LoaderManager - Android 3 0中的新功能

青旅半醒 2022-03-20 13:26 202阅读 0赞

在Android 3.0中提供了一个新概念Loaders，通过LoaderManager类可以很轻松的异步加载数据从Fragment或Activity 中，Loaders提供了回调机制通知最终的运行结果，有点类似AsyncTask类，但由于Loader对于并发可以用过Loader管理器统一管理，所以更适合批量处理多个异步任务的处理(当然内部仍然是多线程)。下面就让Android123一起和大家看下honeycomb中的新特性吧，对于解决多重异步I/O加快Android平板应用的运行是十分有效的。  
  
**一、LoaderManager**  
  
LoaderManager类位于android.app.LoaderManager，提供了以下几个方法：  
  
abstract void destroyLoader(int id) //停止并移除loader通过ID  
abstract void dump(String prefix, FileDescriptor fd, PrintWriter writer, String\[\] args) //打印LoaderManager的状态到一个流中  
static void enableDebugLogging(boolean enabled) //启用debug记录  
abstract <D> Loader<D> getLoader(int id) //返回找到的ID或没有匹配的在Loader中  
abstract <D> Loader<D> initLoader(int id, Bundle args, LoaderCallbacks<D> callback) //初始化Loader使其成为活动状态  
abstract <D> Loader<D> restartLoader(int id, Bundle args, LoaderCallbacks<D> callback) //启动一个新的或重启一个存在的Loader在管理器中  
  
同时LoaderManager还有一个回调接口android.app.LoaderManager.LoaderCallbacks<D> 用于和LoaderManager交互：  
  
abstract Loader<D> onCreateLoader(int id, Bundle args) //举例并返回一个新Loader通过ID  
abstract void onLoadFinished(Loader<D> loader, D data) //当前面一个Loader已经完成时回调  
abstract void onLoaderReset(Loader<D> loader) //当一个新的loader或存在的loader重启时回调  
  
**二、Loader**  
  
Loader类位于android.content.Loader<D>，整体比较复杂，主要成员有  
  
1. 构造方法 Loader(Context context) //作为唯一实例化方法参数只有一个Context  
  
2. Public Methods  
  
void abandon() //高速Loader他在绑定  
String dataToString(D data) //用于调试，转换一个Loader数据类的实例为字符串用于打印  
void deliverResult(D data) //发送一个load注册的listener结果  
void dump(String prefix, FileDescriptor fd, PrintWriter writer, String\[\] args) //打印loader状态通过给定的流  
void forceLoad() //强制一个异步载入  
Context getContext() //返回Context实例  
int getId()  
boolean isAbandoned() //判断是否已经绑定  
boolean isReset() //判断是否已经重启  
boolean isStarted() //判断是否已经执行  
void onContentChanged() //内容变化回调  
  
registerListener(int id, OnLoadCompleteListener<D> listener)  
void reset() //重置一个Loader的状态  
final void startLoading() //启动一个异步的载入从Loader的数据  
void stopLoading() //停止载入  
boolean takeContentChanged() String toString()  
void unregisterListener(OnLoadCompleteListener<D> listener)  
  
提供的子类 android.content.Loader.ForceLoadContentObserver 和 接口 android.content.Loader.OnLoadCompleteListener<D>  
  
为了更清晰的表达Android开发网给出一个SDK例子完整代码，来作分析：

public class LoaderThrottle extends Activity \{ static final String TAG = "LoaderThrottle"; public static final String AUTHORITY = "com.example.android.apis.app.LoaderThrottle"; public static final class MainTable implements BaseColumns \{ // This class cannot be instantiated private MainTable() \{\} public static final String TABLE\_NAME = "main"; public static final Uri CONTENT\_URI = Uri.parse("content://" + AUTHORITY + "/main"); public static final Uri CONTENT\_ID\_URI\_BASE = Uri.parse("content://" + AUTHORITY + "/main/"); public static final String CONTENT\_TYPE = "vnd.android.cursor.dir/vnd.example.api-demos-throttle"; public static final String CONTENT\_ITEM\_TYPE = "vnd.android.cursor.item/vnd.example.api-demos-throttle"; public static final String DEFAULT\_SORT\_ORDER = "data COLLATE LOCALIZED ASC"; public static final String COLUMN\_NAME\_DATA = "data"; \} static class DatabaseHelper extends SQLiteOpenHelper \{ private static final String DATABASE\_NAME = "loader\_throttle.db"; private static final int DATABASE\_VERSION = 2; DatabaseHelper(Context context) \{ // calls the super constructor, requesting the default cursor factory. super(context, DATABASE\_NAME, null, DATABASE\_VERSION); \} @Override public void onCreate(SQLiteDatabase db) \{ db.execSQL("CREATE TABLE " + MainTable.TABLE\_NAME + " (" + MainTable.\_ID + " INTEGER PRIMARY KEY," + MainTable.COLUMN\_NAME\_DATA + " TEXT" + ");"); \} @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) \{ // Logs that the database is being upgraded Log.w(TAG, "Upgrading database from version " + oldVersion + " to " + newVersion + ", which will destroy all old data"); // Kills the table and existing data db.execSQL("DROP TABLE IF EXISTS notes"); // Recreates the database with a new version onCreate(db); \} \} public static class SimpleProvider extends ContentProvider \{ // A projection map used to select columns from the database private final HashMap<String, String> mNotesProjectionMap; // Uri matcher to decode incoming URIs. private final UriMatcher mUriMatcher; // The incoming URI matches the main table URI pattern private static final int MAIN = 1; // The incoming URI matches the main table row ID URI pattern private static final int MAIN\_ID = 2; // Handle to a new DatabaseHelper. private DatabaseHelper mOpenHelper; public SimpleProvider() \{ // Create and initialize URI matcher. mUriMatcher = new UriMatcher(UriMatcher.NO\_MATCH); mUriMatcher.addURI(AUTHORITY, MainTable.TABLE\_NAME, MAIN); mUriMatcher.addURI(AUTHORITY, MainTable.TABLE\_NAME + "/\#", MAIN\_ID); // Create and initialize projection map for all columns. This is // simply an identity mapping. mNotesProjectionMap = new HashMap<String, String>(); mNotesProjectionMap.put(MainTable.\_ID, MainTable.\_ID); mNotesProjectionMap.put(MainTable.COLUMN\_NAME\_DATA, MainTable.COLUMN\_NAME\_DATA); \} @Override public boolean onCreate() \{ mOpenHelper = new DatabaseHelper(getContext()); // Assumes that any failures will be reported by a thrown exception. return true; \} @Override public Cursor query(Uri uri, String\[\] projection, String selection, String\[\] selectionArgs, String sortOrder) \{ // Constructs a new query builder and sets its table name SQLiteQueryBuilder qb = new SQLiteQueryBuilder(); qb.setTables(MainTable.TABLE\_NAME); switch (mUriMatcher.match(uri)) \{ case MAIN: // If the incoming URI is for main table. qb.setProjectionMap(mNotesProjectionMap); break; case MAIN\_ID: // The incoming URI is for a single row. qb.setProjectionMap(mNotesProjectionMap); qb.appendWhere(MainTable.\_ID + "=?"); selectionArgs = DatabaseUtils.appendSelectionArgs(selectionArgs, new String\[\] \{ uri.getLastPathSegment() \}); break; default: throw new IllegalArgumentException("Unknown URI " + uri); \} if (TextUtils.isEmpty(sortOrder)) \{ sortOrder = MainTable.DEFAULT\_SORT\_ORDER; \} SQLiteDatabase db = mOpenHelper.getReadableDatabase(); Cursor c = qb.query(db, projection, selection, selectionArgs, null /\* no group \*/, null /\* no filter \*/, sortOrder); c.setNotificationUri(getContext().getContentResolver(), uri); return c; \} @Override public String getType(Uri uri) \{ switch (mUriMatcher.match(uri)) \{ case MAIN: return MainTable.CONTENT\_TYPE; case MAIN\_ID: return MainTable.CONTENT\_ITEM\_TYPE; default: throw new IllegalArgumentException("Unknown URI " + uri); \} \} @Override public Uri insert(Uri uri, ContentValues initialValues) \{ if (mUriMatcher.match(uri) != MAIN) \{ // Can only insert into to main URI. throw new IllegalArgumentException("Unknown URI " + uri); \} ContentValues values; if (initialValues != null) \{ values = new ContentValues(initialValues); \} else \{ values = new ContentValues(); \} if (values.containsKey(MainTable.COLUMN\_NAME\_DATA) == false) \{ values.put(MainTable.COLUMN\_NAME\_DATA, ""); \} SQLiteDatabase db = mOpenHelper.getWritableDatabase(); long rowId = db.insert(MainTable.TABLE\_NAME, null, values); // If the insert succeeded, the row ID exists. if (rowId > 0) \{ Uri noteUri = ContentUris.withAppendedId(MainTable.CONTENT\_ID\_URI\_BASE, rowId); getContext().getContentResolver().notifyChange(noteUri, null); return noteUri; \} throw new SQLException("Failed to insert row into " + uri); \} @Override public int delete(Uri uri, String where, String\[\] whereArgs) \{ SQLiteDatabase db = mOpenHelper.getWritableDatabase(); String finalWhere; int count; switch (mUriMatcher.match(uri)) \{ case MAIN: count = db.delete(MainTable.TABLE\_NAME, where, whereArgs); break; case MAIN\_ID: finalWhere = DatabaseUtils.concatenateWhere( MainTable.\_ID + " = " + ContentUris.parseId(uri), where); count = db.delete(MainTable.TABLE\_NAME, finalWhere, whereArgs); break; default: throw new IllegalArgumentException("Unknown URI " + uri); \} getContext().getContentResolver().notifyChange(uri, null); return count; \} @Override public int update(Uri uri, ContentValues values, String where, String\[\] whereArgs) \{ SQLiteDatabase db = mOpenHelper.getWritableDatabase(); int count; String finalWhere; switch (mUriMatcher.match(uri)) \{ case MAIN: // If URI is main table, update uses incoming where clause and args. count = db.update(MainTable.TABLE\_NAME, values, where, whereArgs); break; case MAIN\_ID: // If URI is for a particular row ID, update is based on incoming // data but modified to restrict to the given ID. finalWhere = DatabaseUtils.concatenateWhere( MainTable.\_ID + " = " + ContentUris.parseId(uri), where); count = db.update(MainTable.TABLE\_NAME, values, finalWhere, whereArgs); break; default: throw new IllegalArgumentException("Unknown URI " + uri); \} getContext().getContentResolver().notifyChange(uri, null); return count; \} \} @Override protected void onCreate(Bundle savedInstanceState) \{ super.onCreate(savedInstanceState); FragmentManager fm = getFragmentManager(); if (fm.findFragmentById(android.R.id.content) == null) \{ ThrottledLoaderListFragment list = new ThrottledLoaderListFragment(); fm.beginTransaction().add(android.R.id.content, list).commit(); \} \} public static class ThrottledLoaderListFragment extends ListFragment implements LoaderManager.LoaderCallbacks<Cursor> \{ static final int POPULATE\_ID = Menu.FIRST; static final int CLEAR\_ID = Menu.FIRST+1; SimpleCursorAdapter mAdapter; String mCurFilter; AsyncTask<Void, Void, Void> mPopulatingTask; @Override public void onActivityCreated(Bundle savedInstanceState) \{ super.onActivityCreated(savedInstanceState); setEmptyText("No data. Select 'Populate' to fill with data from Z to A at a rate of 4 per second."); setHasOptionsMenu(true); // Create an empty adapter we will use to display the loaded data. mAdapter = new SimpleCursorAdapter(getActivity(), android.R.layout.simple\_list\_item\_1, null, new String\[\] \{ MainTable.COLUMN\_NAME\_DATA \}, new int\[\] \{ android.R.id.text1 \}, 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) \{ menu.add(Menu.NONE, POPULATE\_ID, 0, "Populate") .setShowAsAction(MenuItem.SHOW\_AS\_ACTION\_IF\_ROOM); menu.add(Menu.NONE, CLEAR\_ID, 0, "Clear") .setShowAsAction(MenuItem.SHOW\_AS\_ACTION\_IF\_ROOM); \} @Override public boolean onOptionsItemSelected(MenuItem item) \{ final ContentResolver cr = getActivity().getContentResolver(); switch (item.getItemId()) \{ case POPULATE\_ID: if (mPopulatingTask != null) \{ mPopulatingTask.cancel(false); \} mPopulatingTask = new AsyncTask<Void, Void, Void>() \{ @Override protected Void doInBackground(Void... params) \{ for (char c='Z'; c>='A'; c--) \{ if (isCancelled()) \{ break; \} StringBuilder builder = new StringBuilder("Data "); builder.append(c); ContentValues values = new ContentValues(); values.put(MainTable.COLUMN\_NAME\_DATA, builder.toString()); cr.insert(MainTable.CONTENT\_URI, values); // Wait a bit between each insert. try \{ Thread.sleep(250); \} catch (InterruptedException e) \{ \} \} return null; \} \}; mPopulatingTask.executeOnExecutor( AsyncTask.THREAD\_POOL\_EXECUTOR, (Void\[\])null); return true; case CLEAR\_ID: if (mPopulatingTask != null) \{ mPopulatingTask.cancel(false); mPopulatingTask = null; \} AsyncTask<Void, Void, Void> task = new AsyncTask<Void, Void, Void>() \{ @Override protected Void doInBackground(Void... params) \{ cr.delete(MainTable.CONTENT\_URI, null, null); return null; \} \}; task.execute((Void\[\])null); return true; default: return super.onOptionsItemSelected(item); \} \} @Override public void onListItemClick(ListView l, View v, int position, long id) \{ // Insert desired behavior here. Log.i(TAG, "Item clicked: " + id); \} // These are the rows that we will retrieve. static final String\[\] PROJECTION = new String\[\] \{ MainTable.\_ID, MainTable.COLUMN\_NAME\_DATA, \}; public Loader<Cursor> onCreateLoader(int id, Bundle args) \{ CursorLoader cl = new CursorLoader(getActivity(), MainTable.CONTENT\_URI, PROJECTION, null, null, null); cl.setUpdateThrottle(2000); // update at most every 2 seconds. return cl; \} public void onLoadFinished(Loader<Cursor> loader, Cursor data) \{ mAdapter.swapCursor(data); \} public void onLoaderReset(Loader<Cursor> loader) \{ mAdapter.swapCursor(null); \} \}\}

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