class ConfigCacheTag

class ConfigCacheTag implements EventSubscriberInterface {
    
    /**
     * The theme handler.
     *
     * @var \Drupal\Core\Extension\ThemeHandlerInterface
     */
    protected $themeHandler;
    
    /**
     * The cache tags invalidator.
     *
     * @var \Drupal\Core\Cache\CacheTagsInvalidatorInterface
     */
    protected $cacheTagsInvalidator;
    
    /**
     * Constructs a ConfigCacheTag object.
     *
     * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
     *   The theme handler.
     * @param \Drupal\Core\Cache\CacheTagsInvalidatorInterface $cache_tags_invalidator
     *   The cache tags invalidator.
     */
    public function __construct(ThemeHandlerInterface $theme_handler, CacheTagsInvalidatorInterface $cache_tags_invalidator) {
        $this->themeHandler = $theme_handler;
        $this->cacheTagsInvalidator = $cache_tags_invalidator;
    }
    
    /**
     * Invalidate cache tags when particular system config objects are saved.
     *
     * @param \Drupal\Core\Config\ConfigCrudEvent $event
     *   The Event to process.
     */
    public function onSave(ConfigCrudEvent $event) {
        $config_name = $event->getConfig()
            ->getName();
        // Changing the site settings may mean a different route is selected for the
        // front page. Additionally a change to the site name or similar must
        // invalidate the render cache since this could be used anywhere.
        if ($config_name === 'system.site') {
            $this->cacheTagsInvalidator
                ->invalidateTags([
                'route_match',
                'rendered',
            ]);
        }
        // Theme configuration and global theme settings.
        if (in_array($config_name, [
            'system.theme',
            'system.theme.global',
        ], TRUE)) {
            $this->cacheTagsInvalidator
                ->invalidateTags([
                'rendered',
            ]);
        }
        // Theme-specific settings, check if this matches a theme settings
        // configuration object (THEME_NAME.settings), in that case, clear the
        // rendered cache tag.
        if (preg_match('/^([^\\.]*)\\.settings$/', $config_name, $matches)) {
            if ($this->themeHandler
                ->themeExists($matches[1])) {
                $this->cacheTagsInvalidator
                    ->invalidateTags([
                    'rendered',
                ]);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() {
        $events[ConfigEvents::SAVE][] = [
            'onSave',
        ];
        return $events;
    }

}