PlaceholderingRenderCache.php

<?php

namespace Drupal\Core\Render;

use Drupal\Core\Cache\Context\CacheContextsManager;
use Drupal\Core\Cache\VariationCacheFactoryInterface;
use Symfony\Component\HttpFoundation\RequestStack;

/**
 * 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.
 *
 * @see sites/default/default.services.yml
 *
 * 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.
 */
class PlaceholderingRenderCache extends RenderCache {
    
    /**
     * 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 $requestStack
     *   The request stack.
     * @param \Drupal\Core\Cache\VariationCacheFactoryInterface $cacheFactory
     *   The variation cache factory.
     * @param \Drupal\Core\Cache\Context\CacheContextsManager $cacheContextsManager
     *   The cache contexts manager.
     * @param \Drupal\Core\Render\PlaceholderGeneratorInterface $placeholderGenerator
     *   The placeholder generator.
     */
    public function __construct(RequestStack $requestStack, VariationCacheFactoryInterface $cacheFactory, CacheContextsManager $cacheContextsManager, PlaceholderGeneratorInterface $placeholderGenerator) {
        parent::__construct($requestStack, $cacheFactory, $cacheContextsManager);
    }
    
    /**
     * {@inheritdoc}
     */
    public function get(array $elements) {
        // When rendering placeholders, special case auto-placeholdered elements:
        // avoid retrieving them from cache again, or rendering them again.
        if (isset($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, $pre_bubbling_elements);
        // Writes to the render cache are disabled on uncacheable HTTP requests, to
        // prevent very low hit rate items from being written. If we're not writing
        // to the cache, there's also no benefit to placeholdering either.
        if (!$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;
    }

}