class Fast404ExceptionHtmlSubscriber

class Fast404ExceptionHtmlSubscriber extends HttpExceptionSubscriberBase {

  /**
   * The config factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;

  /**
   * The cache tags invalidator.
   *
   * @var \Drupal\Core\Cache\CacheTagsInvalidatorInterface
   */
  protected $cacheTagsInvalidator;

  /**
   * Constructs a new Fast404ExceptionHtmlSubscriber.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The configuration factory.
   * @param \Drupal\Core\Cache\CacheTagsInvalidatorInterface $cache_tags_invalidator
   *   The cache tags invalidator.
   */
  public function __construct(ConfigFactoryInterface $config_factory, CacheTagsInvalidatorInterface $cache_tags_invalidator) {
    $this->configFactory = $config_factory;
    $this->cacheTagsInvalidator = $cache_tags_invalidator;
  }

  /**
   * {@inheritdoc}
   */
  protected static function getPriority() {

    // A very high priority so that it can take precedent over anything else,
    // and thus be fast.
    return 200;
  }

  /**
   * {@inheritdoc}
   */
  protected function getHandledFormats() {
    return [
      'html',
    ];
  }

  /**
   * Handles a 404 error for HTML.
   *
   * @param \Symfony\Component\HttpKernel\Event\ExceptionEvent $event
   *   The event to process.
   */
  public function on404(ExceptionEvent $event) {
    $request = $event
      ->getRequest();
    $config = $this->configFactory
      ->get('system.performance');
    $exclude_paths = $config
      ->get('fast_404.exclude_paths');
    if ($config
      ->get('fast_404.enabled') && $exclude_paths && !preg_match($exclude_paths, $request
      ->getPathInfo())) {
      $fast_paths = $config
        ->get('fast_404.paths');
      if ($fast_paths && preg_match($fast_paths, $request
        ->getPathInfo())) {
        $fast_404_html = strtr($config
          ->get('fast_404.html'), [
          '@path' => Html::escape($request
            ->getUri()),
        ]);
        $response = new HtmlResponse($fast_404_html, Response::HTTP_NOT_FOUND);

        // Some routes such as system.files conditionally throw a
        // NotFoundHttpException depending on URL parameters instead of just the
        // route and route parameters, so add the URL cache context to account
        // for this.
        $cacheable_metadata = new CacheableMetadata();
        $cacheable_metadata
          ->setCacheContexts([
          'url',
        ]);
        $cacheable_metadata
          ->addCacheTags([
          '4xx-response',
        ]);
        $response
          ->addCacheableDependency($cacheable_metadata);
        $event
          ->setResponse($response);
      }
    }
  }

  /**
   * Invalidates 4xx-response cache tag if fast 404 config is changed.
   *
   * @param \Drupal\Core\Config\ConfigCrudEvent $event
   *   The configuration event.
   */
  public function onConfigSave(ConfigCrudEvent $event) : void {
    $saved_config = $event
      ->getConfig();
    if ($saved_config
      ->getName() === 'system.performance' && $event
      ->isChanged('fast_404')) {
      $this->cacheTagsInvalidator
        ->invalidateTags([
        '4xx-response',
      ]);
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {
    $events = parent::getSubscribedEvents();
    $events[ConfigEvents::SAVE] = 'onConfigSave';
    return $events;
  }

}