Android 装载器---启动装载器

LoaderManager管理Activity或Fragment内部的一个或多个装载器实例，每个Activity或Fragment只有一个LoaderManager对象。

通常在Activity的onCreate()方法或Fragment的onActivityCreated()内初始化一个装载器，如：

// Prepare the loader.  Either re-connect with an existing one,// or start a new one.getLoaderManager().initLoader(0, null, this);

initLoader方法带有以下参数：

1.  标识这个装载器的唯一ID，这个例子中ID是“0”；

2.  可选参数，它是给装载器提供的在构造时所需的参数，这个例子中是null；

3.  一个LoaderManager.LoaderCallbacks的实现，LoaderManager会调用这个实现来报告装载器事件。在这个例子中，这个类实现了LoaderManager.LoaderCallbacks接口，因此它把它自己（this）的引用传给了initLoader()方法。

initLoader()调用确保了装载器被初始化和激活。它有两种可能的结果：

1.  如果通过ID指定的装载器已经存在，那么最后被创建的装载器被重用。

2.  如果通过ID指定的装载器不存在，initLoader()方法会触发LoaderManager.LoaderCallbacks的onCreateLoader()方法。这是你实现初始化代码的地方，并且返回一个新的装载器。关于更多的讨论，请看onCreateLoader()相关章节。

在这两种情况下，都给LoaderManager.LoaderCallbacks实现了与装载器的关联，并且在装载器状态改变的时候，相关的回调方法会被调用。如果在调用的时点调用者是在被启动的状态，并且被请求的装载器已经存在且已经生成了相关的数据，那么系统会立即调用onLoadFinished()(在initLoader()方执行期间)，因此你必须为这种情况做好准备。对于这个回调的更多讨论，请看onloadFinished()回调。

 

注意：initLoader()方法返回一个被创建的Loader对象，但是你不需要获取关于这个对象的引用，因为LoaderManager对象自动的管理着装载器的生存。LoaderManager对象在需要的时候启动和终止装载器，并且保持着装载器的状态和相关匹配的内容。这就意味着，你很少直接跟装载器交互（虽然可以使用装载器方法微调装载器的行为，请参阅例子LoaderThrottle），大多数情况都是使用LoaderManagerLoaderCallbacks的回调方法在加载过程中发生特定事件时来进行干预。关于更多的讨论，可参阅使用LoaderManager回调方法（Using the LoaderManager Callbacks）

例子：LoaderThrottle.java

/* * Copyright (C) 2010 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * *      http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */package com.example.android.apis.app;import android.app.Activity;import android.app.FragmentManager;import android.app.ListFragment;import android.app.LoaderManager;import android.content.ContentProvider;import android.content.ContentResolver;import android.content.ContentUris;import android.content.ContentValues;import android.content.Context;import android.content.CursorLoader;import android.content.Loader;import android.content.UriMatcher;import android.database.Cursor;import android.database.DatabaseUtils;import android.database.SQLException;import android.database.sqlite.SQLiteDatabase;import android.database.sqlite.SQLiteOpenHelper;import android.database.sqlite.SQLiteQueryBuilder;import android.net.Uri;import android.os.AsyncTask;import android.os.Bundle;import android.provider.BaseColumns;import android.text.TextUtils;import android.util.Log;import android.view.Menu;import android.view.MenuInflater;import android.view.MenuItem;import android.view.View;import android.widget.ListView;import android.widget.SimpleCursorAdapter;import java.util.HashMap;/** * Demonstration of bottom to top implementation of a content provider holding * structured data through displaying it in the UI, using throttling to reduce * the number of queries done when its data changes. */public class LoaderThrottle extends Activity {    // Debugging.    static final String TAG = "LoaderThrottle";    /**     * The authority we use to get to our sample provider.     */    public static final String AUTHORITY = "com.example.android.apis.app.LoaderThrottle";    /**     * Definition of the contract for the main table of our provider.     */    public static final class MainTable implements BaseColumns {        // This class cannot be instantiated        private MainTable() {}        /**         * The table name offered by this provider         */        public static final String TABLE_NAME = "main";        /**         * The content:// style URL for this table         */        public static final Uri CONTENT_URI =  Uri.parse("content://" + AUTHORITY + "/main");        /**         * The content URI base for a single row of data. Callers must         * append a numeric row id to this Uri to retrieve a row         */        public static final Uri CONTENT_ID_URI_BASE                = Uri.parse("content://" + AUTHORITY + "/main/");        /**         * The MIME type of {@link #CONTENT_URI}.         */        public static final String CONTENT_TYPE                = "vnd.android.cursor.dir/vnd.example.api-demos-throttle";        /**         * The MIME type of a {@link #CONTENT_URI} sub-directory of a single row.         */        public static final String CONTENT_ITEM_TYPE                = "vnd.android.cursor.item/vnd.example.api-demos-throttle";        /**         * The default sort order for this table         */        public static final String DEFAULT_SORT_ORDER = "data COLLATE LOCALIZED ASC";        /**         * Column name for the single column holding our data.         * <P>Type: TEXT</P>         */        public static final String COLUMN_NAME_DATA = "data";    }    /**     * This class helps open, create, and upgrade the database file.     */   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);       }       /**        *        * Creates the underlying database with table name and column names taken from the        * NotePad class.        */       @Override       public void onCreate(SQLiteDatabase db) {           db.execSQL("CREATE TABLE " + MainTable.TABLE_NAME + " ("                   + MainTable._ID + " INTEGER PRIMARY KEY,"                   + MainTable.COLUMN_NAME_DATA + " TEXT"                   + ");");       }       /**        *        * Demonstrates that the provider must consider what happens when the        * underlying datastore is changed. In this sample, the database is upgraded the database        * by destroying the existing data.        * A real application should upgrade the database in place.        */       @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);       }   }    /**     * A very simple implementation of a content provider.     */    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;        /**         * Global provider initialization.         */        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);        }        /**         * Perform provider creation.         */        @Override        public boolean onCreate() {            mOpenHelper = new DatabaseHelper(getContext());            // Assumes that any failures will be reported by a thrown exception.            return true;        }        /**         * Handle incoming queries.         */        @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;        }        /**         * Return the MIME type for an known URI in the provider.         */        @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);            }        }        /**         * Handler inserting new data.         */        @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);        }        /**         * Handle deleting data.         */        @Override        public int delete(Uri uri, String where, String[] whereArgs) {            SQLiteDatabase db = mOpenHelper.getWritableDatabase();            String finalWhere;            int count;            switch (mUriMatcher.match(uri)) {                case MAIN:                    // If URI is main table, delete uses incoming where clause and args.                    count = db.delete(MainTable.TABLE_NAME, where, whereArgs);                    break;                    // If the incoming URI matches a single note ID, does the delete based on the                    // incoming data, but modifies the where clause to restrict it to the                    // particular note ID.                case MAIN_ID:                    // If URI is for a particular row ID, delete is based on incoming                    // data but modified to restrict to the given 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;        }        /**         * Handle updating data.         */        @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();        // Create the list fragment and add it as our sole content.        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> {        // Menu identifiers        static final int POPULATE_ID = Menu.FIRST;        static final int CLEAR_ID = Menu.FIRST+1;        // 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;        // Task we have running to populate the database.        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);            // 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);        }        @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);            // The list should now be shown.            if (isResumed()) {                setListShown(true);            } else {                setListShownNoAnimation(true);            }        }        public void onLoaderReset(Loader<Cursor> loader) {            mAdapter.swapCursor(null);        }    }}

注：本人转载系个人觉得翻译的很好，值得收藏，且自己回头看着方便。

如有兴趣请访问作者官方博客http://blog.csdn.net/FireOfStar