本文主要是介绍ARTSPConnection::postReceiveReponseEvent启动发送请求循环,希望对大家解决编程问题提供一定的参考价值,需要的开发者们随着小编来一起学习吧!
下面贴出安卓N版本ARTSPConnection是如何启动向服务端发送请求,接收服务端的响应这样一个循环的:
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void ARTSPConnection::postReceiveReponseEvent() {//mReceiveResponseEventPending条件变量一旦postReceiveReponseEvent被调用就设置为true,知道整个循环调用达到末端该mReceiveResponseEventPending才会被设置为false,这样保证一次只存在一个这样的循环处理if (mReceiveResponseEventPending) {return;}//新建一个消息msg,消息名为kWhatReceiveResponse,消息的处理者为this,也即ARTSPConnection//则该消息的处理发生在ARTSPConnection::onMessageReceived的case kWhatReceiveResponse处理分支sp<AMessage> msg = new AMessage(kWhatReceiveResponse, this);msg->post();//将mReceiveResponseEventPending变量设置为true,表示当前这样的一个循环正在进行中,不能再产生一次这样的循环。mReceiveResponseEventPending = true;
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void ARTSPConnection::onMessageReceived(const sp<AMessage> &msg) {switch (msg->what()) {case kWhatConnect:onConnect(msg);break;case kWhatDisconnect:onDisconnect(msg);break;case kWhatCompleteConnection:onCompleteConnection(msg);break;case kWhatSendRequest:onSendRequest(msg);break;case kWhatReceiveResponse://调用onReceiveResponse()函数对消息名为kWhatReceiveResponse的消息进行处理onReceiveResponse();break;case kWhatObserveBinaryData:{CHECK(msg->findMessage("reply", &mObserveBinaryMessage));break;}default:TRESPASS();break;}
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void ARTSPConnection::onReceiveResponse() {//这样的一次循环调用接近到了末端,mReceiveResponseEventPending设置为false,表示可以再进行一次这样的循环调用了mReceiveResponseEventPending = false;if (mState != CONNECTED) {return;}//创建一个timeval类型的时间结构体变量struct timeval tv;tv.tv_sec = 0;tv.tv_usec = kSelectTimeoutUs;//创建一个套接字描述符集合rs//并将该套接字描述符集合内存清零fd_set rs;FD_ZERO(&rs);//将之前创建的套接字描述符mSocket添加到该套接字描述符集合中FD_SET(mSocket, &rs);//调用select函数检查套接字描述符集合rs里添加的套接字的状态,//即查询它的可读性、可写性及错误状态信息//在这里查询可读性传入了 &rs,其他两个查询可写性及错误状态信息都是传入的NULL//说明在这里调用者只检查套接字描述符集合rs里套接字的可读性int res = select(mSocket + 1, &rs, NULL, NULL, &tv);if (res == 1) {//刚才只通过FD_SET(mSocket, &rs)向套接字描述符集合rs里添加了一个我们创建的套接字描述符mSocket//所以select函数的返回值res为1说明,套接字描述符mSocket描述的套接口具有可读性//调用MakeSocketBlocking(mSocket, true)函数阻塞该套接口,因为接下来需要从该套接口获取从服务端回应的消息MakeSocketBlocking(mSocket, true);//真正的从套接口获取服务端回应的消息是在receiveRTSPReponse函数里完成//后续文章为大家介绍receiveRTSPReponse函数bool success = receiveRTSPReponse();//成功从该套接口获取到服务端回应的消息后调用MakeSocketBlocking(mSocket, false)函数将该套接口设置为非阻塞状态MakeSocketBlocking(mSocket, false);if (!success) {// Something horrible, irreparable has happened.flushPendingRequests();return;}}//调用postReceiveReponseEvent()函数再次开启这样一次处理,//如此就形成了一个循环postReceiveReponseEvent();
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小结:该循环的建立流程是:postReceiveReponseEvent ==> onReceiveResponse ==> postReceiveReponseEvent。这是一个设计技巧,是循环处理某些事务的机制,机制的原理就是形成一个循环调用圈。
select函数的百科介绍:select(),确定一个或多个套接口的状态,本函数用于确定一个或多个套接口的状态,对每一个套接口,调用者可查询它的可读性、可写性及错误状态信息,用fd_set结构来表示一组等待检查的套接口,在调用返回时,这个结构存有满足一定条件的套接口组的子集,并且select()返回满足条件的套接口的数目。
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