class RouteSubscriber

class RouteSubscriber extends RouteSubscriberBase {
    
    /**
     * The entity type manager service.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * The module handler.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * The route collection for adding routes.
     *
     * @var \Symfony\Component\Routing\RouteCollection
     */
    protected $collection;
    
    /**
     * The current base path.
     *
     * @var string
     */
    protected $basePath;
    
    /**
     * The BC base path.
     *
     * @var string
     */
    protected $basePathBc;
    
    /**
     * The redirect controller.
     *
     * @var string
     */
    protected $controller;
    
    /**
     * Constructs a RouteSubscriber object.
     *
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager service.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler.
     */
    public function __construct(EntityTypeManagerInterface $entity_type_manager, ModuleHandlerInterface $module_handler) {
        $this->entityTypeManager = $entity_type_manager;
        $this->moduleHandler = $module_handler;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function alterRoutes(RouteCollection $collection) {
        $this->collection = $collection;
        // @see block_content.routing.yml
        if ($this->setUpBaseRoute('entity.block_content_type.collection')) {
            $this->addRedirectRoute('block_content.type_add');
        }
        $entity_type = $this->entityTypeManager
            ->getDefinition('block_content');
        if ($this->setUpBaseRoute($entity_type->get('field_ui_base_route'))) {
            foreach ($this->childRoutes($entity_type) as $route_name) {
                $this->addRedirectRoute($route_name);
            }
        }
    }
    
    /**
     * Gets parameters from a base route and saves them in class variables.
     *
     * @param string $base_route_name
     *   The name of a base route that already has a BC variant.
     *
     * @return bool
     *   TRUE if all parameters are set, FALSE if not.
     */
    protected function setUpBaseRoute(string $base_route_name) : bool {
        $base_route = $this->collection
            ->get($base_route_name);
        $base_route_bc = $this->collection
            ->get("{$base_route_name}.bc");
        if (empty($base_route) || empty($base_route_bc)) {
            return FALSE;
        }
        $this->basePath = $base_route->getPath();
        $this->basePathBc = $base_route_bc->getPath();
        $this->controller = $base_route_bc->getDefault('_controller');
        if (empty($this->basePath) || empty($this->basePathBc) || empty($this->controller) || $this->basePathBc === $this->basePath) {
            return FALSE;
        }
        return TRUE;
    }
    
    /**
     * Adds a redirect route.
     *
     * @param string $route_name
     *   The name of a route whose path has changed.
     */
    protected function addRedirectRoute(string $route_name) : void {
        // Exit early if the BC route is already there.
        if (!empty($this->collection
            ->get("{$route_name}.bc"))) {
            return;
        }
        $route = $this->collection
            ->get($route_name);
        if (empty($route)) {
            return;
        }
        $new_path = $route->getPath();
        if (!str_starts_with($new_path, $this->basePath)) {
            return;
        }
        $bc_route = clone $route;
        // Set the path to what it was in earlier versions of Drupal.
        $bc_route->setPath($this->basePathBc . substr($new_path, strlen($this->basePath)));
        if ($bc_route->getPath() === $route->getPath()) {
            return;
        }
        // Replace the handler with the stored redirect controller.
        $defaults = array_diff_key($route->getDefaults(), array_flip([
            '_entity_form',
            '_entity_list',
            '_entity_view',
            '_form',
        ]));
        $defaults['_controller'] = $this->controller;
        $bc_route->setDefaults($defaults);
        $this->collection
            ->add("{$route_name}.bc", $bc_route);
    }
    
    /**
     * Creates a list of routes that need BC redirects.
     *
     * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
     *   The entity type.
     *
     * @return string[]
     *   A list of route names.
     */
    protected function childRoutes(EntityTypeInterface $entity_type) : array {
        $route_names = [];
        if ($field_ui_base_route = $entity_type->get('field_ui_base_route')) {
            $updated_routes = new RouteCollection();
            $updated_routes->add($field_ui_base_route, $this->collection
                ->get($field_ui_base_route));
            $event = new RouteBuildEvent($updated_routes);
            // Apply route subscribers that add routes based on field_ui_base_route,
            // in the order of their weights.
            $subscribers = [
                'field_ui' => 'field_ui.subscriber',
                'content_translation' => 'content_translation.subscriber',
            ];
            foreach ($subscribers as $module_name => $service_name) {
                if ($this->moduleHandler
                    ->moduleExists($module_name)) {
                    \Drupal::service($service_name)->onAlterRoutes($event);
                }
            }
            $updated_routes->remove($field_ui_base_route);
            $route_names = array_merge($route_names, array_keys($updated_routes->all()));
            $route_names = array_merge($route_names, [
                // @see \Drupal\config_translation\Routing\RouteSubscriber::alterRoutes()
"config_translation.item.add.{$field_ui_base_route}",
                "config_translation.item.edit.{$field_ui_base_route}",
                "config_translation.item.delete.{$field_ui_base_route}",
            ]);
        }
        if ($entity_type_id = $entity_type->getBundleEntityType()) {
            $route_names = array_merge($route_names, [
                // @see \Drupal\Core\Entity\Routing\DefaultHtmlRouteProvider::getRoutes()
"entity.{$entity_type_id}.delete_form",
                // @see \Drupal\config_translation\Routing\RouteSubscriber::alterRoutes()
"entity.{$entity_type_id}.config_translation_overview",
                // @see \Drupal\user\Entity\EntityPermissionsRouteProvider::getRoutes()
"entity.{$entity_type_id}.entity_permissions_form",
            ]);
        }
        if ($entity_id = $entity_type->id()) {
            $route_names = array_merge($route_names, [
                // @see \Drupal\config_translation\Routing\RouteSubscriber::alterRoutes()
"entity.field_config.config_translation_overview.{$entity_id}",
                "config_translation.item.add.entity.field_config.{$entity_id}_field_edit_form",
                "config_translation.item.edit.entity.field_config.{$entity_id}_field_edit_form",
                "config_translation.item.delete.entity.field_config.{$entity_id}_field_edit_form",
                // @see \Drupal\layout_builder\Plugin\SectionStorage\DefaultsSectionStorage::buildRoutes()
"layout_builder.defaults.{$entity_id}.disable",
                "layout_builder.defaults.{$entity_id}.discard_changes",
                "layout_builder.defaults.{$entity_id}.view",
            ]);
        }
        return $route_names;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        $events = parent::getSubscribedEvents();
        // Go after ContentTranslationRouteSubscriber.
        $events[RoutingEvents::ALTER] = [
            'onAlterRoutes',
            -300,
        ];
        return $events;
    }

}