code: public function registerSubscriber($class, callable $callback)
      {
          $service_id = "event." . strtolower(str_replace("\\", ".", $class));
  
          $this[$service_id] = $callback;
  
          $this["dispatcher"]->addSubscriberService($service_id, $class);
      }
docs: register our event subscriber
  @param string $class
  @param callable $callback