2 live555源码分析（二）——live555 基础运行环境

一、基础组件总览

本篇开始分析live555的基础组件，也就是基础运行环境所需要的组件类。UasgeEnvionment相关类的关系如下图所示

UsageEnvironment类是个抽象类，提供基础的运行环境，它依赖于TaskScheduler类来进行任务的调度。
BasicUsageEnvironment0继承自UsageEnvironment，是对UsageEnvironment的初步实现，但是也是个抽象类，真正的实现在BasicUsageEnvironment类。
BasicUsageEnvironment真正实现了基础运行环境，主要是实现了控制台对信息和错误的打印。

TaskScheduler类是任务调度的基础抽象类，里面有对三种任务（延迟任务、事件任务和后台IO任务）调度的函数抽象。
BasicTaskScheduler0类继承自TaskScheduler，是对TaskScheduler的初步实现，但是也是个抽象类，里面对三种任务对应的成员变量做了定义。
BasicTaskScheduler真正实现了任务的调度，分别是对延迟任务、事件任务和后台IO任务的调度。

DelayQueue类定义了延迟任务的队列，包含了对延迟任务的增删减除等功能。
DelayQueueEntry类其实就是DelayQueue队列的元素，也就是对延迟任务的定义。
AlarmHandler类是对DelayQueueEntry的真正实现，BasicTaskScheduler真正使用的是AlarmHandler。
DelayInterval是对时间差的一个封装，供DelayQueue相关类使用。
Timeval是一个工具类，可以实现时间的算术运算和比较大小。

二、UsageEnvironment

UsageEnvironment代码定义在目录UsageEnvironment/include/UsageEnvironment.h下，这是一个抽象类。头文件：

class UsageEnvironment {
public:
  Boolean reclaim();
      // returns True iff we were actually able to delete our object

  // task scheduler:
  TaskScheduler& taskScheduler() const {return fScheduler;}

  // 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;

  // 'console' output:
  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;
};

有三个成员变量，分别是：
liveMediaPriv ：媒体相关的状态
groupsockPriv：网络相关的状态
fScheduler：任务调度器，负责任务的调度
有一系列成员函数，基本都是做信息打印用的，都比较简单，这里不做过多分析。除了信息打印的外，还有个 reclaim()函数，做资源回收用，但是只有在媒体资源和网络资源为NULL时才会执行。

Boolean UsageEnvironment::reclaim() {
  // We delete ourselves only if we have no remainining state:
  if (liveMediaPriv == NULL && groupsockPriv == NULL) {
    delete this;
    return True;
  }
    return False;
}

三、BasicUsageEnvironment0

BasicUsageEnvironment0继承自UsageEnvironment，是对UsageEnvironment的初步实现，但是也是个抽象类，真正的实现在BasicUsageEnvironment类。它主要实现了将消息信息放进消息buffer里。主要通过函数appendToResultMsg来实现：

void BasicUsageEnvironment0::appendToResultMsg(MsgString msg) {
  char* curPtr = &fResultMsgBuffer[fCurBufferSize];
  unsigned spaceAvailable = fBufferMaxSize - fCurBufferSize;
  unsigned msgLength = strlen(msg);

  // Copy only enough of "msg" as will fit:
  if (msgLength > spaceAvailable-1) {
    msgLength = spaceAvailable-1;
  }

  memmove(curPtr, (char*)msg, msgLength);
  fCurBufferSize += msgLength;
  fResultMsgBuffer[fCurBufferSize] = '\0';
}

看起来也比较简单，先是计算出缓冲区剩余大小，然后跟消息长度做比较，如果消息太长，就丢弃过长的部分不做缓冲，防止缓冲区溢出。

四、BasicUsageEnvironment

BasicUsageEnvironment真正实现了基础运行环境，主要是通过重载输出运算符实现了对不同类型的消息打印到stderr的功能。

UsageEnvironment& BasicUsageEnvironment::operator<<(char const* str) {
  if (str == NULL) str = "(NULL)"; // sanity check
  fprintf(stderr, "%s", str);
  return *this;
}

UsageEnvironment& BasicUsageEnvironment::operator<<(int i) {
  fprintf(stderr, "%d", i);
  return *this;
}

UsageEnvironment& BasicUsageEnvironment::operator<<(unsigned u) {
  fprintf(stderr, "%u", u);
  return *this;
}

UsageEnvironment& BasicUsageEnvironment::operator<<(double d) {
  fprintf(stderr, "%f", d);
  return *this;
}

UsageEnvironment& BasicUsageEnvironment::operator<<(void* p) {
  fprintf(stderr, "%p", p);
  return *this;
}