code: public function eventstream(EventStreamInterface $eventstream = null) : EventStreamInterface
      {
          if ($eventstream){
              $this->eventstream = $eventstream;
          }
          if (!$this->eventstream){
              $this->eventstream = new NullEventStream;
          }
          return $this->eventstream;
      }
docs: Set/get event stream
  
  @param EventStreamInterface $eventstream
  
  @return EventStreamInterface