public function on($event_name, $handler_callback)
    {
        $event = null;
        if (!isset($this->event_queue[$event_name])) {
            $event = new Event($event_name);
            $this->event_queue[$event_name] = $event;
        }

        $event = $this->event_queue[$event_name];
        $event->setHandlerCallback($handler_callback);
    }