DynamicPageCacheSubscriber::onResponse

public DynamicPageCacheSubscriber::onResponse(FilterResponseEvent $event)

public function onResponse(FilterResponseEvent $event) {
  $response = $event->getResponse();

  // Dynamic Page Cache only works with cacheable responses. It does not work
  // with plain Response objects. (Dynamic Page Cache needs to be able to
  // access and modify the cacheability metadata associated with the
  // response.)
  if (!$response instanceof CacheableResponseInterface) {
    return;
  }

  // There's no work left to be done if this is a Dynamic Page Cache hit.
  if ($response->headers->get(self::HEADER) === 'HIT') {
    return;
  }

  // There's no work left to be done if this is an uncacheable response.
  if (!$this->shouldCacheResponse($response)) {
    // The response is uncacheable, mark it as such.
    $response->headers->set(self::HEADER, 'UNCACHEABLE');
    return;
  }

  // Don't cache the response if Dynamic Page Cache's request subscriber did
  // not fire, because that means it is impossible to have a Dynamic Page
  // Cache hit. This can happen when the master request is for example a 403
  // or 404, in which case a subrequest is performed by the router. In that
  // case, it is the subrequest's response that is cached by Dynamic Page
  // Cache, because the routing happens in a request subscriber earlier than
  // Dynamic Page Cache's and immediately sets a response, i.e. the one
  // returned by the subrequest, and thus causes Dynamic Page Cache's request
  // subscriber to not fire for the master request.
  // @see \Drupal\Core\Routing\AccessAwareRouter::checkAccess()
  // @see \Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber::on403()
  $request = $event->getRequest();
  if (!isset($this->requestPolicyResults[$request])) {
    return;
  }

  // Don't cache the response if the Dynamic Page Cache request & response
  // policies are not met.
  // @see onRouteMatch()
  if ($this->requestPolicyResults[$request] === RequestPolicyInterface::DENY || $this->responsePolicy->check($response, $request) === ResponsePolicyInterface::DENY) {
    return;
  }

  // Embed the response object in a render array so that RenderCache is able
  // to cache it, handling cache redirection for us.
  $response_as_render_array = $this->responseToRenderArray($response);
  $this->renderCache->set($response_as_render_array, $this->dynamicPageCacheRedirectRenderArray);

  // The response was generated, mark the response as a cache miss. The next
  // time, it will be a cache hit.
  $response->headers->set(self::HEADER, 'MISS');
}