Live555主要用到的类介绍：

1.UsageEnvironment类：抽象基类

class UsageEnvironment {
public:
  void reclaim();

  TaskScheduler& taskScheduler() const {return fScheduler;}//返回TaskScheduler

  // result message handling:
  typedef char const* MsgString;
  virtual MsgString getResultMsg() const = 0;

  virtual void setResultMsg(MsgString msg) = 0;
  virtual void setResultMsg(MsgString msg1, MsgString msg2) = 0;
  virtual void setResultMsg(MsgString msg1, MsgString msg2, MsgString msg3) = 0;
  virtual void setResultErrMsg(MsgString msg, int err = 0) = 0;
	// like setResultMsg(), except that an 'errno' message is appended.  (If "err == 0", the "getErrno()" code is used instead.)

  virtual void appendToResultMsg(MsgString msg) = 0;

  virtual void reportBackgroundError() = 0;
	// used to report a (previously set) error message within
	// a background event

  virtual void internalError(); // used to 'handle' a 'should not occur'-type error condition within the library.

  // 'errno'
  virtual int getErrno() const = 0;

  //重载"<<"进行一些错误信息的输出
  virtual UsageEnvironment& operator<<(char const* str) = 0;
  virtual UsageEnvironment& operator<<(int i) = 0;
  virtual UsageEnvironment& operator<<(unsigned u) = 0;
  virtual UsageEnvironment& operator<<(double d) = 0;
  virtual UsageEnvironment& operator<<(void* p) = 0;

  // a pointer to additional, optional, client-specific state
  void* liveMediaPriv;
  void* groupsockPriv;

protected:
  UsageEnvironment(TaskScheduler& scheduler); // abstract base class
  virtual ~UsageEnvironment(); // we are deleted only by reclaim()

private:
  TaskScheduler& fScheduler;//任务调度类
};

2.BasicUsageEnvironment继承自BasicUsageEnvironment0，BasicUsageEnvironment0又继承自UsageEnvironment。

class BasicUsageEnvironment: public BasicUsageEnvironment0 {
public:
  static BasicUsageEnvironment* createNew(TaskScheduler& taskScheduler);//创建UsageEnvironment对象

  // redefined virtual functions:
  virtual int getErrno() const;

  virtual UsageEnvironment& operator<<(char const* str);
  virtual UsageEnvironment& operator<<(int i);
  virtual UsageEnvironment& operator<<(unsigned u);
  virtual UsageEnvironment& operator<<(double d);
  virtual UsageEnvironment& operator<<(void* p);

protected:
  BasicUsageEnvironment(TaskScheduler& taskScheduler);
      // called only by "createNew()" (or subclass constructors)
  virtual ~BasicUsageEnvironment();
};

3.BasicUsageEnvironment0的实现，实现了一些错误信息的处理。

class BasicUsageEnvironment0: public UsageEnvironment {
public:
  // redefined virtual functions:
  virtual MsgString getResultMsg() const;

  virtual void setResultMsg(MsgString msg);
  virtual void setResultMsg(MsgString msg1,
		    MsgString msg2);
  virtual void setResultMsg(MsgString msg1,
		    MsgString msg2,
		    MsgString msg3);
  virtual void setResultErrMsg(MsgString msg, int err = 0);

  virtual void appendToResultMsg(MsgString msg);

  virtual void reportBackgroundError();

protected:
  BasicUsageEnvironment0(TaskScheduler& taskScheduler);
  virtual ~BasicUsageEnvironment0();

private:
  void reset();

  char fResultMsgBuffer[RESULT_MSG_BUFFER_MAX];
  unsigned fCurBufferSize;
  unsigned fBufferMaxSize;
};

4.TaskScheduler类则提供了任务调度功能，是整个应用程序的运行发动机，调度任务，执行任务。TaskScheduler在全局中只有一个，所以保存在了UsageEnvironment中，并且所有的类都保存了UsageEnvironment的指针，所以谁都可以将自己的任务加入调度中，很容易。

class TaskScheduler {
public:
  virtual ~TaskScheduler();

  virtual TaskToken scheduleDelayedTask(int64_t microseconds, TaskFunc* proc,
					void* clientData) = 0;
	// Schedules a task to occur (after a delay) when we next
	// reach a scheduling point.
	// (Does not delay if "microseconds" <= 0)
	// Returns a token that can be used in a subsequent call to
	// unscheduleDelayedTask()

  virtual void unscheduleDelayedTask(TaskToken& prevTask) = 0;
	// (Has no effect if "prevTask" == NULL)
        // Sets "prevTask" to NULL afterwards.

  virtual void rescheduleDelayedTask(TaskToken& task,
				     int64_t microseconds, TaskFunc* proc,
				     void* clientData);
  // Combines "unscheduleDelayedTask()" with "scheduleDelayedTask()"
  // (setting "task" to the new task token).

  // For handling socket operations in the background (from the event loop):
  typedef void BackgroundHandlerProc(void* clientData, int mask);
    // Possible bits to set in "mask".  (These are deliberately defined
    // the same as those in Tcl, to make a Tcl-based subclass easy.)
    #define SOCKET_READABLE    (1<<1)
    #define SOCKET_WRITABLE    (1<<2)
    #define SOCKET_EXCEPTION   (1<<3)
  virtual void setBackgroundHandling(int socketNum, int conditionSet, BackgroundHandlerProc* handlerProc, void* clientData) = 0;
  void disableBackgroundHandling(int socketNum) { setBackgroundHandling(socketNum, 0, NULL, NULL); }
  virtual void moveSocketHandling(int oldSocketNum, int newSocketNum) = 0;
        // Changes any socket handling for "oldSocketNum" so that occurs with "newSocketNum" instead.

  virtual void doEventLoop(char* watchVariable = NULL) = 0;
      // Causes further execution to take place within the event loop.
      // Delayed tasks, background I/O handling, and other events are handled, sequentially (as a single thread of control).
      // (If "watchVariable" is not NULL, then we return from this routine when *watchVariable != 0)

  virtual EventTriggerId createEventTrigger(TaskFunc* eventHandlerProc) = 0;
      // Creates a 'trigger' for an event, which - if it occurs - will be handled (from the event loop) using "eventHandlerProc".
      // (Returns 0 iff no such trigger can be created (e.g., because of implementation limits on the number of triggers).)
  virtual void deleteEventTrigger(EventTriggerId eventTriggerId) = 0;

  virtual void triggerEvent(EventTriggerId eventTriggerId, void* clientData = NULL) = 0;
      // Causes the (previously-registered) handler function for the specified event to be handled (from the event loop).
      // The handler function is called with "clientData" as parameter.
      // Note: This function (unlike other library functions) may be called from an external thread - to signal an external event.

  // The following two functions are deprecated, and are provided for backwards-compatibility only:
  void turnOnBackgroundReadHandling(int socketNum, BackgroundHandlerProc* handlerProc, void* clientData) {
    setBackgroundHandling(socketNum, SOCKET_READABLE, handlerProc, clientData);
  }
  void turnOffBackgroundReadHandling(int socketNum) { disableBackgroundHandling(socketNum); }

  virtual void internalError(); // used to 'handle' a 'should not occur'-type error condition within the library.

protected:
  TaskScheduler(); // abstract base class
};

5.BasicTaskScheduler类。任务调度的一个子类，实现了任务调度的创建。继承自BasicTaskScheduler0,BasicTaskScheduler0又继承自TaskScheduler。

class BasicTaskScheduler: public BasicTaskScheduler0 {
public:
  static BasicTaskScheduler* createNew(unsigned maxSchedulerGranularity = 10000/*microseconds*/);//参数描述了在返回到事件循环去处理非sock或者非计时器事件，比如"triggered events"之前等待的时间。

  virtual ~BasicTaskScheduler();

protected:
  BasicTaskScheduler(unsigned maxSchedulerGranularity);//protected属性

  static void schedulerTickTask(void* clientData);
  void schedulerTickTask();

protected:
  // Redefined virtual functions:
  virtual void SingleStep(unsigned maxDelayTime);

  virtual void setBackgroundHandling(int socketNum, int conditionSet, BackgroundHandlerProc* handlerProc, void* clientData);
  virtual void moveSocketHandling(int oldSocketNum, int newSocketNum);

protected:
  unsigned fMaxSchedulerGranularity;

  // To implement background operations:
  int fMaxNumSockets;
  fd_set fReadSet;
  fd_set fWriteSet;
  fd_set fExceptionSet;
};

createNew事实上就是创建了一个TaskScheduler对象，此处用到的是单例模式

BasicTaskScheduler* BasicTaskScheduler::createNew(unsigned maxSchedulerGranularity) {//
	return new BasicTaskScheduler(maxSchedulerGranularity);
}

6.BasicTaskScheduler0类的实现。

class BasicTaskScheduler0: public TaskScheduler {
public:
  virtual ~BasicTaskScheduler0();

  virtual void SingleStep(unsigned maxDelayTime = 0) = 0;
      // "maxDelayTime" is in microseconds.  It allows a subclass to impose a limit
      // on how long "select()" can delay, in case it wants to also do polling.
      // 0 (the default value) means: There's no maximum; just look at the delay queue

public:
  // Redefined virtual functions:
  virtual TaskToken scheduleDelayedTask(int64_t microseconds, TaskFunc* proc,
				void* clientData);
  virtual void unscheduleDelayedTask(TaskToken& prevTask);

  virtual void doEventLoop(char* watchVariable);

  virtual EventTriggerId createEventTrigger(TaskFunc* eventHandlerProc);
  virtual void deleteEventTrigger(EventTriggerId eventTriggerId);
  virtual void triggerEvent(EventTriggerId eventTriggerId, void* clientData = NULL);

protected:
  BasicTaskScheduler0();

protected:
  // To implement delayed operations:
  DelayQueue fDelayQueue;

  // To implement background reads:
  HandlerSet* fHandlers;
  int fLastHandledSocketNum;

  // To implement event triggers:
  EventTriggerId fTriggersAwaitingHandling, fLastUsedTriggerMask; // implemented as 32-bit bitmaps
  TaskFunc* fTriggeredEventHandlers[MAX_NUM_EVENT_TRIGGERS];
  void* fTriggeredEventClientDatas[MAX_NUM_EVENT_TRIGGERS];
  unsigned fLastUsedTriggerNum; // in the range [0,MAX_NUM_EVENT_TRIGGERS)
};