Loader

starting  a  loader:

   

    The LoaderManager manages one or more Loader instances within an Activity or Fragment. There is only one LoaderManager per activity or fragment.

    你在Activity中oncreate()方法中，或是在fragment中的 onActivityCreated()方法中实现

  代码如下：

   getLoaderManager().initLoader(0, null, this);其中第一个参数：loader的唯一标识符，第二个是一个可选参数提供给Loader的构造函数，第三个参数：对LoaderManager.LoaderCallbacks的实现，调用loaderManager报告loader事件。

   LoaderManager会自动的管理Loader，很少直接对loader进行操作，通常是使用LoaderManager.LoaderCallbacks方法来处理loading过程。

Restarting a Loader：

   当你想抛弃你的数据的时候，需要调用restartLoader().

   例如：

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;
}

 

LoaderManager.LoaderCallbacks

LoaderManager.LoaderCallbacks 包括如下的方法：

  onCreateLoader（）：返回一个给定ID号的Loader；

  onLoaderFinished（）：当先前的Loader完成了load的时候被调用

  onLoaderReset（）：数据难以获取的时候，重启先前的Loader

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); } }