private function serializeFields($props, $event) {
        array_map(function (ReflectionProperty $prop) use (&$fields, $event) {
            /** @var ReflectionProperty $prop */
            $prop->setAccessible(true);
            $fields[$prop->getName()] = $this->valueSerializer->serialize($prop->getValue($event));
        }, $props);

        return $fields;
    }