code: def _watchdog(self):
          
          if not self.is_alive:
              logger.debug("Player has already terminated.")
              self._exit_event.set()
              return
          logger.debug("Watching %s[%d]",
                       self._player_command, self.sub_proc.pid)
          returncode = self.sub_proc.wait()
          logger.debug("%s[%d] exit with code %d",
                       self._player_command, self.sub_proc.pid, returncode)
          self._exit_event.set()
docs: 监控正在运行的播放器（独立线程）
  
          播放器退出后将会设置 _exit_event