code: public function getSubscribers($eventName = null)
      {
          if ($eventName !== null) {
              if (!isset($this->subscribers[$eventName])) {
                  return array();
              }
              if (!isset($this->sorted[$eventName])) {
                  $this->sortSubscribers($eventName);
              }
              return $this->sorted[$eventName];
          }
          // return all subscribers
          foreach (\array_keys($this->subscribers) as $eventName) {
              if (!isset($this->sorted[$eventName])) {
                  $this->sortSubscribers($eventName);
              }
          }
          return \array_filter($this->sorted);
      }
docs: Gets the subscribers of a specific event or all subscribers sorted by descending priority.
  
  If event name is not specified, subscribers for all events will be returned
  
  @param string $eventName The name of the event
  
  @return array The event subscribers for the specified event, or all event subscribers by event name