HtmlResponsePlaceholderStrategySubscriber.php

<?php

namespace Drupal\Core\EventSubscriber;

use Drupal\Core\Render\HtmlResponse;
use Drupal\Core\Render\Placeholder\PlaceholderStrategyInterface;
use Symfony\Component\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * HTML response subscriber to allow for different placeholder strategies.
 *
 * This allows core and contrib to coordinate how to render placeholders;
 * e.g. an EsiRenderStrategy could replace the placeholders with ESI tags,
 * while e.g. a BigPipeRenderStrategy could store the placeholders in a
 * BigPipe service and render them after the main content has been sent to
 * the client.
 */
class HtmlResponsePlaceholderStrategySubscriber implements EventSubscriberInterface {
    
    /**
     * The placeholder strategy to use.
     *
     * @var \Drupal\Core\Render\Placeholder\PlaceholderStrategyInterface
     */
    protected $placeholderStrategy;
    
    /**
     * Constructs a HtmlResponsePlaceholderStrategySubscriber object.
     *
     * @param \Drupal\Core\Render\Placeholder\PlaceholderStrategyInterface $placeholder_strategy
     *   The placeholder strategy to use.
     */
    public function __construct(PlaceholderStrategyInterface $placeholder_strategy) {
        $this->placeholderStrategy = $placeholder_strategy;
    }
    
    /**
     * Processes placeholders for HTML responses.
     *
     * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
     *   The event to process.
     */
    public function onRespond(ResponseEvent $event) {
        $response = $event->getResponse();
        if (!$response instanceof HtmlResponse) {
            return;
        }
        $attachments = $response->getAttachments();
        if (empty($attachments['placeholders'])) {
            return;
        }
        $attachments['placeholders'] = $this->placeholderStrategy
            ->processPlaceholders($attachments['placeholders']);
        $response->setAttachments($attachments);
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        // Run shortly before HtmlResponseSubscriber.
        $events[KernelEvents::RESPONSE][] = [
            'onRespond',
            5,
        ];
        return $events;
    }

}