class RoutePreloader

class RoutePreloader implements EventSubscriberInterface {

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

  /**
   * The state key value store.
   *
   * @var \Drupal\Core\State\StateInterface
   */
  protected $state;

  /**
   * Contains the non-admin routes while rebuilding the routes.
   *
   * @var array
   */
  protected $nonAdminRoutesOnRebuild = [];

  /**
   * Constructs a new RoutePreloader.
   *
   * @param \Drupal\Core\Routing\RouteProviderInterface $route_provider
   *   The route provider.
   * @param \Drupal\Core\State\StateInterface $state
   *   The state key value store.
   */
  public function __construct(RouteProviderInterface $route_provider, StateInterface $state) {
    $this->routeProvider = $route_provider;
    $this->state = $state;
    if (func_num_args() > 2) {
      @trigger_error(sprintf('Passing a cache bin to %s is deprecated in drupal:10.3.0 and will be removed before drupal:11.0.0. Caching is now managed by the state service. See https://www.drupal.org/node/3177901', __METHOD__), E_USER_DEPRECATED);
    }
  }

  /**
   * Loads all non-admin routes right before the actual page is rendered.
   *
   * @param \Symfony\Component\HttpKernel\Event\KernelEvent $event
   *   The event to process.
   */
  public function onRequest(KernelEvent $event) {

    // Only preload on normal HTML pages, as they will display menu links.
    if ($this->routeProvider instanceof PreloadableRouteProviderInterface && $event
      ->getRequest()
      ->getRequestFormat() == 'html') {
      $routes = $this->state
        ->get('routing.non_admin_routes', []);
      if ($routes) {

        // Preload all the non-admin routes at once.
        $this->routeProvider
          ->preLoadRoutes($routes);
      }
    }
  }

  /**
   * Alters existing routes for a specific collection.
   *
   * @param \Drupal\Core\Routing\RouteBuildEvent $event
   *   The route build event.
   */
  public function onAlterRoutes(RouteBuildEvent $event) {
    $collection = $event
      ->getRouteCollection();
    foreach ($collection
      ->all() as $name => $route) {
      if (!str_starts_with($route
        ->getPath(), '/admin/') && $route
        ->getPath() != '/admin' && static::isGetAndHtmlRoute($route)) {
        $this->nonAdminRoutesOnRebuild[] = $name;
      }
    }
    $this->nonAdminRoutesOnRebuild = array_unique($this->nonAdminRoutesOnRebuild);
  }

  /**
   * Store the non admin routes in state when the route building is finished.
   */
  public function onFinishedRoutes() {
    $this->state
      ->set('routing.non_admin_routes', $this->nonAdminRoutesOnRebuild);
    $this->nonAdminRoutesOnRebuild = [];
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {

    // Set a really low priority to catch as many as possible routes.
    $events[RoutingEvents::ALTER] = [
      'onAlterRoutes',
      -1024,
    ];
    $events[RoutingEvents::FINISHED] = [
      'onFinishedRoutes',
    ];

    // Load the routes before the controller is executed (which happens after
    // the kernel request event).
    $events[KernelEvents::REQUEST][] = [
      'onRequest',
    ];
    return $events;
  }

  /**
   * Determines whether the given route is a GET and HTML route.
   *
   * @param \Symfony\Component\Routing\Route $route
   *   The route to analyze.
   *
   * @return bool
   *   TRUE if GET is a valid method and HTML is a valid format for this route.
   */
  protected static function isGetAndHtmlRoute(Route $route) {
    $methods = $route
      ->getMethods() ?: [
      'GET',
    ];

    // If a route has no explicit format, then HTML is valid.
    // @see \Drupal\Core\Routing\RequestFormatRouteFilter::getAvailableFormats()
    $format = $route
      ->hasRequirement('_format') ? explode('|', $route
      ->getRequirement('_format')) : [
      'html',
    ];
    return in_array('GET', $methods, TRUE) && in_array('html', $format, TRUE);
  }

}