ContentProvider。

3.1. 启动加载器

在 Activity 的 onCreate() 方法或片段的onActivityCreated() 方法内初始化 Loader

// Prepare the loader.  Either re-connect with an existing one,

// or start a new one.

// 标识ID + 构建时给加载器的可选参数 + LoaderManager.LoaderCallbacks 实现

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

// 如果 ID 指定的加载器已存在，则将重复使用上次创建的加载器。

// 如果 ID 指定的加载器不存在，则 initLoader() 将触发 LoaderManager.LoaderCallbacks 方法 onCreateLoader()

3.2. 重启加载器

public boolean onQueryTextChanged(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;

}

3.3. 使用LoaderManager回调

LoaderManager.LoaderCallbacks 是一个支持客户端与 LoaderManager 交互的回调接口。

• 加载器（特别是 CursorLoader）在停止运行后，仍需保留其数据。这样，应用即可保留 Activity 或片段的 onStop() 和 onStart() 方法中的数据。当用户返回应用时，无需等待它重新加载这些数据。您可使用 LoaderManager.LoaderCallbacks 方法了解何时创建新加载器，并告知应用何时停止使用加载器的数据。
• LoaderManager.LoaderCallbacks 包括以下方法：
  • onCreateLoader()：针对指定的 ID 进行实例化并返回新的 Loader
  • onLoadFinished() ：将在先前创建的加载器完成加载时调用
  • onLoaderReset()：将在先前创建的加载器重置且其数据因此不可用时调用

4. 实例： 访问用户联系人

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);

    }

}