Loaders
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,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);
}
}
}