public function attach($event, $callback) {
        $events = is_array($event) ? $event : array($event);
        $listeners = $this->getOption(self::OPTION_LISTENERS);
        foreach ($events as $event) {
            $eventObject = is_object($event) ? $event : new GenericEvent($event);
            if (!isset($listeners[$eventObject->getName()])) {
                $listeners[$eventObject->getName()] = array();
            }

            if (!in_array($callback, $listeners[$eventObject->getName()], true)) {
                $listeners[$eventObject->getName()][] = $callback;
            }
        }
        $this->setOption(self::OPTION_LISTENERS, $listeners);
    }