public function register($event, $listener, \Closure $callback)
    {
        if (!isset($this->events[$event])) {
            $this->events[$event] = [];
        }

        if ($listener) {
            $this->events[$event][$listener] = $callback;
        } else {
            $this->events[$event][] = $callback;
            $keys = array_keys($this->events[$event]);
            $listener = array_pop($keys);
        }

        return $listener;
    }