private function getEtag(ResourceObject $ro, HttpCache $httpCache = null) : string
    {
        $etag = $httpCache instanceof HttpCache && $httpCache->etag ? $this->getEtagByPartialBody($httpCache, $ro) : $this->getEtagByEitireView($ro);

        return (string) \crc32(\get_class($ro) . $etag);
    }