CacheTagPreloadSubscriber.php

<?php

namespace Drupal\Core\Cache\EventSubscriber;

use Drupal\Core\Cache\CacheTagsChecksumInterface;
use Drupal\Core\Cache\CacheTagsChecksumPreloadInterface;
use Drupal\Core\Site\Settings;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\RequestEvent;
use Symfony\Component\HttpKernel\KernelEvents;

/**
 * Preloads frequently used cache tags.
 */
class CacheTagPreloadSubscriber implements EventSubscriberInterface {
    public function __construct(CacheTagsChecksumInterface $cacheTagsChecksum) {
    }
    
    /**
     * Preloads frequently used cache tags.
     *
     * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event
     *   The request event.
     */
    public function onRequest(RequestEvent $event) : void {
        if ($event->isMainRequest() && $this->cacheTagsChecksum instanceof CacheTagsChecksumPreloadInterface) {
            $default_preload_cache_tags = array_merge([
                'route_match',
                'access_policies',
                'routes',
                'router',
                'entity_types',
                'entity_field_info',
                'entity_bundles',
                'local_task',
                'library_info',
            ], Settings::get('cache_preload_tags', []));
            $this->cacheTagsChecksum
                ->registerCacheTagsForPreload($default_preload_cache_tags);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        $events[KernelEvents::REQUEST][] = [
            'onRequest',
            500,
        ];
        return $events;
    }

}