class PlaceholderingRenderCache in Drupal 9

Adds automatic placeholdering to the RenderCache.

This automatic placeholdering is performed to ensure the containing elements and overarching response are as cacheable as possible. Elements whose subtree bubble either max-age=0 or high-cardinality cache contexts (such as 'user' and 'session') are considered poorly cacheable.

Automatic placeholdering is performed only on elements whose subtree was generated using a #lazy_builder callback and whose bubbled cacheability meets the auto-placeholdering conditions as configured in the renderer.config container parameter.

This RenderCache implementation automatically replaces an element with a placeholder:

• on render cache hit, i.e. ::get()
• on render cache miss, i.e. ::set() (in subsequent requests, it will be a cache hit)

In either case, the render cache is guaranteed to contain the to-be-rendered placeholder, so replacing (rendering) the placeholder will be very fast.

Finally, in case the render cache item disappears between the time it is decided to automatically placeholder the element and the time where the placeholder is replaced (rendered), that is guaranteed to not be problematic. Because this only automatically placeholders elements that have a #lazy_builder callback set, which means that in the worst case, it will need to be re-rendered.

Hierarchy

• class \Drupal\Core\Render\RenderCache implements RenderCacheInterface
  • class \Drupal\Core\Render\PlaceholderingRenderCache

See also

sites/default/default.services.yml

1 file declares its use of PlaceholderingRenderCache

RendererTestBase.php in core/tests/Drupal/Tests/Core/Render/RendererTestBase.php

Contains \Drupal\Tests\Core\Render\RendererTestBase.

1 string reference to 'PlaceholderingRenderCache'

core.services.yml in core/core.services.yml

core/core.services.yml

1 service uses PlaceholderingRenderCache

render_cache in core/core.services.yml

Drupal\Core\Render\PlaceholderingRenderCache

File

core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php, line 40

Namespace

Drupal\Core\Render

class PlaceholderingRenderCache extends RenderCache {

  /**
   * The placeholder generator.
   *
   * @var \Drupal\Core\Render\PlaceholderGeneratorInterface
   */
  protected $placeholderGenerator;

  /**
   * Stores rendered results for automatically placeholdered elements.
   *
   * This allows us to avoid talking to the cache twice per auto-placeholdered
   * element, or in case of an uncacheable element, to render it twice.
   *
   * Scenario A. The double cache read would happen because:
   * 1. when rendering, cache read, but auto-placeholdered
   * 2. when rendering placeholders, again cache read
   *
   * Scenario B. The cache write plus read would happen because:
   * 1. when rendering, cache write, but auto-placeholdered
   * 2. when rendering placeholders, cache read
   *
   * Scenario C. The double rendering for an uncacheable element would happen because:
   * 1. when rendering, not cacheable, but auto-placeholdered
   * 2. when rendering placeholders, rendered again
   *
   * In all three scenarios, this static cache avoids the second step, thus
   * avoiding expensive work.
   *
   * @var array
   */
  protected $placeholderResultsCache = [];

  /**
   * Constructs a new PlaceholderingRenderCache object.
   *
   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
   *   The request stack.
   * @param \Drupal\Core\Cache\CacheFactoryInterface $cache_factory
   *   The cache factory.
   * @param \Drupal\Core\Cache\Context\CacheContextsManager $cache_contexts_manager
   *   The cache contexts manager.
   * @param \Drupal\Core\Render\PlaceholderGeneratorInterface $placeholder_generator
   *   The placeholder generator.
   */
  public function __construct(RequestStack $request_stack, CacheFactoryInterface $cache_factory, CacheContextsManager $cache_contexts_manager, PlaceholderGeneratorInterface $placeholder_generator) {
    parent::__construct($request_stack, $cache_factory, $cache_contexts_manager);
    $this->placeholderGenerator = $placeholder_generator;
  }

  /**
   * {@inheritdoc}
   */
  public function get(array $elements) {

    // @todo remove this check when https://www.drupal.org/node/2367555 lands.
    if (!$this->requestStack
      ->getCurrentRequest()
      ->isMethodCacheable()) {
      return FALSE;
    }

    // When rendering placeholders, special case auto-placeholdered elements:
    // avoid retrieving them from cache again, or rendering them again.
    if (isset($elements['#create_placeholder']) && $elements['#create_placeholder'] === FALSE) {
      $cached_placeholder_result = $this
        ->getFromPlaceholderResultsCache($elements);
      if ($cached_placeholder_result !== FALSE) {
        return $cached_placeholder_result;
      }
    }
    $cached_element = parent::get($elements);
    if ($cached_element === FALSE) {
      return FALSE;
    }
    else {
      if ($this->placeholderGenerator
        ->canCreatePlaceholder($elements) && $this->placeholderGenerator
        ->shouldAutomaticallyPlaceholder($cached_element)) {
        return $this
          ->createPlaceholderAndRemember($cached_element, $elements);
      }
      return $cached_element;
    }
  }

  /**
   * {@inheritdoc}
   */
  public function set(array &$elements, array $pre_bubbling_elements) {
    $result = parent::set($elements, $pre_bubbling_elements);

    // @todo remove this check when https://www.drupal.org/node/2367555 lands.
    if (!$this->requestStack
      ->getCurrentRequest()
      ->isMethodCacheable()) {
      return FALSE;
    }
    if ($this->placeholderGenerator
      ->canCreatePlaceholder($pre_bubbling_elements) && $this->placeholderGenerator
      ->shouldAutomaticallyPlaceholder($elements)) {

      // Overwrite $elements with a placeholder. The Renderer (which called this
      // method) will update the context with the bubbleable metadata of the
      // overwritten $elements.
      $elements = $this
        ->createPlaceholderAndRemember($this
        ->getCacheableRenderArray($elements), $pre_bubbling_elements);
    }
    return $result;
  }

  /**
   * Create a placeholder for a renderable array and remember in a static cache.
   *
   * @param array $rendered_elements
   *   A fully rendered renderable array.
   * @param array $pre_bubbling_elements
   *   A renderable array corresponding to the state (in particular, the
   *   cacheability metadata) of $rendered_elements prior to the beginning of
   *   its rendering process, and therefore before any bubbling of child
   *   information has taken place. Only the #cache property is used by this
   *   function, so the caller may omit all other properties and children from
   *   this array.
   *
   * @return array
   *   Renderable array with placeholder markup and the attached placeholder
   *   replacement metadata.
   */
  protected function createPlaceholderAndRemember(array $rendered_elements, array $pre_bubbling_elements) {
    $placeholder_element = $this->placeholderGenerator
      ->createPlaceholder($pre_bubbling_elements);

    // Remember the result for this placeholder to avoid double work.
    $placeholder = (string) $placeholder_element['#markup'];
    $this->placeholderResultsCache[$placeholder] = $rendered_elements;
    return $placeholder_element;
  }

  /**
   * Retrieves an auto-placeholdered renderable array from the static cache.
   *
   * @param array $elements
   *   A renderable array.
   *
   * @return array|false
   *   A renderable array, with the original element and all its children pre-
   *   rendered, or FALSE if no cached copy of the element is available.
   */
  protected function getFromPlaceholderResultsCache(array $elements) {
    $placeholder_element = $this->placeholderGenerator
      ->createPlaceholder($elements);
    $placeholder = (string) $placeholder_element['#markup'];
    if (isset($this->placeholderResultsCache[$placeholder])) {
      return $this->placeholderResultsCache[$placeholder];
    }
    return FALSE;
  }

}

Members

Name	Modifiers	Type	Description	Overrides
PlaceholderingRenderCache::$placeholderGenerator	protected	property	The placeholder generator.
PlaceholderingRenderCache::$placeholderResultsCache	protected	property	Stores rendered results for automatically placeholdered elements.
PlaceholderingRenderCache::createPlaceholderAndRemember	protected	function	Create a placeholder for a renderable array and remember in a static cache.
PlaceholderingRenderCache::get	public	function	Gets the cached, pre-rendered element of a renderable element from cache. Overrides RenderCache::get
PlaceholderingRenderCache::getFromPlaceholderResultsCache	protected	function	Retrieves an auto-placeholdered renderable array from the static cache.
PlaceholderingRenderCache::set	public	function	Caches the rendered output of a renderable array. Overrides RenderCache::set
PlaceholderingRenderCache::__construct	public	function	Constructs a new PlaceholderingRenderCache object. Overrides RenderCache::__construct
RenderCache::$cacheContextsManager	protected	property	The cache contexts manager.
RenderCache::$cacheFactory	protected	property	The cache factory.
RenderCache::$requestStack	protected	property	The request stack.
RenderCache::createCacheID	protected	function	Creates the cache ID for a renderable element.
RenderCache::getCacheableRenderArray	public	function	Gets a cacheable render array for a render array and its rendered output. Overrides RenderCacheInterface::getCacheableRenderArray
RenderCache::maxAgeToExpire	protected	function	Maps a #cache[max-age] value to an "expire" value for the Cache API.