Android 装载器---启动装载器

长平狐 发布于 2012/10/16 10:19
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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);
        }
    }
}


原文链接:http://blog.csdn.net/fireofstar/article/details/7280791
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