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
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。
现代魔法 推荐于 2013-02-27 10:23