public function unsubscribe($eventType, $callbackId)
    {
        $success = false;
        if (!isset($this->eventList[$eventType]) || !is_array($this->eventList[$eventType])) {
            $this->eventList[$eventType] = array_filter(
                $this->eventList[$eventType],
                function ($cb) use ($callbackId) {
                    return $cb['id'] !== $callbackId;
                }
            );
            $success = true;
        }
        return $success;
    }