class DynamicPageCacheSubscriber

class DynamicPageCacheSubscriber implements EventSubscriberInterface {

  /**
   * Name of Dynamic Page Cache's response header.
   */
  const HEADER = 'X-Drupal-Dynamic-Cache';

  /**
   * A request policy rule determining the cacheability of a response.
   *
   * @var \Drupal\Core\PageCache\RequestPolicyInterface
   */
  protected $requestPolicy;

  /**
   * A response policy rule determining the cacheability of the response.
   *
   * @var \Drupal\Core\PageCache\ResponsePolicyInterface
   */
  protected $responsePolicy;

  /**
   * The variation cache.
   *
   * @var \Drupal\Core\Cache\VariationCacheInterface
   */
  protected $cache;

  /**
   * The default cache contexts to vary every cache item by.
   *
   * @var string[]
   */
  protected $cacheContexts = [
    'route',
    // Some routes' controllers rely on the request format (they don't have
    // a separate route for each request format). Additionally, a controller
    // may be returning a domain object that a KernelEvents::VIEW subscriber
    // must turn into an actual response, but perhaps a format is being
    // requested that the subscriber does not support.
    // @see \Drupal\Core\EventSubscriber\RenderArrayNonHtmlSubscriber::onResponse()
    'request_format',
  ];

  /**
   * The cache contexts manager service.
   *
   * @var \Drupal\Core\Cache\Context\CacheContextsManager
   */
  protected $cacheContextsManager;

  /**
   * The renderer configuration array.
   *
   * @var array
   */
  protected $rendererConfig;

  /**
   * Internal cache of request policy results.
   *
   * @var \SplObjectStorage
   */
  protected $requestPolicyResults;

  /**
   * Constructs a new DynamicPageCacheSubscriber object.
   *
   * @param \Drupal\Core\PageCache\RequestPolicyInterface $request_policy
   *   A policy rule determining the cacheability of a request.
   * @param \Drupal\Core\PageCache\ResponsePolicyInterface $response_policy
   *   A policy rule determining the cacheability of the response.
   * @param \Drupal\Core\Cache\VariationCacheInterface $cache
   *   The variation cache.
   * @param \Drupal\Core\Cache\Context\CacheContextsManager $cache_contexts_manager
   *   The cache contexts manager service.
   * @param array $renderer_config
   *   The renderer configuration array.
   */
  public function __construct(RequestPolicyInterface $request_policy, ResponsePolicyInterface $response_policy, VariationCacheInterface $cache, CacheContextsManager $cache_contexts_manager, array $renderer_config) {
    $this->requestPolicy = $request_policy;
    $this->responsePolicy = $response_policy;
    $this->cache = $cache;
    $this->cacheContextsManager = $cache_contexts_manager;
    $this->rendererConfig = $renderer_config;
    $this->requestPolicyResults = new \SplObjectStorage();
  }

  /**
   * Sets a response in case of a Dynamic Page Cache hit.
   *
   * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event
   *   The event to process.
   */
  public function onRequest(RequestEvent $event) {

    // Don't cache the response if the Dynamic Page Cache request policies are
    // not met. Store the result in a static keyed by current request, so that
    // onResponse() does not have to redo the request policy check.
    $request = $event
      ->getRequest();
    $request_policy_result = $this->requestPolicy
      ->check($request);
    $this->requestPolicyResults[$request] = $request_policy_result;
    if ($request_policy_result === RequestPolicyInterface::DENY) {
      return;
    }

    // Sets the response for the current route, if cached.
    $cached = $this->cache
      ->get([
      'response',
    ], (new CacheableMetadata())
      ->setCacheContexts($this->cacheContexts));
    if ($cached) {
      $response = $cached->data;
      $response->headers
        ->set(self::HEADER, 'HIT');
      $event
        ->setResponse($response);
    }
  }

  /**
   * Stores a response in case of a Dynamic Page Cache miss, if cacheable.
   *
   * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
   *   The event to process.
   */
  public function onResponse(ResponseEvent $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 onRequest()
    if ($this->requestPolicyResults[$request] === RequestPolicyInterface::DENY || $this->responsePolicy
      ->check($response, $request) === ResponsePolicyInterface::DENY) {
      return;
    }
    $cacheable_metadata = CacheableMetadata::createFromObject($response
      ->getCacheableMetadata());
    $this->cache
      ->set([
      'response',
    ], $response, $cacheable_metadata
      ->addCacheContexts($this->cacheContexts), (new CacheableMetadata())
      ->setCacheContexts($this->cacheContexts));

    // 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');
  }

  /**
   * Whether the given response should be cached by Dynamic Page Cache.
   *
   * We consider any response that has cacheability metadata meeting the auto-
   * placeholdering conditions to be uncacheable. Because those conditions
   * indicate poor cacheability, and if it doesn't make sense to cache parts of
   * a page, then neither does it make sense to cache an entire page.
   *
   * But note that auto-placeholdering avoids such cacheability metadata ever
   * bubbling to the response level: while rendering, the Renderer checks every
   * subtree to see if meets the auto-placeholdering conditions. If it does, it
   * is automatically placeholdered, and consequently the cacheability metadata
   * of the placeholdered content does not bubble up to the response level.
   *
   * @param \Drupal\Core\Cache\CacheableResponseInterface $response
   *   The response whose cacheability to analyze.
   *
   * @return bool
   *   Whether the given response should be cached.
   *
   * @see \Drupal\Core\Render\Renderer::shouldAutomaticallyPlaceholder()
   */
  protected function shouldCacheResponse(CacheableResponseInterface $response) {
    $conditions = $this->rendererConfig['auto_placeholder_conditions'];

    // Create a new CacheableMetadata to avoid changing the response itself.
    $cacheability = CacheableMetadata::createFromObject($response
      ->getCacheableMetadata());

    // Response's max-age is at or below the configured threshold.
    if ($cacheability
      ->getCacheMaxAge() !== Cache::PERMANENT && $cacheability
      ->getCacheMaxAge() <= $conditions['max-age']) {
      return FALSE;
    }

    // Optimize the contexts and let them affect the cache tags to mimic what
    // happens to the cacheability in the variation cache.
    $cacheability
      ->addCacheableDependency($this->cacheContextsManager
      ->convertTokensToKeys($cacheability
      ->getCacheContexts()));
    $cacheability
      ->setCacheContexts($this->cacheContextsManager
      ->optimizeTokens($cacheability
      ->getCacheContexts()));

    // Response has a high-cardinality cache context.
    if (array_intersect($cacheability
      ->getCacheContexts(), $conditions['contexts'])) {
      return FALSE;
    }

    // Response has a high-invalidation frequency cache tag.
    if (array_intersect($cacheability
      ->getCacheTags(), $conditions['tags'])) {
      return FALSE;
    }
    return TRUE;
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {
    $events = [];

    // Run after AuthenticationSubscriber (necessary for the 'user' cache
    // context; priority 300) and MaintenanceModeSubscriber (Dynamic Page Cache
    // should not be polluted by maintenance mode-specific behavior; priority
    // 30), but before ContentControllerSubscriber (updates _controller, but
    // that is a no-op when Dynamic Page Cache runs; priority 25).
    $events[KernelEvents::REQUEST][] = [
      'onRequest',
      27,
    ];

    // Run before HtmlResponseSubscriber::onRespond(), which has priority 0.
    $events[KernelEvents::RESPONSE][] = [
      'onResponse',
      100,
    ];
    return $events;
  }

}