編輯:關於Android編程
Loaders,中文可理解為“加載器”,在Android3.0中新增。從字面含義可見其功能,即提供數據加載。特別地,加載數據的方式為異步。它具有以下特點:
Loaders用於所有的Activity和Fragment; 提供異步數據裝載機制;監控他們的來源數據變化情況,在數據發生變化的時候傳遞新的結果;自動重連到最後一個數據加載器游標,因此不需要重新查詢數據
如何在應用中使用Loaders
使用Loaders的先決條件:
需要一個Activity 或者 Fragmnet一個LoaderManager實例一個用於加載數據的的CursorLoader對象(依賴於ContentProvider)一個LoaderManager.LoaderCallbacks的實現類.一個數據展現適配器,比如SimpleCursorAdapter一個數據源,比如ContentProvider
啟動數據加載器Loaders
LoaderManager管理者一個Activity或者Fragment中的一個或多個Loader實例,每個Activity或者Fragment只有對應一個LoaserManager。
一般在Activity的onCreate方法或者Fragment的onActivityCreated方法中初始化一個Loader:
getLoaderManager().initLoader(0, null, this);
參數:
1、 第一個參數:0 為Loader的唯一標識ID;
2、 第二個參數: 為Loader的構造器可選參數,這裡為null;
3、 第三個參數:this,這裡表示當前Activity對象或者Fragment對象,提供給LoaderManager對象進行數據匯報。
InitLoader()方法保證了Loader初始化及對象激活,執行這個方法有2個可能的結果:
1、 如果ID存在,則重復利用;
2、 如果ID不存在,則出發LoaderManager.LoaderCallbacks的onCreateLoader()方法新創建一個Loader並返回;
不管在什麼情況下,只有Loader狀態發生了變化,與之關聯的LoaderManager.LoaderCallbacks實現類都會被告知;
你可能注意到了,initLoader返回的Loader對象並未與任何變量關聯,那是因為LoaderManager有自動的Loader管理功能;LoaderManager在必要的時候自動啟動及停止數據加載操作,並且維護者Loader的狀態;這就意味著,你很少直接與Loader進行交互。一般地,使用LoaderManager.LoaderCallbacks的onCreateLoader()方法去干預數據加載的過程中的特殊事件。
如何重啟數據加載器Loaders
在上面創建Loaders時,如果ID不存在則創建,否則使用舊的Loader,但有些時候,我們需要清理掉舊的數據重新開始。
使用restartLoaser()可以做到。比如,SearchView.OnQueryTextListener的實現類,在查詢條件發生改變時重啟Loaders,以便獲取最新的查詢結果。
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 接口是客戶端與LoaderManager進行交互的腰帶。
Loader ,特別是CursorLoader,期望在停止狀態後保存它們的狀態。這樣的話,用戶在交互過程中,就避免了數據的重新加載而導致UI卡死的局面。使用回調函數,就可以知道什麼時候去創建一個新的Loader,並且告知應用程序什麼時候停止使用Loader加載的數據。
回調方法有:
onCreateLoader():根據給定的ID創建新的Loader;onLoaderFinished():當Loader完成數據加載後調用;onLoaderReset():Loader重置,使之前的數據無效;
// If non-null, this is the current filter the user has provided. String mCurFilter; ... public LoaderonCreateLoader(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); }
完整的實例
public static class CursorLoaderListFragment extends ListFragment implements OnQueryTextListener, LoaderManager.LoaderCallbacks{ // 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 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 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 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); } }
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