ActiveLinkResponseFilter::onResponse

public ActiveLinkResponseFilter::onResponse(FilterResponseEvent $event)

public function onResponse(FilterResponseEvent $event) {
  // Only care about HTML responses.
  if (stripos($event->getResponse()->headers->get('Content-Type'), 'text/html') === FALSE) {
    return;
  }

  // For authenticated users, the 'is-active' class is set in JavaScript.
  // @see system_page_attachments()
  if ($this->currentUser->isAuthenticated()) {
    return;
  }

  $response = $event->getResponse();
  $response->setContent(static::setLinkActiveClass(
  $response->getContent(), 
  ltrim($this->currentPath->getPath(), '/'), 
  $this->pathMatcher->isFrontPage(), 
  $this->languageManager->getCurrentLanguage(LanguageInterface::TYPE_URL)->getId(), 
  $event->getRequest()->query->all()
  ));
}