live555学习-FramedSource详解

FramedSource讲解

FramedSource是一个抽象类，继承自mediaSource继承自medium，里面有纯虚函数virtual void doGetNextFrame() = 0;

主要作用就是从文件中获得数据，只是获得数据，交给分析H264or5VideoStreamParser分析获得NALU

关于有关牵涉到文件操作的基本都是直接或者间接继承自该类：例如H26VideoFileSourceMediaSubsession

主要函数流程：获得下一包数据getNextFrame

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;//存放读取到数据的内存指针  fMaxSize = maxSize;//允许的最大数据量，即fTo指针指向的内存区间的大小  fNumTruncatedBytes = 0; // by default; could be changed by doGetNextFrame()   表示实际读取数据大于<span style="font-family: Arial, Helvetica, sans-serif; font-size: 12px;">fMaxSize的时候，多了多少字节</span>  fDurationInMicroseconds = 0; // by default; could be changed by doGetNextFrame()  fAfterGettingFunc = afterGettingFunc;  fAfterGettingClientData = afterGettingClientData;  fOnCloseFunc = onCloseFunc;  fOnCloseClientData = onCloseClientData;  fIsCurrentlyAwaitingData = True;  doGetNextFrame();}

由此可见，此函数初始化了几个必要参数，并调用了纯虚函数doGetNextFrame();此函数留给派生类实现，例如ByteStreamFileSource类实现如下

void ByteStreamFileSource::doGetNextFrame() {  if (feof(fFid) || ferror(fFid) || (fLimitNumBytesToStream && fNumBytesToStream == 0)) {    handleClosure();    return;  }#ifdef READ_FROM_FILES_SYNCHRONOUSLY  doReadFromFile();#else  if (!fHaveStartedReading) {    // Await readable data from the file:    envir().taskScheduler().turnOnBackgroundReadHandling(fileno(fFid),       (TaskScheduler::BackgroundHandlerProc*)&fileReadableHandler, this);    fHaveStartedReading = True;  }#endif}

void ByteStreamFileSource::doReadFromFile() {  // Try to read as many bytes as will fit in the buffer provided (or "fPreferredFrameSize" if less)  if (fLimitNumBytesToStream && fNumBytesToStream < (u_int64_t)fMaxSize) {    fMaxSize = (unsigned)fNumBytesToStream;  }  if (fPreferredFrameSize > 0 && fPreferredFrameSize < fMaxSize) {    fMaxSize = fPreferredFrameSize;  }#ifdef READ_FROM_FILES_SYNCHRONOUSLY  fFrameSize = fread(fTo, 1, fMaxSize, fFid);#else  if (fFidIsSeekable) {    fFrameSize = fread(fTo, 1, fMaxSize, fFid);  } else {    // For non-seekable files (e.g., pipes), call "read()" rather than "fread()", to ensure that the read doesn't block:    fFrameSize = read(fileno(fFid), fTo, fMaxSize);  }#endif  if (fFrameSize == 0) {    handleClosure();    return;  }  fNumBytesToStream -= fFrameSize;  // Set the 'presentation time':  if (fPlayTimePerFrame > 0 && fPreferredFrameSize > 0) {    if (fPresentationTime.tv_sec == 0 && fPresentationTime.tv_usec == 0) {      // This is the first frame, so use the current time:      gettimeofday(&fPresentationTime, NULL);    } else {      // Increment by the play time of the previous data:      unsigned uSeconds= fPresentationTime.tv_usec + fLastPlayTime;      fPresentationTime.tv_sec += uSeconds/1000000;      fPresentationTime.tv_usec = uSeconds%1000000;    }    // Remember the play time of this data:    fLastPlayTime = (fPlayTimePerFrame*fFrameSize)/fPreferredFrameSize;    fDurationInMicroseconds = fLastPlayTime;  } else {    // We don't know a specific play time duration for this data,    // so just record the current time as being the 'presentation time':    gettimeofday(&fPresentationTime, NULL);  }  // Inform the reader that he has data:#ifdef READ_FROM_FILES_SYNCHRONOUSLY  // To avoid possible infinite recursion, we need to return to the event loop to do this:  nextTask() = envir().taskScheduler().scheduleDelayedTask(0,(TaskFunc*)FramedSource::afterGetting, this);#else  // Because the file read was done from the event loop, we can call the  // 'after getting' function directly, without risk of infinite recursion:  FramedSource::afterGetting(this);#endif}

在以上函数中关于几个重要参数主要是通过这里传送的nextTask() = envir().taskScheduler().scheduleDelayedTask(0,
(TaskFunc*)FramedSource::afterGetting, this);

FramedSource::afterGetting 再回到FramedSource中，发现afterGetting是个静态函数，所以在这里可以直接的调用，FramedSource中定义如下

void FramedSource::afterGetting(FramedSource* source) {  source->fIsCurrentlyAwaitingData = False;      // indicates that we can be read again      // Note that this needs to be done here, in case the "fAfterFunc"      // called below tries to read another frame (which it usually will)  if (source->fAfterGettingFunc != NULL) {    (*(source->fAfterGettingFunc))(source->fAfterGettingClientData,   source->fFrameSize, source->fNumTruncatedBytes,   source->fPresentationTime,   source->fDurationInMicroseconds);  }}

由上可见，afterGetting实际上就是把getNextFrame和doGetNextFrame();中初始化或者改变的参数包装了一下，用nextTask() = envir().taskScheduler().scheduleDelayedTask(0,
(TaskFunc*)FramedSource::afterGetting, this);传递下去

关于参数还有一个叫fFrameSize的，此参数代表实际读取的字节数，肯定这个数值要小于fMaxSize