class FinishResponseSubscriber

class FinishResponseSubscriber implements EventSubscriberInterface {
    
    /**
     * The language manager object for retrieving the correct language code.
     *
     * @var \Drupal\Core\Language\LanguageManagerInterface
     */
    protected $languageManager;
    
    /**
     * A config object for the system performance configuration.
     *
     * @var \Drupal\Core\Config\Config
     */
    protected $config;
    
    /**
     * A policy rule determining the cacheability of a request.
     *
     * @var \Drupal\Core\PageCache\RequestPolicyInterface
     */
    protected $requestPolicy;
    
    /**
     * A policy rule determining the cacheability of the response.
     *
     * @var \Drupal\Core\PageCache\ResponsePolicyInterface
     */
    protected $responsePolicy;
    
    /**
     * The cache contexts manager service.
     *
     * @var \Drupal\Core\Cache\Context\CacheContextsManager
     */
    protected $cacheContexts;
    
    /**
     * Whether to send cacheability headers for debugging purposes.
     *
     * @var bool
     */
    protected $debugCacheabilityHeaders = FALSE;
    
    /**
     * Constructs a FinishResponseSubscriber object.
     *
     * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
     *   The language manager object for retrieving the correct language code.
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   A config factory for retrieving required config objects.
     * @param \Drupal\Core\PageCache\RequestPolicyInterface $request_policy
     *   A policy rule determining the cacheability of a request.
     * @param \Drupal\Core\PageCache\ResponsePolicyInterface $response_policy
     *   A policy rule determining the cacheability of a response.
     * @param \Drupal\Core\Cache\Context\CacheContextsManager $cache_contexts_manager
     *   The cache contexts manager service.
     * @param bool $http_response_debug_cacheability_headers
     *   (optional) Whether to send cacheability headers for debugging purposes.
     */
    public function __construct(LanguageManagerInterface $language_manager, ConfigFactoryInterface $config_factory, RequestPolicyInterface $request_policy, ResponsePolicyInterface $response_policy, CacheContextsManager $cache_contexts_manager, $http_response_debug_cacheability_headers = FALSE) {
        $this->languageManager = $language_manager;
        $this->config = $config_factory->get('system.performance');
        $this->requestPolicy = $request_policy;
        $this->responsePolicy = $response_policy;
        $this->cacheContextsManager = $cache_contexts_manager;
        $this->debugCacheabilityHeaders = $http_response_debug_cacheability_headers;
    }
    
    /**
     * Sets extra headers on any responses, also subrequest ones.
     *
     * @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
     *   The event to process.
     */
    public function onAllResponds(FilterResponseEvent $event) {
        $response = $event->getResponse();
        // Always add the 'http_response' cache tag to be able to invalidate every
        // response, for example after rebuilding routes.
        if ($response instanceof CacheableResponseInterface) {
            $response->getCacheableMetadata()
                ->addCacheTags([
                'http_response',
            ]);
        }
    }
    
    /**
     * Sets extra headers on successful responses.
     *
     * @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
     *   The event to process.
     */
    public function onRespond(FilterResponseEvent $event) {
        if (!$event->isMasterRequest()) {
            return;
        }
        $request = $event->getRequest();
        $response = $event->getResponse();
        // Set the X-UA-Compatible HTTP header to force IE to use the most recent
        // rendering engine.
        $response->headers
            ->set('X-UA-Compatible', 'IE=edge', FALSE);
        // Set the Content-language header.
        $response->headers
            ->set('Content-language', $this->languageManager
            ->getCurrentLanguage()
            ->getId());
        // Prevent browsers from sniffing a response and picking a MIME type
        // different from the declared content-type, since that can lead to
        // XSS and other vulnerabilities.
        // https://www.owasp.org/index.php/List_of_useful_HTTP_headers
        $response->headers
            ->set('X-Content-Type-Options', 'nosniff', FALSE);
        $response->headers
            ->set('X-Frame-Options', 'SAMEORIGIN', FALSE);
        // If the current response isn't an implementation of the
        // CacheableResponseInterface, we assume that a Response is either
        // explicitly not cacheable or that caching headers are already set in
        // another place.
        if (!$response instanceof CacheableResponseInterface) {
            if (!$this->isCacheControlCustomized($response)) {
                $this->setResponseNotCacheable($response, $request);
            }
            // HTTP/1.0 proxies do not support the Vary header, so prevent any caching
            // by sending an Expires date in the past. HTTP/1.1 clients ignore the
            // Expires header if a Cache-Control: max-age directive is specified (see
            // RFC 2616, section 14.9.3).
            if (!$response->headers
                ->has('Expires')) {
                $this->setExpiresNoCache($response);
            }
            return;
        }
        if ($this->debugCacheabilityHeaders) {
            // Expose the cache contexts and cache tags associated with this page in a
            // X-Drupal-Cache-Contexts and X-Drupal-Cache-Tags header respectively.
            $response_cacheability = $response->getCacheableMetadata();
            $response->headers
                ->set('X-Drupal-Cache-Tags', implode(' ', $response_cacheability->getCacheTags()));
            $response->headers
                ->set('X-Drupal-Cache-Contexts', implode(' ', $this->cacheContextsManager
                ->optimizeTokens($response_cacheability->getCacheContexts())));
        }
        $is_cacheable = $this->requestPolicy
            ->check($request) === RequestPolicyInterface::ALLOW && $this->responsePolicy
            ->check($response, $request) !== ResponsePolicyInterface::DENY;
        // Add headers necessary to specify whether the response should be cached by
        // proxies and/or the browser.
        if ($is_cacheable && $this->config
            ->get('cache.page.max_age') > 0) {
            if (!$this->isCacheControlCustomized($response)) {
                // Only add the default Cache-Control header if the controller did not
                // specify one on the response.
                $this->setResponseCacheable($response, $request);
            }
        }
        else {
            // If either the policy forbids caching or the sites configuration does
            // not allow to add a max-age directive, then enforce a Cache-Control
            // header declaring the response as not cacheable.
            $this->setResponseNotCacheable($response, $request);
        }
    }
    
    /**
     * Determine whether the given response has a custom Cache-Control header.
     *
     * Upon construction, the ResponseHeaderBag is initialized with an empty
     * Cache-Control header. Consequently it is not possible to check whether the
     * header was set explicitly by simply checking its presence. Instead, it is
     * necessary to examine the computed Cache-Control header and compare with
     * values known to be present only when Cache-Control was never set
     * explicitly.
     *
     * When neither Cache-Control nor any of the ETag, Last-Modified, Expires
     * headers are set on the response, ::get('Cache-Control') returns the value
     * 'no-cache, private'. If any of ETag, Last-Modified or Expires are set but
     * not Cache-Control, then 'private, must-revalidate' (in exactly this order)
     * is returned.
     *
     * @see \Symfony\Component\HttpFoundation\ResponseHeaderBag::computeCacheControlValue()
     *
     * @param \Symfony\Component\HttpFoundation\Response $response
     *
     * @return bool
     *   TRUE when Cache-Control header was set explicitly on the given response.
     */
    protected function isCacheControlCustomized(Response $response) {
        $cache_control = $response->headers
            ->get('Cache-Control');
        return $cache_control != 'no-cache, private' && $cache_control != 'private, must-revalidate';
    }
    
    /**
     * Add Cache-Control and Expires headers to a response which is not cacheable.
     *
     * @param \Symfony\Component\HttpFoundation\Response $response
     *   A response object.
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   A request object.
     */
    protected function setResponseNotCacheable(Response $response, Request $request) {
        $this->setCacheControlNoCache($response);
        $this->setExpiresNoCache($response);
        // There is no point in sending along headers necessary for cache
        // revalidation, if caching by proxies and browsers is denied in the first
        // place. Therefore remove ETag, Last-Modified and Vary in that case.
        $response->setEtag(NULL);
        $response->setLastModified(NULL);
        $response->headers
            ->remove('Vary');
    }
    
    /**
     * Add Cache-Control and Expires headers to a cacheable response.
     *
     * @param \Symfony\Component\HttpFoundation\Response $response
     *   A response object.
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   A request object.
     */
    protected function setResponseCacheable(Response $response, Request $request) {
        // HTTP/1.0 proxies do not support the Vary header, so prevent any caching
        // by sending an Expires date in the past. HTTP/1.1 clients ignore the
        // Expires header if a Cache-Control: max-age directive is specified (see
        // RFC 2616, section 14.9.3).
        if (!$response->headers
            ->has('Expires')) {
            $this->setExpiresNoCache($response);
        }
        $max_age = $this->config
            ->get('cache.page.max_age');
        $response->headers
            ->set('Cache-Control', 'public, max-age=' . $max_age);
        // In order to support HTTP cache-revalidation, ensure that there is a
        // Last-Modified and an ETag header on the response.
        if (!$response->headers
            ->has('Last-Modified')) {
            $timestamp = REQUEST_TIME;
            $response->setLastModified(new \DateTime(gmdate(DateTimePlus::RFC7231, REQUEST_TIME)));
        }
        else {
            $timestamp = $response->getLastModified()
                ->getTimestamp();
        }
        $response->setEtag($timestamp);
        // Allow HTTP proxies to cache pages for anonymous users without a session
        // cookie. The Vary header is used to indicates the set of request-header
        // fields that fully determines whether a cache is permitted to use the
        // response to reply to a subsequent request for a given URL without
        // revalidation.
        if (!$response->hasVary() && !Settings::get('omit_vary_cookie')) {
            $response->setVary('Cookie', FALSE);
        }
    }
    
    /**
     * Disable caching in the browser and for HTTP/1.1 proxies and clients.
     *
     * @param \Symfony\Component\HttpFoundation\Response $response
     *   A response object.
     */
    protected function setCacheControlNoCache(Response $response) {
        $response->headers
            ->set('Cache-Control', 'no-cache, must-revalidate');
    }
    
    /**
     * Disable caching in ancient browsers and for HTTP/1.0 proxies and clients.
     *
     * HTTP/1.0 proxies do not support the Vary header, so prevent any caching by
     * sending an Expires date in the past. HTTP/1.1 clients ignore the Expires
     * header if a Cache-Control: max-age= directive is specified (see RFC 2616,
     * section 14.9.3).
     *
     * @param \Symfony\Component\HttpFoundation\Response $response
     *   A response object.
     */
    protected function setExpiresNoCache(Response $response) {
        $response->setExpires(\DateTime::createFromFormat('j-M-Y H:i:s T', '19-Nov-1978 05:00:00 UTC'));
    }
    
    /**
     * Registers the methods in this class that should be listeners.
     *
     * @return array
     *   An array of event listener definitions.
     */
    public static function getSubscribedEvents() {
        $events[KernelEvents::RESPONSE][] = [
            'onRespond',
        ];
        // There is no specific reason for choosing 16 beside it should be executed
        // before ::onRespond().
        $events[KernelEvents::RESPONSE][] = [
            'onAllResponds',
            16,
        ];
        return $events;
    }

}