class RouteProviderLazyBuilder

class RouteProviderLazyBuilder implements PreloadableRouteProviderInterface, EventSubscriberInterface {

  /**
   * The route provider service.
   *
   * @var \Drupal\Core\Routing\RouteProviderInterface
   */
  protected $routeProvider;

  /**
   * The route building service.
   *
   * @var \Drupal\Core\Routing\RouteBuilderInterface
   */
  protected $routeBuilder;

  /**
   * Flag to determine if the router has been rebuilt.
   *
   * @var bool
   */
  protected $rebuilt = FALSE;

  /**
   * Flag to determine if router is currently being rebuilt.
   *
   * Used to prevent recursive router rebuilds during module installation.
   * Recursive rebuilds can occur when route information is required by alter
   * hooks that are triggered during a rebuild, for example,
   * hook_menu_links_discovered_alter().
   *
   * @var bool
   */
  protected $rebuilding = FALSE;

  /**
   * RouteProviderLazyBuilder constructor.
   *
   * @param \Drupal\Core\Routing\RouteProviderInterface $route_provider
   *   The route provider service.
   * @param \Drupal\Core\Routing\RouteBuilderInterface $route_builder
   *   The route building service.
   */
  public function __construct(RouteProviderInterface $route_provider, RouteBuilderInterface $route_builder) {
    $this->routeProvider = $route_provider;
    $this->routeBuilder = $route_builder;
  }

  /**
   * Gets the real route provider service and rebuilds the router id necessary.
   *
   * @return \Drupal\Core\Routing\RouteProviderInterface
   *   The route provider service.
   */
  protected function getRouteProvider() {
    if (!$this->rebuilt && !$this->rebuilding) {
      $this->routeBuilder
        ->rebuild();
    }
    return $this->routeProvider;
  }

  /**
   * {@inheritdoc}
   */
  public function getRouteCollectionForRequest(Request $request) {
    return $this
      ->getRouteProvider()
      ->getRouteCollectionForRequest($request);
  }

  /**
   * {@inheritdoc}
   */
  public function getRouteByName($name) {
    return $this
      ->getRouteProvider()
      ->getRouteByName($name);
  }

  /**
   * {@inheritdoc}
   */
  public function preLoadRoutes($names) {
    return $this
      ->getRouteProvider()
      ->preLoadRoutes($names);
  }

  /**
   * {@inheritdoc}
   */
  public function getRoutesByNames($names) {
    return $this
      ->getRouteProvider()
      ->getRoutesByNames($names);
  }

  /**
   * {@inheritdoc}
   */
  public function getRoutesByPattern($pattern) {
    return $this
      ->getRouteProvider()
      ->getRoutesByPattern($pattern);
  }

  /**
   * {@inheritdoc}
   */
  public function getAllRoutes() {
    return $this
      ->getRouteProvider()
      ->getAllRoutes();
  }

  /**
   * {@inheritdoc}
   */
  public function reset() {

    // Don't call getRouteProvider as this is results in recursive rebuilds.
    return $this->routeProvider
      ->reset();
  }

  /**
   * Determines if the router has been rebuilt.
   *
   * @return bool
   *   TRUE is the router has been rebuilt, FALSE if not.
   */
  public function hasRebuilt() {
    return $this->rebuilt;
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {
    $events[RoutingEvents::DYNAMIC][] = [
      'routerRebuilding',
      3000,
    ];
    $events[RoutingEvents::FINISHED][] = [
      'routerRebuildFinished',
      -3000,
    ];
    return $events;
  }

  /**
   * Sets the router rebuilding flag to TRUE.
   */
  public function routerRebuilding() {
    $this->rebuilding = TRUE;
  }

  /**
   * Sets the router rebuilding flag to FALSE.
   */
  public function routerRebuildFinished() {
    $this->rebuilding = FALSE;
    $this->rebuilt = TRUE;
  }

}