class ThemeInfoRebuildSubscriber

class ThemeInfoRebuildSubscriber implements EventSubscriberInterface {
  use StringTranslationTrait;

  /**
   * Internal flag for handle user notification.
   *
   * @var string
   */
  protected $notificationFlag = 'devel.rebuild_theme_warning';

  /**
   * The devel config.
   *
   * @var \Drupal\Core\Config\Config;
   */
  protected $config;

  /**
   * The current user.
   *
   * @var \Drupal\Core\Session\AccountProxyInterface
   */
  protected $account;

  /**
   * The theme handler.
   *
   * @var \Drupal\Core\Extension\ThemeHandlerInterface
   */
  protected $themeHandler;

  /**
   * Constructs a ThemeInfoRebuildSubscriber object.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config
   *   The config factory.
   * @param \Drupal\Core\Session\AccountProxyInterface $account
   *   The current user.
   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
   *   The theme handler.
   */
  public function __construct(ConfigFactoryInterface $config, AccountProxyInterface $account, ThemeHandlerInterface $theme_handler) {
    $this->config = $config
      ->get('devel.settings');
    $this->account = $account;
    $this->themeHandler = $theme_handler;
  }

  /**
   * Forces the system to rebuild the theme registry.
   *
   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
   *   The event to process.
   */
  public function rebuildThemeInfo(GetResponseEvent $event) {
    if ($this->config
      ->get('rebuild_theme')) {

      // Update the theme registry.
      drupal_theme_rebuild();

      // Refresh theme data.
      $this->themeHandler
        ->refreshInfo();

      // Resets the internal state of the theme handler and clear the 'system
      // list' cache; this allow to properly register, if needed, PSR-4
      // namespaces for theme extensions after refreshing the info data.
      $this->themeHandler
        ->reset();

      // Notify the user that the theme info are rebuilt on every request.
      $this
        ->triggerWarningIfNeeded($event
        ->getRequest());
    }
  }

  /**
   * Notifies the user that the theme info are rebuilt on every request.
   *
   * The warning message is shown only to users with adequate permissions and
   * only once per session.
   *
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The request.
   */
  protected function triggerWarningIfNeeded(Request $request) {
    if ($this->account && $this->account
      ->hasPermission('access devel information')) {
      $session = $request
        ->getSession();
      if ($session && !$session
        ->has($this->notificationFlag)) {
        $session
          ->set($this->notificationFlag, TRUE);
        $message = $this
          ->t('The theme information is being rebuilt on every request. Remember to <a href=":url">turn off</a> this feature on production websites.', [
          ':url' => Url::fromRoute('devel.admin_settings')
            ->toString(),
        ]);
        drupal_set_message($message, 'warning', TRUE);
      }
    }
  }

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

    // Set high priority value to start as early as possible.
    $events[KernelEvents::REQUEST][] = [
      'rebuildThemeInfo',
      256,
    ];
    return $events;
  }

}