Android之Activity--Loaders

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特性：
Available For Every Activity and Fragment
Asynchronous loading data
monitor source data and deliever new results when the content changes
reconnnect to the last loader's cursor.

Loader API Summary

• LoaderManager, 抽象类，关联到Activity（or Fragment）, 用于管理LoaderManager的实例。每个Activity或者Fragmente只拥有一个LoaderManager，但一个LoadManager可拥有多个loaders， 常用的有CursoLoader
• LoaderManager.LoaderCallbacks， 接口类，用于Client与LoaderManager交互
• Loader, 抽象类， 执行异步loading data, 具体类CursorLoader, 它会监视源文件，并在内容变化时传递变化结果
• AsyncTaskLoader, 抽象Loader, 提供一个AsyncTask 以便执行任务。
• CursorLoader， AsyncTaskLoader子类，查询ContentResolver 并返回Cursor。实现了Loader协议，并使用AsyncTaskLoader将任务放至后台进行处理。最好的异步加载数据 from ContentResolver的解决方案

Starting a Loader

1. onCreate()

getLoaderManaer().initLoader(int id, Bundle args, LoaderCallbacks<D> callback)
其中
id设为0，标示loader， 若该id已被使用，则直接连接上使用该id的loader,否则create one.
args设为null, 无配置数据
callback 设为this, loadmanager将汇报loader event。

当检索条件改变，希望重新查询的时候，使用getLoaderManager().restartLoader(0, null, this);
具体initLoader所做工作，可看源码追踪的时序图。

2. Using the LoaderManager Callbacks

在实现LoaderManager.LoaderCallbacks接口时，三个方法必须实现:
- onCreateLoader() — 根据所给id 实例化loader and 返回一个新的loader
- onLoadFinished() — 数据加载完成时, 参数data即是返回的结果。
- onLoaderReset() — 当数据源更新时。

可在onLoadFinished方法中，发送EVENT至Handler，然后在Handler中处理数据刷新，更新UI等工作。

Tracking Source Code

当开发者在实现LoaderCallbacks接口的onCreateLoader()方法，返回CursorLoader实例时，
在LoadManager.LoadManagerImpl.LoaderInfo.start()方法中会调用
mLoader = mCallbacks.onCreateLoader(mId, mArgs);
这样就将我们CursorLoader的实例传入了LoaderInfo的实例中。
其中mCallbacks在LoaderInfo的构造方法中进行赋值,也就是传入我们实现的LoaderCallbacks接口的实例。
public LoaderInfo(int id, Bundle args, LoaderManager.LoaderCallbacks<Object> callbacks)
接下来会调用
mLoader.startLoading()
而Loader类的startLoading()方法会调用onStartLoading()，它是一个空方法。
CursorLoader 作为Loader的子类，重写了onStartLoading()方法，代码如下：

Java@Override
    protected void onStartLoading() {
        if (mCursor != null) {
            deliverResult(mCursor);
        }
        if (takeContentChanged() || mCursor == null) {
            forceLoad();
        }
    }

可见，当mCursor为空时，或者contentChanged时，都将调用forceLoad()。也即数据源更新时，CursorLoader会自动调用onLoadComplete()，所以开发者只需在此处发送事件给Handler，让其更新数据即可。
当mCursor不为空，则直接传递结果，此处将会调用onLoadFinish()方法，将数据传递给客户端onLoadComplete()。
具体代码流程如图所示。

对于数据源更新，如何自动调用onLoadComplete()方法，简单来说，就是在进行onCreateLoader()方法中，会注册观察者，使其在数据更新时，将takeContentChanged()置为true。不再进行详细展开。

Example

Javapublic 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);
    }
}