class AnonymousUserResponseSubscriber

class AnonymousUserResponseSubscriber implements EventSubscriberInterface {
    
    /**
     * The current user.
     *
     * @var \Drupal\Core\Session\AccountInterface
     */
    protected $currentUser;
    
    /**
     * Constructs an AnonymousUserResponseSubscriber object.
     *
     * @param \Drupal\Core\Session\AccountInterface $current_user
     *   The current user.
     */
    public function __construct(AccountInterface $current_user) {
        $this->currentUser = $current_user;
    }
    
    /**
     * Adds a cache tag if the 'user.permissions' cache context is present.
     *
     * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
     *   The event to process.
     */
    public function onRespond(ResponseEvent $event) {
        if (!$event->isMainRequest()) {
            return;
        }
        if (!$this->currentUser
            ->isAnonymous()) {
            return;
        }
        $response = $event->getResponse();
        if (!$response instanceof CacheableResponseInterface) {
            return;
        }
        // The 'user.permissions' cache context ensures that if the permissions for
        // a role are modified, users are not served stale render cache content.
        // But, when entire responses are cached in reverse proxies, the value for
        // the cache context is never calculated, causing the stale response to not
        // be invalidated. Therefore, when varying by permissions and the current
        // user is the anonymous user, also add the cache tag for the 'anonymous'
        // role.
        if (in_array('user.permissions', $response->getCacheableMetadata()
            ->getCacheContexts())) {
            $per_permissions_response_for_anon = new CacheableMetadata();
            $per_permissions_response_for_anon->setCacheTags([
                'config:user.role.anonymous',
            ]);
            $response->addCacheableDependency($per_permissions_response_for_anon);
        }
    }
    
    /**
     * Registers the methods in this class that should be listeners.
     *
     * @return array
     *   An array of event listener definitions.
     */
    public static function getSubscribedEvents() : array {
        // Priority 5, so that it runs before FinishResponseSubscriber, but after
        // event subscribers that add the associated cacheability metadata (which
        // have priority 10). This one is conditional, so must run after those.
        $events[KernelEvents::RESPONSE][] = [
            'onRespond',
            5,
        ];
        return $events;
    }

}