使用传统Android组件实现高效数据加载

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本文主要介绍使用ContentProvider + Sqlite + Loader等Android的基本组件实现高内聚低耦合的数据加载的数据设计模式，这是一种传统而高效的数据加载，熟悉这种模式同时也是对ContentProvider的更好掌握。

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优势：

1.ContentProvider可以非常简单的调整数据源而不影响其他程序。外部程序不关心数据，只关心返回的Cursor.举个例子，ContentProvider指向的数据库可以随时改变不会影响使用者，同时ContentProvider也可以返回网络数据，使用者只要求结果是Cursor,但是对于数据来自哪里并不关心，这样就实现了数据读取和使用的分离。

2.提供数据安全保护与权限管理，ContentProvider使用授权机制，对数据在方便访问的同时进行了很好地保护。

3.强大的API支持，Google官方提供了强大的API.

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Loader特点：

1.与Activity管理同步，与Activity/Fragment生命周期同步，创建与销毁都会受到Activity/Fragment生命周期的管理。这就意味着我们不需要再去考虑何时去加载数据，使用Loader之后会结合Activity或者Fragment的生命周期自行进行加载和更新。

2.内部线程异步加载，我们也就不需要再去开启线程获取数据解析数据，大大减少了代码量。

3.数据源发生改变时实时更新，貌似我们自己的数据数据是无法自动更新的，不过不要紧，只需要一行简单的reStartLoader就可以手动重新加载一下。

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数据分层：

Loader 写操作

ContentProvider

SQLite

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下面就使用这种模式一步一步实现一下。

1.定义一个关系类，声明表结构,Uri,授权

    public class WebContract {    public static final String Tb_History = "tb_history";    public static final String Tb_BookMark = "tb_bookmark";    public static final String Authority = "com.march.db_browser";    public static class History implements BaseColumns {    //content://com.march.db_browser/tb_history    public static final Uri ContentUri = Uri.parse(    "content://" + Authority).buildUpon().appendPath(Tb_History).build();    public static final String Link = "link";    public static final String Icon = "icon";    public static final String Title = "title";    public static final String Time = "time";    }    public static class BookMark implements BaseColumns {    //content://com.march.db_browser/tb_history    public static final Uri ContentUri = Uri.parse(    "content://" + Authority).buildUpon().appendPath(Tb_BookMark).build();    public static final String Link = "link";    public static final String Icon = "icon";    public static final String Title = "title";    public static final String Time = "time";    }    }

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2.继承ContentProvider实现数据提供者，设定好Code作为表资源的唯一标示，使用UriMacher匹配

    public class WebContentProvider extends ContentProvider {    /*为可以访问的数据库表资源定义代号，区分哪个表可以被访问*/    private static final int CODE_BOOKMARK = 0x1;    private static final int CODE_HISTORY = 0x2;    /*使用UriMacher生成访问的Uri*/    private static UriMatcher uriMatcher;    /*使用静态代码块初始化UriMacher,添加*/    static {    uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);    uriMatcher.addURI(WebContract.Authority, WebContract.Tb_BookMark, CODE_BOOKMARK);    uriMatcher.addURI(WebContract.Authority, WebContract.Tb_History, CODE_HISTORY);    }    private MySqliteOpenHelper mySqliteOpenHelper;    private SQLiteDatabase db;    @Override    public boolean onCreate() {    mySqliteOpenHelper = new MySqliteOpenHelper(getContext(), "db_browser", 1);    return false;    }    @Override    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {    Cursor cursor = null;    if (uri != null) {    int code = uriMatcher.match(uri);    db = mySqliteOpenHelper.getWritableDatabase();    switch (code) {    case CODE_BOOKMARK:    cursor = db.query(WebContract.Tb_BookMark, null, null, null, null, null, null);    break;    case CODE_HISTORY:    cursor = db.query(WebContract.Tb_History, null, null, null, null, null, null);    break;    }    }    if (cursor == null) {    Log.i("chendong", "cursor is null");    }       // db.close();    //读操作不能关闭连接，写操作需要关闭连接    return cursor;    }    @Override    public String getType(Uri uri) {    return null;    }    @Override    public Uri insert(Uri uri, ContentValues values) {    Uri uri_return = null;    if (uri != null) {    int code = uriMatcher.match(uri);    db = mySqliteOpenHelper.getWritableDatabase();    long id = -1;    switch (code) {    case CODE_BOOKMARK:    id = db.insert(WebContract.Tb_BookMark, null, values);    break;    case CODE_HISTORY:    id = db.insert(WebContract.Tb_History, null, values);    break;    }    if (id != -1) {    uri_return = ContentUris.withAppendedId(uri, id);    }    db.close();    }    return uri_return;    }    @Override    public int delete(Uri uri, String selection, String[] selectionArgs) {    // Implement this to handle requests to delete one or more rows.    throw new UnsupportedOperationException("Not yet implemented");    }    @Override    public int update(Uri uri, ContentValues values, String selection,      String[] selectionArgs) {    // TODO: Implement this to handle requests to update one or more rows.    throw new UnsupportedOperationException("Not yet implemented");    }    }

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3.使用Loader加载数据

    adapter = new SimpleCursorAdapter(this,    R.layout.item_listview,    null,    new String[]{WebContract.History.Title, WebContract.History.Link},    new int[]{R.id.item_listview_title, R.id.item_listview_url},    SimpleCursorAdapter.FLAG_REGISTER_CONTENT_OBSERVER);    listView.setAdapter(adapter);    getLoaderManager().initLoader(0x123, bundle, this);    //这是实现的    implements LoaderManager.LoaderCallbacks<Cursor> 的方法    @Override    public Loader<Cursor> onCreateLoader(int id, Bundle args) {    int action = args.getInt("action");    CursorLoader cursorLoader = null;    Log.i("chendong", "action is " + action);    if (action == 0) {    //history    cursorLoader = new CursorLoader(this, WebContract.History.ContentUri, null, null, null, null);    } else {    cursorLoader = new CursorLoader(this, WebContract.BookMark.ContentUri, null, null, null, null);    }    return cursorLoader;    }    @Override    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {    Log.i("chendong", "get data ");    adapter.changeCursor(data);    }    @Override    public void onLoaderReset(Loader<Cursor> loader) {    }    使用    getLoaderManager().restartLoader(0x123, bundle, this);    重新加载数据