class MaintenanceModeSubscriber

class MaintenanceModeSubscriber implements EventSubscriberInterface {
    use StringTranslationTrait;
    
    /**
     * The maintenance mode.
     *
     * @var \Drupal\Core\Site\MaintenanceModeInterface
     */
    protected $maintenanceMode;
    
    /**
     * The current account.
     *
     * @var \Drupal\Core\Session\AccountInterface
     */
    protected $account;
    
    /**
     * The config factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $config;
    
    /**
     * The URL generator.
     *
     * @var \Drupal\Core\Routing\UrlGeneratorInterface
     */
    protected $urlGenerator;
    
    /**
     * The bare HTML page renderer.
     *
     * @var \Drupal\Core\Render\BareHtmlPageRendererInterface
     */
    protected $bareHtmlPageRenderer;
    
    /**
     * The messenger.
     *
     * @var \Drupal\Core\Messenger\MessengerInterface
     */
    protected $messenger;
    
    /**
     * An event dispatcher instance to use for configuration events.
     *
     * @var \Symfony\Contracts\EventDispatcher\EventDispatcherInterface
     */
    protected $eventDispatcher;
    
    /**
     * Constructs a new MaintenanceModeSubscriber.
     *
     * @param \Drupal\Core\Site\MaintenanceModeInterface $maintenance_mode
     *   The maintenance mode.
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     * @param \Drupal\Core\StringTranslation\TranslationInterface $translation
     *   The string translation.
     * @param \Drupal\Core\Routing\UrlGeneratorInterface $url_generator
     *   The URL generator.
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The current user.
     * @param \Drupal\Core\Render\BareHtmlPageRendererInterface $bare_html_page_renderer
     *   The bare HTML page renderer.
     * @param \Drupal\Core\Messenger\MessengerInterface $messenger
     *   The messenger.
     * @param \Symfony\Contracts\EventDispatcher\EventDispatcherInterface $event_dispatcher
     *   The event dispatcher.
     */
    public function __construct(MaintenanceModeInterface $maintenance_mode, ConfigFactoryInterface $config_factory, TranslationInterface $translation, UrlGeneratorInterface $url_generator, AccountInterface $account, BareHtmlPageRendererInterface $bare_html_page_renderer, MessengerInterface $messenger, EventDispatcherInterface $event_dispatcher) {
        $this->maintenanceMode = $maintenance_mode;
        $this->config = $config_factory;
        $this->stringTranslation = $translation;
        $this->urlGenerator = $url_generator;
        $this->account = $account;
        $this->bareHtmlPageRenderer = $bare_html_page_renderer;
        $this->messenger = $messenger;
        $this->eventDispatcher = $event_dispatcher;
    }
    
    /**
     * Returns the site maintenance page if the site is offline.
     *
     * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event
     *   The event to process.
     */
    public function onKernelRequestMaintenance(RequestEvent $event) {
        $request = $event->getRequest();
        $route_match = RouteMatch::createFromRequest($request);
        if ($this->maintenanceMode
            ->applies($route_match)) {
            // Don't cache maintenance mode pages.
            \Drupal::service('page_cache_kill_switch')->trigger();
            if (!$this->maintenanceMode
                ->exempt($this->account)) {
                // When the account is not exempt, other subscribers handle request.
                $this->eventDispatcher
                    ->dispatch($event, MaintenanceModeEvents::MAINTENANCE_MODE_REQUEST);
            }
            else {
                // Display a message if the logged-in user has access to the site in
                // maintenance mode. Don't show maintenance message:
                // - on AJAX requests.
                // - on Iframe uploads.
                // - on the maintenance mode settings page.
                if ($route_match->getRouteName() != 'system.site_maintenance_mode') {
                    $show_message = $route_match->getRouteName() != 'system.site_maintenance_mode' && !$event->getRequest()
                        ->isXmlHttpRequest() && $event->getRequest()
                        ->get('ajax_iframe_upload', FALSE) === FALSE;
                    if ($show_message) {
                        if ($this->account
                            ->hasPermission('administer site configuration')) {
                            $this->messenger
                                ->addMessage($this->t('Operating in maintenance mode. <a href=":url">Go online.</a>', [
                                ':url' => $this->urlGenerator
                                    ->generate('system.site_maintenance_mode'),
                            ]), 'status', FALSE);
                        }
                        else {
                            $this->messenger
                                ->addMessage($this->t('Operating in maintenance mode.'), 'status', FALSE);
                        }
                    }
                }
            }
        }
    }
    
    /**
     * Returns response when site is in maintenance mode and user is not exempt.
     *
     * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event
     *   The event to process.
     */
    public function onMaintenanceModeRequest(RequestEvent $event) {
        $request = $event->getRequest();
        if ($request->getRequestFormat() !== 'html') {
            $response = new Response($this->maintenanceMode
                ->getSiteMaintenanceMessage(), 503, [
                'Content-Type' => 'text/plain',
            ]);
            // Calling RequestEvent::setResponse() also stops propagation of event.
            $event->setResponse($response);
            return;
        }
        drupal_maintenance_theme();
        $response = $this->bareHtmlPageRenderer
            ->renderBarePage([
            '#markup' => $this->maintenanceMode
                ->getSiteMaintenanceMessage(),
        ], $this->t('Site under maintenance'), 'maintenance_page');
        $response->setStatusCode(503);
        // Calling RequestEvent::setResponse() also stops propagation of the event.
        $event->setResponse($response);
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        $events[KernelEvents::REQUEST][] = [
            'onKernelRequestMaintenance',
            30,
        ];
        $events[KernelEvents::EXCEPTION][] = [
            'onKernelRequestMaintenance',
        ];
        $events[MaintenanceModeEvents::MAINTENANCE_MODE_REQUEST][] = [
            'onMaintenanceModeRequest',
            -1000,
        ];
        return $events;
    }

}