public function indexAction(Request $request)
    {
        $registry = $this->get('ekyna_sitemap.provider_registry');
        $sitemaps = [];

        $lastUpdateDate = null;
        foreach($registry->getSitemaps() as $sitemap) {
            $providers = $registry->getProvidersBySitemap($sitemap);
            $sitemapLastUpdateDate = null;
            /** @var \Ekyna\Bundle\SitemapBundle\Provider\ProviderInterface $provider */
            foreach ($providers as $provider) {
                if (null === $sitemapLastUpdateDate || $sitemapLastUpdateDate < $provider->getLastUpdateDate()) {
                    $sitemapLastUpdateDate = $provider->getLastUpdateDate();
                }
            }
            $sitemaps[$sitemap] = $sitemapLastUpdateDate;
            if (null === $lastUpdateDate || $lastUpdateDate < $sitemapLastUpdateDate) {
                $lastUpdateDate = $sitemapLastUpdateDate;
            }
        }

        $response = new Response();
        $response->setLastModified($lastUpdateDate);
        $response->setPublic();
        if ($response->isNotModified($request)) {
            return $response;
        }
        $response->setMaxAge($this->container->getParameter('ekyna_sitemap.index_ttl'));
        $response->headers->add(['Content-Type' => 'application/xml; charset=UTF-8']);

        return $response->setContent($this->renderView('EkynaSitemapBundle:Sitemap:index.xml.twig', [
            'sitemaps' => $sitemaps,
        ]));
    }