protected function transformUrl($url, $transformation)
    {
        $cachedUrl = $this->pool->getItem(md5($url));

        if ($cachedUrl->isMiss()) {
            $url = $this->provider->$transformation($url);

            $cachedUrl->set($url);
        } else {
            $url = $cachedUrl->get();
        }

        return $url;
    }