class EarlyRenderingControllerWrapperSubscriber

class EarlyRenderingControllerWrapperSubscriber implements EventSubscriberInterface {
    
    /**
     * The argument resolver.
     *
     * @var \Symfony\Component\HttpKernel\Controller\ArgumentResolverInterface
     */
    protected $argumentResolver;
    
    /**
     * The renderer.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $renderer;
    
    /**
     * Constructs a new EarlyRenderingControllerWrapperSubscriber instance.
     *
     * @param \Symfony\Component\HttpKernel\Controller\ArgumentResolverInterface $argument_resolver
     *   The argument resolver.
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer.
     */
    public function __construct(ArgumentResolverInterface $argument_resolver, RendererInterface $renderer) {
        $this->argumentResolver = $argument_resolver;
        $this->renderer = $renderer;
    }
    
    /**
     * Ensures bubbleable metadata from early rendering is not lost.
     *
     * @param \Symfony\Component\HttpKernel\Event\FilterControllerEvent $event
     *   The controller event.
     */
    public function onController(FilterControllerEvent $event) {
        $controller = $event->getController();
        // See \Symfony\Component\HttpKernel\HttpKernel::handleRaw().
        $arguments = $this->argumentResolver
            ->getArguments($event->getRequest(), $controller);
        $event->setController(function () use ($controller, $arguments) {
            return $this->wrapControllerExecutionInRenderContext($controller, $arguments);
        });
    }
    
    /**
     * Wraps a controller execution in a render context.
     *
     * @param callable $controller
     *   The controller to execute.
     * @param array $arguments
     *   The arguments to pass to the controller.
     *
     * @return mixed
     *   The return value of the controller.
     *
     * @throws \LogicException
     *   When early rendering has occurred in a controller that returned a
     *   Response or domain object that cares about attachments or cacheability.
     *
     * @see \Symfony\Component\HttpKernel\HttpKernel::handleRaw()
     */
    protected function wrapControllerExecutionInRenderContext($controller, array $arguments) {
        $context = new RenderContext();
        $response = $this->renderer
            ->executeInRenderContext($context, function () use ($controller, $arguments) {
            // Now call the actual controller, just like HttpKernel does.
            return call_user_func_array($controller, $arguments);
        });
        // If early rendering happened, i.e. if code in the controller called
        // drupal_render() outside of a render context, then the bubbleable metadata
        // for that is stored in the current render context.
        if (!$context->isEmpty()) {
            
            /** @var \Drupal\Core\Render\BubbleableMetadata $early_rendering_bubbleable_metadata */
            $early_rendering_bubbleable_metadata = $context->pop();
            // If a render array or AjaxResponse is returned by the controller, merge
            // the "lost" bubbleable metadata.
            if (is_array($response)) {
                BubbleableMetadata::createFromRenderArray($response)->merge($early_rendering_bubbleable_metadata)
                    ->applyTo($response);
            }
            elseif ($response instanceof AjaxResponse) {
                $response->addAttachments($early_rendering_bubbleable_metadata->getAttachments());
                // @todo Make AjaxResponse cacheable in
                //   https://www.drupal.org/node/956186. Meanwhile, allow contrib
                //   subclasses to be.
                if ($response instanceof CacheableResponseInterface) {
                    $response->addCacheableDependency($early_rendering_bubbleable_metadata);
                }
            }
            elseif ($response instanceof AttachmentsInterface || $response instanceof CacheableResponseInterface || $response instanceof CacheableDependencyInterface) {
                throw new \LogicException(sprintf('The controller result claims to be providing relevant cache metadata, but leaked metadata was detected. Please ensure you are not rendering content too early. Returned object class: %s.', get_class($response)));
            }
            else {
                // A Response or domain object is returned that does not care about
                // attachments nor cacheability; for instance, a RedirectResponse. It is
                // safe to discard any early rendering metadata.
            }
        }
        return $response;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() {
        $events[KernelEvents::CONTROLLER][] = [
            'onController',
        ];
        return $events;
    }

}