Разбираем систему событий в Drupal 8

namespace Drupal\user\EventSubscriber;

use Drupal\Core\Routing\RouteMatch;
use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Site\MaintenanceModeInterface;
use Drupal\Core\Url;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

/**
* Maintenance mode subscriber to log out users.
*/
class MaintenanceModeSubscriber implements EventSubscriberInterface {

 /**
  * The maintenance mode.
  */
 protected $maintenanceMode;

 /**
  * The current account.
  */
 protected $account;

 /**
  * Constructs a new MaintenanceModeSubscriber.
  */
 public function __construct(MaintenanceModeInterface $maintenance_mode, AccountInterface $account) {
   // Подключаем сервисы, которые мы определили в описании сервиса в user.services.yml
   $this->maintenanceMode = $maintenance_mode;
   $this->account = $account;
 }

 /**
  * Метод getSubscribedEvents() является обязательным и служит
  * для описания слушателей.
  */
 public static function getSubscribedEvents() {
   // Подписываемся на событие KernelEvents::REQUEST. 
   // Когда событие произойдет, то запустится метод-слушатель onKernelRequestMaintenance в этом же классе. 
   // 31 - приоритет слушателя.   
   $events[KernelEvents::REQUEST][] = ['onKernelRequestMaintenance', 31];
   return $events;
 }

 /**
  * Logout users if site is in maintenance mode.
  */
 public function onKernelRequestMaintenance(GetResponseEvent $event) {
   // Непосредственно реализация слушателя.
   $request = $event->getRequest();
   $route_match = RouteMatch::createFromRequest($request);
   if ($this->maintenanceMode->applies($route_match)) {
     // If the site is offline, log out unprivileged users.
     if ($this->account->isAuthenticated() && !$this->maintenanceMode->exempt($this->account)) {
       user_logout();
       // Redirect to homepage.
       $event->setResponse(
         new RedirectResponse(Url::fromRoute('<front>')->toString())
       );
     }
   }
 }
}

/**
* Инициализация слушателей.
*/
public static function getSubscribedEvents() {
 $events[ConfigEvents::SAVE][] = ['onConfigSave', 0];
 // Проверка на пустое значение имеет высокий приоритет, 
 // чтобы остановить дальнейшую обработку событий если конфигурация пустая.
 $events[ConfigEvents::IMPORT_VALIDATE][] = ['onConfigImporterValidateNotEmpty', 512];
 return $events;
}

/**
* Если конфигурация пуста, то не нужно её импортировать, т.к. это удалит имеющуюся конфигурацию. 
* Останавливаем процесс импорта на данном этапе.
*/
public function onConfigImporterValidateNotEmpty(ConfigImporterEvent $event) {
 $importList = $event->getConfigImporter()->getStorageComparer()->getSourceStorage()->listAll();
 if (empty($importList)) {
   $event->getConfigImporter()->logError($this->t('This import is empty and if applied would delete all of your configuration, so has been rejected.'));
   $event->stopPropagation();
 }
}

$response = new RedirectResponse($url);
// Слушатель события определенный динамически.
$listener = function ($event) use ($response) {
  $event->setResponse($response);
};
// Добавляем слушатель события в реестр событий используя сервис “event_dispatcher”.
$this->dispatcher->addListener(KernelEvents::RESPONSE, $listener);

namespace Drupal\Core\Routing;

/**
* Contains all events thrown in the core routing component.
*/
final class RoutingEvents {

 /**
  * Name of the event fired during route collection to allow new routes.
  */
 const DYNAMIC = 'routing.route_dynamic';

 /**
  * Name of the event fired during route collection to allow changes to routes.
  */
 const ALTER = 'routing.route_alter';

 /**
  * Name of the event fired to indicate route building has ended.
  */
 const FINISHED = 'routing.route_finished';
}

namespace Drupal\Core\Routing;

use Symfony\Component\EventDispatcher\Event;
use Symfony\Component\Routing\RouteCollection;

/**
* Represents route building information as event.
*/
class RouteBuildEvent extends Event {

 /**
  * The route collection.
  */
 protected $routeCollection;

 /**
  * Constructs a RouteBuildEvent object.
  */
 public function __construct(RouteCollection $route_collection) {
   $this->routeCollection = $route_collection;
 }

 /**
  * Gets the route collection.
  */
 public function getRouteCollection() {
   return $this->routeCollection;
 }
}

namespace Drupal\Core\Routing;

/**
* Managing class for rebuilding the router table.
*/
class RouteBuilder implements RouteBuilderInterface, DestructableInterface {

/**
* The event dispatcher to notify of routes.
*
* @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
*/
protected $dispatcher;

...

public function rebuild() {
  ...
  // DYNAMIC is supposed to be used to add new routes based upon all the
  // static defined ones.
  $this->dispatcher->dispatch(RoutingEvents::DYNAMIC, new RouteBuildEvent($collection));

  // ALTER is the final step to alter all the existing routes. We cannot stop
  // people from adding new routes here, but we define two separate steps to
  // make it clear.
  $this->dispatcher->dispatch(RoutingEvents::ALTER, new RouteBuildEvent($collection));
  $this->checkProvider->setChecks($collection);

  ...
}
}