public function getCategoriesFunction($locale, $parentKey = null)
    {
        return $this->memoizeCache->memoizeById(
            'sulu_categories',
            func_get_args(),
            function($locale, $parentKey = null) {
                $entities = $this->categoryManager->findChildrenByParentKey($parentKey);
                $categories = $this->categoryManager->getApiObjects($entities, $locale);
                $context = SerializationContext::create();
                $context->setSerializeNull(true);

                return $this->serializer->serialize($categories, 'array', $context);
            }
        );
    }