前面提到了加载器Loaders，还有LoaderManager，这里就以具体程序为例，加深对这些概念的理解吧。

下面是官方程序例子：

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

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

    // The SearchView for doing filtering.
    SearchView mSearchView;

    // 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);

        // Start out with a progress indicator.
        setListShown(false);

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

    public static class MySearchView extends SearchView {
        public MySearchView(Context context) {
            super(context);
        }

        // The normal SearchView doesn't clear its search text when
        // collapsed, so we will do this for it.
        @Override
        public void onActionViewCollapsed() {
            setQuery("", false);
            super.onActionViewCollapsed();
        }
    }

    @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
                | MenuItem.SHOW_AS_ACTION_COLLAPSE_ACTION_VIEW);
        mSearchView = new MySearchView(getActivity());
        mSearchView.setOnQueryTextListener(this);
        mSearchView.setOnCloseListener(this);
        mSearchView.setIconifiedByDefault(true);
        item.setActionView(mSearchView);
    }

    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.
        String newFilter = !TextUtils.isEmpty(newText) ? newText : null;
        // Don't do anything if the filter hasn't actually changed.
        // Prevents restarting the loader when restoring state.
        if (mCurFilter == null && newFilter == null) {
            return true;
        }
        if (mCurFilter != null && mCurFilter.equals(newFilter)) {
            return true;
        }
        mCurFilter = newFilter;
        getLoaderManager().restartLoader(0, null, this);
        return true;
    }

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

    @Override
    public boolean onClose() {
        if (!TextUtils.isEmpty(mSearchView.getQuery())) {
            mSearchView.setQuery(null, true);
        }
        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);

        // The list should now be shown.
        if (isResumed()) {
            setListShown(true);
        } else {
            setListShownNoAnimation(true);
        }
    }

    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);
    }
}

注释已经说得比较清楚了，所以这里就不再罗嗦。下面再看看另一个例子：

import android.app.Activity;
import android.app.FragmentManager;
import android.app.LoaderManager;
import android.content.ContentValues;
import android.database.Cursor;
import android.os.Bundle;

public class ToDoListActivity extends Activity implements NewItemFragment.OnNewItemAddedListener,
        LoaderManager.LoaderCallbacks<Cursor>{
    //获得对UI小组件的引用
    private ArrayList<ToDoItem> todoItems;
    private ToDoItemAdapter aa;
    @Override
    // onCreate是创建这个activity的时候会调用的
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        // 获取该Fragment的引用
        FragmentManager fm = getFragmentManager();
        ToDoListFragment todoListFragment = (ToDoListFragment)fm.findFragmentById(R.id.TodoListFragment);

        todoItems = new ArrayList<ToDoItem>();

        // 这里需要将数据库中存储的东西都读取出来
        int resID = R.layout.todolist_item;

        aa = new ToDoItemAdapter(this, resID, todoItems);

        todoListFragment.setListAdapter(aa);
        getLoaderManager().initLoader(0, null, this);
        getLoaderManager().enableDebugLogging(true);
    }

    @Override
    // onResume是暂停以后重新启动这个Activity时候调用
    protected void onResume() {
        super.onResume();
        getLoaderManager().restartLoader(0, null, this);
    }

    @Override
    public void onNewItemAdded(String newItem) {
        ContentResolver cr = getContentResolver();

        ContentValues values = new ContentValues();
        values.put(ToDoContentProvider.KEY_TASK, newItem);

        cr.insert(ToDoContentProvider.CONTENT_URI, values);
        getLoaderManager().restartLoader(0, null, this);
    }

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        CursorLoader loader = new CursorLoader(this, ToDoContentProvider.CONTENT_URI,
                null, null, null, null);
        return loader;
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
    // 当loader查询完成的时候，Cursor会返回到onLoadFinished处理程序。
        int keyTaskIndex = cursor.getColumnIndexOrThrow(ToDoContentProvider.KEY_TASK);
        todoItems.clear();
        while (cursor.moveToNext()) {
            ToDoItem newItem = new ToDoItem(cursor.getString(keyTaskIndex));
            todoItems.add(newItem);
        }
        aa.notifyDataSetChanged();
    }

    @Override
    public void onLoaderReset(Loader<Cursor> arg0) {
        // TODO Auto-generated method stub
    }

}

在这个例子中，在创建activity的时候（onCreate）调用了：

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

这里的第一个参数0是指loader的id，我们并不关注它，所以设置了一个0。第二个参数是给Loader初始化的时候传递的参数（也就是onCreateLoader中的第二个参数）。

这里的第三个参数LoaderCallbacks<D>使用的直接是Activity类，所以这个类需要实现LoaderCallbacks的三个方法：

• onCreateLoader
• onLoadFinished
• onLoaderReset

在onCreateLoader中创建CursorLoader，然后在onLoadFinished中重新渲染ViewList。

在3.0之后Android的官方文档强烈推荐使用Loader来做数据的加载。因此在能使用这个的情况下就尽量使用Loader吧。

使用需要先确定一个类来实现LoaderCallbacks<D>接口，然后实现接口的三个方法。之后使用getLoaderManager来获取LoadManager，再调用initLoader来创建loader，把实际的修改页面的逻辑放在onLoadFinished中。

当然Loader并不只有CursorLoader，你也可以自己定义loader。