code: def start(self, timeout=None, root_object=None):
          
  
          if self._is_running:
              return
  
          if timeout:
              self._timeout = timeout
              self._start_time = int(time())
  
          pushcenter_logger.debug("[NURESTPushCenter] Starting push center on url %s ..." % self.url)
          self._is_running = True
          self.__root_object = root_object
  
          from .nurest_session import NURESTSession
          current_session = NURESTSession.get_current_session()
          args_session = {'session': current_session}
  
          self._thread = StoppableThread(target=self._listen, name='push-center', kwargs=args_session)
          self._thread.daemon = True
          self._thread.start()
docs: Starts listening to events.
  
              Args:
                  timeout (int): number of seconds before timeout. Used for testing purpose only.
                  root_object (bambou.NURESTRootObject): NURESTRootObject object that is listening. Used for testing purpose only.