• 加载器Loaders使用的两个程序样例

    包括官方例子
    服务器君一共花费 12.437 ms 进行了 3 次数据库查询,努力地为您提供了这个页面。
    广告很萌的

    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。

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现代魔法 推荐于 2013-02-27 10:23   

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