AccessRouteAlterSubscriber.php

<?php

namespace Drupal\system\EventSubscriber;

use Drupal\Core\Routing\RouteBuildEvent;
use Drupal\Core\Routing\RoutingEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Alters routes to add necessary requirements.
 *
 * @see \Drupal\system\Access\SystemAdminMenuBlockAccessCheck
 * @see \Drupal\system\Controller\SystemController::systemAdminMenuBlockPage()
 */
class AccessRouteAlterSubscriber implements EventSubscriberInterface {
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        $events[RoutingEvents::ALTER][] = 'accessAdminMenuBlockPage';
        return $events;
    }
    
    /**
     * Adds _access_admin_menu_block_page requirement to routes pointing to SystemController::systemAdminMenuBlockPage.
     *
     * @param \Drupal\Core\Routing\RouteBuildEvent $event
     *   The event to process.
     */
    public function accessAdminMenuBlockPage(RouteBuildEvent $event) {
        $routes = $event->getRouteCollection();
        foreach ($routes as $route) {
            // Do not use a leading slash when comparing to the _controller string
            // because the leading slash in a fully-qualified method name is optional.
            if ($route->hasDefault('_controller')) {
                switch (ltrim($route->getDefault('_controller'), '\\')) {
                    case 'Drupal\\system\\Controller\\SystemController::systemAdminMenuBlockPage':
                        $route->setRequirement('_access_admin_menu_block_page', 'TRUE');
                        break;
                    case 'Drupal\\system\\Controller\\SystemController::overview':
                        $route->setRequirement('_access_admin_overview_page', 'TRUE');
                        break;
                }
            }
        }
    }

}