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;
    }

}