live555 rtsp服务器实战之doGetNextFrame

live555关于RTSP协议交互流程

live555的核心数据结构值之闭环双向链表

live555 rtsp服务器实战之createNewStreamSource

live555 rtsp服务器实战之doGetNextFrame

概要

　　live555用于实际项目开发时，createNewStreamSource和doGetNextFrame是必须要实现的两个虚函数，一般会创建两个类来实现这两个函数：假如这两个类为H264LiveVideoServerMediaSubssion和H264FramedLiveSource；H264LiveVideoServerMediaSubssion为实时视频会话类，用于实现createNewStreamSource虚函数；

　　H264FramedLiveSource为实时视频帧资源类，用户实现doGetNextFrame函数；

　　那么这两个函数是什么时候被调用以及他们的作用是什么呢？本节将详细介绍；

声明：该文章基于H264视频源为基础分析，其他源类似；

　　由于这两个类主要是自定义虚函数，所以存在一定的继承关系，首先需要明确这两个类的继承关系(本章只介绍H264FramedLiveSource)：

　　H264FramedLiveSource:FramedSource:MediaSource:Medium

　　doGetNextFrame调用流程

　　doGetNextFrame函数声明于FrameSource类：

virtual void doGetNextFrame() = 0;

　　该声明为纯虚函数，所以必须有子类对该函数进行实现，H264FramedLiveSource类就是用于实现该函数；doGetNextFrame函数只有一个功能：获取视频帧；

void H264FramedLiveSource::doGetNextFrame()
{
    uint8_t *frame = new uint8_t[1024*1024];
    int len = 0;
    //获取一帧视频帧
    get_frame(&frame, len);
    //将帧长度赋值给父类的成员变量fFrameSize
    fFrameSize = len;
    //将帧数据赋值给父类的成员变量fTo
    memcpy(fTo, frame, len);
    
    delete [] frame;
    // nextTask() = envir().taskScheduler().scheduleDelayedTask(0,(TaskFunc*)FramedSource::afterGetting, this);//表示延迟0秒后再执行 afterGetting 函数
    afterGetting(this);
 
    return;
}

　　fFrameSize和fTo的两个父类变量在流传输时会用到；那么问题来了：doGetNextFrame函数在整个流程中在哪里被调用？什么时候调用？

　　首先doGetNextFrame函数是在PLAY信令交互的时候被调用；调用流程为：

handleCmd_PLAY->startStream->startPlaying(OnDemandServerMediaSubsession)->startPlaying(MediaSink)->continuePlaying(虚函数，子类H264or5VideoRTPSink实现)->continuePlaying(虚函数，子类MultiFramedRTPSink实现)->buildAndSendPacket->packFrame(MultiFramedRTPSink)->getNextFrame->doGetNextFrame

　　下面详解介绍下这个流程，看过我的另一篇文章："live555 rtsp服务器实战之createNewStreamSource" 的都知道handleRequestBytes函数调用了handleCmd_SETUP；同样handleCmd_PLAY函数也是在这里被调用的；handleRequestBytes函数就是用来处理各种客户端信令交互信息的；handleRequestBytes函数如下：

void RTSPServer::RTSPClientConnection::handleRequestBytes(int newBytesRead)
{
    .
    .
    .
    if (urlIsRTSPS != fOurRTSPServer.fOurConnectionsUseTLS)
      {
#ifdef DEBUG
        fprintf(stderr, "Calling handleCmd_redirect()\n");
#endif
        handleCmd_redirect(urlSuffix);
      }
      else if (strcmp(cmdName, "OPTIONS") == 0)
      {
        // If the "OPTIONS" command included a "Session:" id for a session that doesn't exist,
        // then treat this as an error:
        if (requestIncludedSessionId && clientSession == NULL)
        {
#ifdef DEBUG
          fprintf(stderr, "Calling handleCmd_sessionNotFound() (case 1)\n");
#endif
          handleCmd_sessionNotFound();
        }
        else
        {
          // Normal case:
          handleCmd_OPTIONS();
        }
      }
      else if (urlPreSuffix[0] == '\0' && urlSuffix[0] == '*' && urlSuffix[1] == '\0')
      {
        // The special "*" URL means: an operation on the entire server.  This works only for GET_PARAMETER and SET_PARAMETER:
        if (strcmp(cmdName, "GET_PARAMETER") == 0)
        {
          handleCmd_GET_PARAMETER((char const *)fRequestBuffer);
        }
        else if (strcmp(cmdName, "SET_PARAMETER") == 0)
        {
          handleCmd_SET_PARAMETER((char const *)fRequestBuffer);
        }
        else
        {
          handleCmd_notSupported();
        }
      }
      else if (strcmp(cmdName, "DESCRIBE") == 0)
      {
        handleCmd_DESCRIBE(urlPreSuffix, urlSuffix, (char const *)fRequestBuffer);
      }
      else if (strcmp(cmdName, "SETUP") == 0)
      {
        Boolean areAuthenticated = True;
 
        if (!requestIncludedSessionId)
        {
          // No session id was present in the request.
          // So create a new "RTSPClientSession" object for this request.
 
          // But first, make sure that we're authenticated to perform this command:
          char urlTotalSuffix[2 * RTSP_PARAM_STRING_MAX];
          // enough space for urlPreSuffix/urlSuffix'\0'
          urlTotalSuffix[0] = '\0';
          if (urlPreSuffix[0] != '\0')
          {
            strcat(urlTotalSuffix, urlPreSuffix);
            strcat(urlTotalSuffix, "/");
          }
          strcat(urlTotalSuffix, urlSuffix);
          if (authenticationOK("SETUP", urlTotalSuffix, (char const *)fRequestBuffer))
          {
            clientSession = (RTSPServer::RTSPClientSession *)fOurRTSPServer.createNewClientSessionWithId();
          }
          else
          {
            areAuthenticated = False;
          }
        }
        if (clientSession != NULL)
        {
          clientSession->handleCmd_SETUP(this, urlPreSuffix, urlSuffix, (char const *)fRequestBuffer);
          playAfterSetup = clientSession->fStreamAfterSETUP;
        }
        else if (areAuthenticated)
        {
#ifdef DEBUG
          fprintf(stderr, "Calling handleCmd_sessionNotFound() (case 2)\n");
#endif
          handleCmd_sessionNotFound();
        }
      }
      else if (strcmp(cmdName, "TEARDOWN") == 0 || strcmp(cmdName, "PLAY") == 0 || strcmp(cmdName, "PAUSE") == 0 || strcmp(cmdName, "GET_PARAMETER") == 0 || strcmp(cmdName, "SET_PARAMETER") == 0)
      {
        if (clientSession != NULL)
        {
          clientSession->handleCmd_withinSession(this, cmdName, urlPreSuffix, urlSuffix, (char const *)fRequestBuffer);
        }
        else
        {
#ifdef DEBUG
          fprintf(stderr, "Calling handleCmd_sessionNotFound() (case 3)\n");
#endif
          handleCmd_sessionNotFound();
        }
      }
      else if (strcmp(cmdName, "REGISTER") == 0 || strcmp(cmdName, "DEREGISTER") == 0)
      {
        // Because - unlike other commands - an implementation of this command needs
        // the entire URL, we re-parse the command to get it:
        char *url = strDupSize((char *)fRequestBuffer);
        if (sscanf((char *)fRequestBuffer, "%*s %s", url) == 1)
        {
          // Check for special command-specific parameters in a "Transport:" header:
          Boolean reuseConnection, deliverViaTCP;
          char *proxyURLSuffix;
          parseTransportHeaderForREGISTER((const char *)fRequestBuffer, reuseConnection, deliverViaTCP, proxyURLSuffix);
 
          handleCmd_REGISTER(cmdName, url, urlSuffix, (char const *)fRequestBuffer, reuseConnection, deliverViaTCP, proxyURLSuffix);
          delete[] proxyURLSuffix;
        }
        else
        {
          handleCmd_bad();
        }
        delete[] url;
      }
      else
      {
        // The command is one that we don't handle:
        handleCmd_notSupported();
      }
      .
      .
      .
}

　　handleCmd_withinSession函数内部就调用了handleCmd_PLAY函数；仅整理流程，调用途中的函数这里不做解释；直接跳到packFrame(MultiFramedRTPSink)函数：

void MultiFramedRTPSink::packFrame()
{
  // Get the next frame.
 
  // First, skip over the space we'll use for any frame-specific header:
  fCurFrameSpecificHeaderPosition = fOutBuf->curPacketSize();
  fCurFrameSpecificHeaderSize = frameSpecificHeaderSize();
  fOutBuf->skipBytes(fCurFrameSpecificHeaderSize);
  fTotalFrameSpecificHeaderSizes += fCurFrameSpecificHeaderSize;
 
  // See if we have an overflow frame that was too big for the last pkt
  if (fOutBuf->haveOverflowData())
  {
    // Use this frame before reading a new one from the source
    unsigned frameSize = fOutBuf->overflowDataSize();
    struct timeval presentationTime = fOutBuf->overflowPresentationTime();
    unsigned durationInMicroseconds = fOutBuf->overflowDurationInMicroseconds();
    fOutBuf->useOverflowData();
 
    afterGettingFrame1(frameSize, 0, presentationTime, durationInMicroseconds);
  }
  else
  {
    // Normal case: we need to read a new frame from the source
    if (fSource == NULL)
      return;
 
    fSource->getNextFrame(fOutBuf->curPtr(), fOutBuf->totalBytesAvailable(),
                          afterGettingFrame, this, ourHandleClosure, this);
  }
}

　　这里调用了getNextFrame函数，来看下getNextFrame函数的实现：

void FramedSource::getNextFrame(unsigned char* to, unsigned maxSize,
        afterGettingFunc* afterGettingFunc,
        void* afterGettingClientData,
        onCloseFunc* onCloseFunc,
        void* onCloseClientData) {
  // Make sure we're not already being read:
  if (fIsCurrentlyAwaitingData) {
    envir() << "FramedSource[" << this << "]::getNextFrame(): attempting to read more than once at the same time!\n";
    envir().internalError();
  }
 
  fTo = to;
  fMaxSize = maxSize;
  fNumTruncatedBytes = 0; // by default; could be changed by doGetNextFrame()
  fDurationInMicroseconds = 0; // by default; could be changed by doGetNextFrame()
  fAfterGettingFunc = afterGettingFunc;
  fAfterGettingClientData = afterGettingClientData;
  fOnCloseFunc = onCloseFunc;
  fOnCloseClientData = onCloseClientData;
  fIsCurrentlyAwaitingData = True;
 
  doGetNextFrame();
}

　　发现啦！发现doGetNextFrame函数啦！所以doGetNextFrame函数就是在getNextFrame内被调用的；但是问题来了：搜索可以发现在live555中doGetNextFrame函数很多；怎么就确定这里的doGetNextFrame和我们自定义的doGetNextFrame有关呢？

问的好！那这的关键点就在于fSource->getNextFrame的fSource变量到底是什么类的对象；才能确定doGetNextFrame到底调用的是哪个类的函数；fSource属于MediaSink类的成员变量:

FramedSource* fSource;

　　但是FrameSource子类很多；所以需要确定fSource到底指向的是哪个子类；

　　不好意思，内容太多，懒得再拷贝了；具体内容参考上面的链接！

　　后续还会继续更新关于doGetNextFrame函数获取的帧数据是怎么处理和发送的，H264VideoStreamFramer中的doGetNextFrame函数和MPEGVideoStreamFramer中的doGetNextFrame函数都是什么作用！期待的话关注我，了解最新动态！

　　该文章持续更新！如果有错误或者模糊的地方欢迎留言探讨！