class HtmlResponseBigPipeSubscriber

class HtmlResponseBigPipeSubscriber implements EventSubscriberInterface {
    
    /**
     * The BigPipe service.
     *
     * @var \Drupal\big_pipe\Render\BigPipe
     */
    protected $bigPipe;
    
    /**
     * Constructs a HtmlResponseBigPipeSubscriber object.
     *
     * @param \Drupal\big_pipe\Render\BigPipe $big_pipe
     *   The BigPipe service.
     */
    public function __construct(BigPipe $big_pipe) {
        $this->bigPipe = $big_pipe;
    }
    
    /**
     * Adds markers to the response necessary for the BigPipe render strategy.
     *
     * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
     *   The event to process.
     */
    public function onRespondEarly(ResponseEvent $event) {
        $response = $event->getResponse();
        if (!$response instanceof HtmlResponse) {
            return;
        }
        // Wrap the scripts_bottom placeholder with a marker before and after,
        // because \Drupal\big_pipe\Render\BigPipe needs to be able to extract that
        // markup if there are no-JS BigPipe placeholders.
        // @see \Drupal\big_pipe\Render\BigPipe::sendPreBody()
        $attachments = $response->getAttachments();
        if (isset($attachments['html_response_attachment_placeholders']['scripts_bottom'])) {
            $scripts_bottom_placeholder = $attachments['html_response_attachment_placeholders']['scripts_bottom'];
            $content = $response->getContent();
            $content = str_replace($scripts_bottom_placeholder, '<drupal-big-pipe-scripts-bottom-marker>' . $scripts_bottom_placeholder . '<drupal-big-pipe-scripts-bottom-marker>', $content);
            $response->setContent($content);
        }
    }
    
    /**
     * Transforms a HtmlResponse to a BigPipeResponse.
     *
     * @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 = $response->getAttachments();
        // If there are no no-JS BigPipe placeholders, unwrap the scripts_bottom
        // markup.
        // @see onRespondEarly()
        // @see \Drupal\big_pipe\Render\BigPipe::sendPreBody()
        if (empty($attachments['big_pipe_nojs_placeholders'])) {
            $content = $response->getContent();
            $content = str_replace('<drupal-big-pipe-scripts-bottom-marker>', '', $content);
            $response->setContent($content);
        }
        // If there are neither BigPipe placeholders nor no-JS BigPipe placeholders,
        // there isn't anything dynamic in this response, and we can return early:
        // there is no point in sending this response using BigPipe.
        if (empty($attachments['big_pipe_placeholders']) && empty($attachments['big_pipe_nojs_placeholders'])) {
            return;
        }
        $big_pipe_response = new BigPipeResponse($response);
        $big_pipe_response->setBigPipeService($this->getBigPipeService($event));
        $event->setResponse($big_pipe_response);
    }
    
    /**
     * Returns the BigPipe service to use to send the current response.
     *
     * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
     *   A response event.
     *
     * @return \Drupal\big_pipe\Render\BigPipe
     *   The BigPipe service.
     */
    protected function getBigPipeService(ResponseEvent $event) {
        return $this->bigPipe;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        // Run after HtmlResponsePlaceholderStrategySubscriber (priority 5), i.e.
        // after BigPipeStrategy has been applied, but before normal (priority 0)
        // response subscribers have been applied, because by then it'll be too late
        // to transform it into a BigPipeResponse.
        $events[KernelEvents::RESPONSE][] = [
            'onRespondEarly',
            3,
        ];
        // Run as the last possible subscriber.
        $events[KernelEvents::RESPONSE][] = [
            'onRespond',
            -10000,
        ];
        return $events;
    }

}