gistfile1.txt

  /**
   * Sets expires and max-age for bubbled-up max-age values that are > 0.
   *
   * @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
   *   The response event.
   *
   * @throws \Exception
   *   Thrown when \DateTime() cannot create a new date object from the
   *   arguments passed in.
   */
  public function onResponse(FilterResponseEvent $event) {
    // Don't bother proceeding on sub-requests.
    if (!$event->isMasterRequest()) {
      return;
    }
    $response = $event->getResponse();

    // Nothing to do here if there isn't cacheable metadata available.
    if (!($response instanceof CacheableResponseInterface)) {
      return;
    }

    // Bail out early if this isn't an anonymous request.
    if ($this->user->isAuthenticated()) {
      return;
    }

    // Do some other crazy business logic, if necessary.

    $max_age = (int) $response->getCacheableMetadata()->getCacheMaxAge();
    if ($max_age !== Cache::PERMANENT) {
      // Here we do 2 things: 1) we forward the bubbled max-age to the response
      // Cache-Control "max-age" directive (which would otherwise take the
      // site-wide `system.performance:cache.page.max_age` value; and 2) we
      // replicate that into the "Expires" header, which is unfortunately what
      // Drupal's internal page cache will respect. The former is for the outer
      // world (proxies, CDNs, etc), and the latter for our own page cache.
      $response->setMaxAge($max_age);
      $date = new \DateTime('@' . ($this->time->getRequestTime() + $max_age));
      $response->setExpires($date);
    }
  }