MaintenanceModeSubscriber::onKernelRequestMaintenance

public MaintenanceModeSubscriber::onKernelRequestMaintenance(GetResponseEvent $event)

public function onKernelRequestMaintenance(GetResponseEvent $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)) {
      // Deliver the 503 page if the site is in maintenance mode and the
      // logged in user is not allowed to bypass it.

      // If the request format is not 'html' then show default maintenance
      // mode page else show a text/plain page with maintenance message.
      if ($request->getRequestFormat() !== 'html') {
        $response = new Response($this->getSiteMaintenanceMessage(), 503, array('Content-Type' => 'text/plain'));
        $event->setResponse($response);
        return;
      }
      drupal_maintenance_theme();
      $response = $this->bareHtmlPageRenderer->renderBarePage(['#markup' => $this->getSiteMaintenanceMessage()], $this->t('Site under maintenance'), 'maintenance_page');
      $response->setStatusCode(503);
      $event->setResponse($response);
    }
    else {
      // Display a message if the logged in user has access to the site in
      // maintenance mode. However, suppress it on the maintenance mode
      // settings page.
      if ($route_match->getRouteName() != 'system.site_maintenance_mode') {
        if ($this->account->hasPermission('administer site configuration')) {
          $this->drupalSetMessage($this->t('Operating in maintenance mode. <a href=":url">Go online.</a>', array(':url' => $this->urlGenerator->generate('system.site_maintenance_mode'))), 'status', FALSE);
        }
        else {
          $this->drupalSetMessage($this->t('Operating in maintenance mode.'), 'status', FALSE);
        }
      }
    }
  }
}