class ResourceObjectNormalizationCacher

class ResourceObjectNormalizationCacher implements EventSubscriberInterface {
  
  /**
   * Key for the base subset.
   *
   * The base subset contains the parts of the normalization that are always
   * present. The presence or absence of these are not affected by the requested
   * sparse field sets. This typically includes the resource type name, and the
   * resource ID.
   */
  const RESOURCE_CACHE_SUBSET_BASE = 'base';
  
  /**
   * Key for the fields subset.
   *
   * The fields subset contains the parts of the normalization that can appear
   * in a normalization based on the selected field set. This subset is
   * incrementally built across different requests for the same resource object.
   * A given field is normalized and put into the cache whenever there is a
   * cache miss for that field.
   */
  const RESOURCE_CACHE_SUBSET_FIELDS = 'fields';
  
  /**
   * The variation cache.
   *
   * @var \Drupal\Core\Cache\VariationCacheInterface
   */
  protected $variationCache;
  
  /**
   * The request stack.
   *
   * @var \Symfony\Component\HttpFoundation\RequestStack
   */
  protected $requestStack;
  
  /**
   * The things to cache after the response has been sent.
   *
   * @var array
   */
  protected $toCache = [];
  
  /**
   * Sets the variation cache.
   *
   * @param \Drupal\Core\Cache\VariationCacheInterface $variation_cache
   *   The variation cache.
   */
  public function setVariationCache(VariationCacheInterface $variation_cache) {
    $this->variationCache = $variation_cache;
  }
  
  /**
   * Sets the request stack.
   *
   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
   *   The request stack.
   */
  public function setRequestStack(RequestStack $request_stack) {
    $this->requestStack = $request_stack;
  }
  
  /**
   * Reads an entity normalization from cache.
   *
   * The returned normalization may only be a partial normalization because it
   * was previously normalized with a sparse fieldset.
   *
   * @param \Drupal\jsonapi\JsonApiResource\ResourceObject $object
   *   The resource object for which to generate a cache item.
   *
   * @return array|false
   *   The cached normalization parts, or FALSE if not yet cached.
   *
   * @see \Drupal\dynamic_page_cache\EventSubscriber\DynamicPageCacheSubscriber::renderArrayToResponse()
   */
  public function get(ResourceObject $object) {
    // @todo Investigate whether to cache POST and PATCH requests.
    // @todo Follow up on https://www.drupal.org/project/drupal/issues/3381898.
    if (!$this->requestStack
      ->getCurrentRequest()
      ->isMethodCacheable()) {
      return FALSE;
    }
    $cached = $this->variationCache
      ->get($this->generateCacheKeys($object), new CacheableMetadata());
    if (!$cached) {
      return FALSE;
    }
    // When a cache hit occurs, we first calculate the remaining time before the
    // cached record expires, ensuring that we do not reset the expiration with
    // one that might have been generated on an earlier timestamp. This is done
    // by subtracting the current timestamp from the cached record's expiration
    // timestamp. If the max-age is set, we adjust it by merging the calculated
    // remaining time with the original max-age of the cached item, ensuring
    // that the expiration remains accurate based on the current cache state
    // and timestamp.
    $normalizer_values = $cached->data;
    assert(is_array($normalizer_values));
    if ($cached->expire >= 0) {
      $max_age = max($cached->expire - $this->requestStack
        ->getCurrentRequest()->server
        ->get('REQUEST_TIME'), 0);
      $cacheability = new CacheableMetadata();
      $cacheability->setCacheMaxAge($max_age);
      $subsets = [
        ResourceObjectNormalizationCacher::RESOURCE_CACHE_SUBSET_BASE,
        ResourceObjectNormalizationCacher::RESOURCE_CACHE_SUBSET_FIELDS,
      ];
      foreach ($subsets as $subset) {
        foreach ($normalizer_values[$subset] as $name => $normalization) {
          assert($normalization instanceof CacheableNormalization);
          if ($normalization->getCacheMaxAge() > 0) {
            $normalizer_values[$subset][$name] = $normalization->withCacheableDependency($cacheability);
          }
        }
      }
    }
    return $normalizer_values;
  }
  
  /**
   * Adds a normalization to be cached after the response has been sent.
   *
   * @param \Drupal\jsonapi\JsonApiResource\ResourceObject $object
   *   The resource object for which to generate a cache item.
   * @param array $normalization_parts
   *   The normalization parts to cache.
   */
  public function saveOnTerminate(ResourceObject $object, array $normalization_parts) {
    assert(array_keys($normalization_parts) === [
      static::RESOURCE_CACHE_SUBSET_BASE,
      static::RESOURCE_CACHE_SUBSET_FIELDS,
    ]);
    $resource_type = $object->getResourceType();
    $key = $resource_type->getTypeName() . ':' . $object->getId();
    $this->toCache[$key] = [
      $object,
      $normalization_parts,
    ];
  }
  
  /**
   * Writes normalizations of entities to cache, if any were created.
   *
   * @param \Symfony\Component\HttpKernel\Event\TerminateEvent $event
   *   The Event to process.
   */
  public function onTerminate(TerminateEvent $event) {
    foreach ($this->toCache as $value) {
      [$object, $normalization_parts] = $value;
      $this->set($object, $normalization_parts);
    }
  }
  
  /**
   * Writes a normalization to cache.
   *
   * @param \Drupal\jsonapi\JsonApiResource\ResourceObject $object
   *   The resource object for which to generate a cache item.
   * @param array $normalization_parts
   *   The normalization parts to cache.
   */
  protected function set(ResourceObject $object, array $normalization_parts) {
    // @todo Investigate whether to cache POST and PATCH requests.
    // @todo Follow up on https://www.drupal.org/project/drupal/issues/3381898.
    if (!$this->requestStack
      ->getCurrentRequest()
      ->isMethodCacheable()) {
      return;
    }
    // Merge the entity's cacheability metadata with that of the normalization
    // parts, so that VariationCache can take care of cache redirects for us.
    $cacheability = CacheableMetadata::createFromObject($object)->merge(static::mergeCacheableDependencies($normalization_parts[static::RESOURCE_CACHE_SUBSET_BASE]))
      ->merge(static::mergeCacheableDependencies($normalization_parts[static::RESOURCE_CACHE_SUBSET_FIELDS]));
    $this->variationCache
      ->set($this->generateCacheKeys($object), $normalization_parts, $cacheability, new CacheableMetadata());
  }
  
  /**
   * Generates the cache keys for a normalization.
   *
   * @param \Drupal\jsonapi\JsonApiResource\ResourceObject $object
   *   The resource object for which to generate the cache keys.
   *
   * @return string[]
   *   The cache keys to pass to the variation cache.
   *
   * @see \Drupal\dynamic_page_cache\EventSubscriber\DynamicPageCacheSubscriber::$dynamicPageCacheRedirectRenderArray
   */
  protected static function generateCacheKeys(ResourceObject $object) {
    return [
      $object->getResourceType()
        ->getTypeName(),
      $object->getId(),
      $object->getLanguage()
        ->getId(),
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {
    $events[KernelEvents::TERMINATE][] = [
      'onTerminate',
    ];
    return $events;
  }
  
  /**
   * Determines the joint cacheability of all provided dependencies.
   *
   * @param \Drupal\Core\Cache\CacheableDependencyInterface|object[] $dependencies
   *   The dependencies.
   *
   * @return \Drupal\Core\Cache\CacheableMetadata
   *   The cacheability of all dependencies.
   *
   * @see \Drupal\Core\Cache\RefinableCacheableDependencyInterface::addCacheableDependency()
   */
  protected static function mergeCacheableDependencies(array $dependencies) {
    $merged_cacheability = new CacheableMetadata();
    array_walk($dependencies, function ($dependency) use ($merged_cacheability) {
      $merged_cacheability->addCacheableDependency($dependency);
    });
    return $merged_cacheability;
  }

}