LoaderManager - Android 3.0新特性

在Android 3.0中提供了一個新概念Loaders，通過LoaderManager類可以很輕松的異步加載數據從Fragment或Activity中，Loaders提供了回調機制通知最終的運行結果，有點類似AsyncTask類，但由於Loader對於並發可以用過Loader管理器統一管理，所以更適合批量處理多個異步任務的處理(當然內部仍然是多線程)。下面就讓Android123一起和大家看下honeycomb中的新特性吧，對於解決多重異步I/O加快android平板應用的運行是十分有效的。

  一、LoaderManager

  LoaderManager類位於android.app.LoaderManager，提供了以下幾個方法

abstract void  destroyLoader(int id) //停止並移除loader通過ID
abstract void  dump(String prefix, FileDescriptor fd, PrintWriter writer, String[] args)  //打印LoaderManager的狀態到一個流中
static void  enableDebugLogging(boolean enabled) //啟用debug記錄
abstract <D> Loader<D>  getLoader(int id) //返回找到的ID或沒有匹配的在Loader中
abstract <D> Loader<D>  initLoader(int id, Bundle args, LoaderCallbacks<D> callback)  //初始化Loader使其成為活動狀態
abstract <D> Loader<D>  restartLoader(int id, Bundle args, LoaderCallbacks<D> callback)  //啟動一個新的或重啟一個存在的Loader在管理器中

同時LoaderManager還有一個回調接口android.app.LoaderManager.LoaderCallbacks<D> 用於和LoaderManager交互

abstract Loader<D>  onCreateLoader(int id, Bundle args)  //舉例並返回一個新Loader通過ID
abstract void  onLoadFinished(Loader<D> loader, D data)  //當前面一個Loader已經完成時回調
abstract void  onLoaderReset(Loader<D> loader)  //當一個新的loader或存在的loader重啟時回調

 二、Loader

 Loader類位於android.content.Loader<D>，整體比較復雜，主要成員有

 1. 構造方法  Loader(Context context) //作為唯一實例化方法參數只有一個Context

 2. Public Methods

void  abandon()  //高速Loader他在綁定
String  dataToString(D data)  //用於調試，轉換一個Loader數據類的實例為字符串用於打印
void  deliverResult(D data) //發送一個load注冊的listener結果
void  dump(String prefix, FileDescriptor fd, PrintWriter writer, String[] args) //打印loader狀態通過給定的流
void  forceLoad()  //強制一個異步載入
Context  getContext()   //返回Context實例
int  getId() 
boolean  isAbandoned()  //判斷是否已經綁定
boolean  isReset()  //判斷是否已經重啟
boolean  isStarted()  //判斷是否已經執行
void  onContentChanged() //內容變化回調

registerListener(int id, OnLoadCompleteListener<D> listener)
void  reset() //重置一個Loader的狀態
final void  startLoading()  //啟動一個異步的載入從Loader的數據
void  stopLoading()  //停止載入
boolean  takeContentChanged() String  toString()
void  unregisterListener(OnLoadCompleteListener<D> listener) 

 提供的子類 android.content.Loader.ForceLoadContentObserver 和 接口 android.content.Loader.OnLoadCompleteListener<D>

 為了更清晰的表達android開發網給出一個SDK例子完整代碼，來作分析

  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);
        }
    }
}