AsyncQueryHandler是用于ContentProvider上执行异步的CRUD操作。

1. AsyncQueryHandler类

AsyncQueryHandler类继承Handler类，通过封装ContentResolver来实现对ContentProvider的异步操作。

public AsyncQueryHandler(ContentResolver cr) {super();mResolver = new WeakReference<ContentResolver>(cr);synchronized (AsyncQueryHandler.class) {if (sLooper == null) {HandlerThread thread = new HandlerThread("AsyncQueryWorker");thread.start();sLooper = thread.getLooper();}}mWorkerThreadHandler = createHandler(sLooper);
}protected Handler createHandler(Looper looper) {return new WorkerHandler(looper);
}

2. WorkerHandler类

WorkerHandler类同样继承Handler类，对CRUD操作进行异步处理，最后返回处理结果。

protected class WorkerHandler extends Handler {public WorkerHandler(Looper looper) {super(looper);}@Overridepublic void handleMessage(Message msg) {final ContentResolver resolver = mResolver.get();if (resolver == null) return;WorkerArgs args = (WorkerArgs) msg.obj;int token = msg.what;int event = msg.arg1;switch (event) {case EVENT_ARG_QUERY:Cursor cursor;try {cursor = resolver.query(args.uri, args.projection,args.selection, args.selectionArgs,args.orderBy);// Calling getCount() causes the cursor window to be filled,// which will make the first access on the main thread a lot faster.if (cursor != null) {cursor.getCount();}} catch (Exception e) {Log.w(TAG, "Exception thrown during handling EVENT_ARG_QUERY", e);cursor = null;}args.result = cursor;break;case EVENT_ARG_INSERT:args.result = resolver.insert(args.uri, args.values);break;case EVENT_ARG_UPDATE:args.result = resolver.update(args.uri, args.values, args.selection,args.selectionArgs);break;case EVENT_ARG_DELETE:args.result = resolver.delete(args.uri, args.selection, args.selectionArgs);break;}// passing the original token value back to the caller// on top of the event values in arg1.Message reply = args.handler.obtainMessage(token);reply.obj = args;reply.arg1 = msg.arg1;if (localLOGV) {Log.d(TAG, "WorkerHandler.handleMsg: msg.arg1=" + msg.arg1+ ", reply.what=" + reply.what);}// AsyncQueryHandler处理回调事件reply.sendToTarget();}
}

3. 事件流程

查询事件，由WorkerHandler进行异步处理

public void startQuery(int token, Object cookie, Uri uri,String[] projection, String selection, String[] selectionArgs,String orderBy) {// Use the token as what so cancelOperations works properlyMessage msg = mWorkerThreadHandler.obtainMessage(token);msg.arg1 = EVENT_ARG_QUERY;WorkerArgs args = new WorkerArgs();args.handler = this; // 处理回调事件args.uri = uri;args.projection = projection;args.selection = selection;args.selectionArgs = selectionArgs;args.orderBy = orderBy;args.cookie = cookie;msg.obj = args;mWorkerThreadHandler.sendMessage(msg);
}

查询结束，onQueryComplete()处理回调事件

@Override
public void handleMessage(Message msg) {WorkerArgs args = (WorkerArgs) msg.obj;if (localLOGV) {Log.d(TAG, "AsyncQueryHandler.handleMessage: msg.what=" + msg.what+ ", msg.arg1=" + msg.arg1);}int token = msg.what;int event = msg.arg1;// pass token back to caller on each callback.switch (event) {case EVENT_ARG_QUERY:onQueryComplete(token, args.cookie, (Cursor) args.result);break;case EVENT_ARG_INSERT:onInsertComplete(token, args.cookie, (Uri) args.result);break;case EVENT_ARG_UPDATE:onUpdateComplete(token, args.cookie, (Integer) args.result);break;case EVENT_ARG_DELETE:onDeleteComplete(token, args.cookie, (Integer) args.result);break;}
}

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