public function subscribe($eventType, $callback)
    {
        $callback = $this->makeEventCallback($callback);
        if (!isset($this->eventList[$eventType]) || !is_array($this->eventList[$eventType])) {
            $this->eventList[$eventType] = [];
        }
        array_push($this->eventList[$eventType], $callback);
        return $callback['id'];
    }