class MainContentViewSubscriber

class MainContentViewSubscriber implements EventSubscriberInterface {
  
  /**
   * The class resolver service.
   *
   * @var \Drupal\Core\DependencyInjection\ClassResolverInterface
   */
  protected $classResolver;
  
  /**
   * The current route match.
   *
   * @var \Drupal\Core\Routing\RouteMatchInterface
   */
  protected $routeMatch;
  
  /**
   * The available main content renderer services, keyed per format.
   *
   * @var array
   */
  protected $mainContentRenderers;
  
  /**
   * URL query attribute to indicate the wrapper used to render a request.
   *
   * The wrapper format determines how the HTML is wrapped, for example in a
   * modal dialog.
   */
  const WRAPPER_FORMAT = '_wrapper_format';
  
  /**
   * Constructs a new MainContentViewSubscriber object.
   *
   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
   *   The class resolver service.
   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
   *   The current route match.
   * @param array $main_content_renderers
   *   The available main content renderer service IDs, keyed by format.
   */
  public function __construct(ClassResolverInterface $class_resolver, RouteMatchInterface $route_match, array $main_content_renderers) {
    $this->classResolver = $class_resolver;
    $this->routeMatch = $route_match;
    $this->mainContentRenderers = $main_content_renderers;
  }
  
  /**
   * Sets a response given a (main content) render array.
   *
   * @param \Symfony\Component\HttpKernel\Event\ViewEvent $event
   *   The event to process.
   */
  public function onViewRenderArray(ViewEvent $event) {
    $request = $event->getRequest();
    $result = $event->getControllerResult();
    // Render the controller result into a response if it's a render array.
    if (is_array($result) && ($request->query
      ->has(static::WRAPPER_FORMAT) || $request->getRequestFormat() == 'html')) {
      $wrapper = $request->query
        ->get(static::WRAPPER_FORMAT, 'html');
      // Fall back to HTML if the requested wrapper envelope is not available.
      $wrapper = isset($this->mainContentRenderers[$wrapper]) ? $wrapper : 'html';
      $renderer = $this->classResolver
        ->getInstanceFromDefinition($this->mainContentRenderers[$wrapper]);
      $response = $renderer->renderResponse($result, $request, $this->routeMatch);
      // The main content render array is rendered into a different Response
      // object, depending on the specified wrapper format.
      if ($response instanceof CacheableResponseInterface) {
        $main_content_view_subscriber_cacheability = (new CacheableMetadata())->setCacheContexts([
          'url.query_args:' . static::WRAPPER_FORMAT,
        ]);
        $response->addCacheableDependency($main_content_view_subscriber_cacheability);
      }
      $event->setResponse($response);
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {
    $events[KernelEvents::VIEW][] = [
      'onViewRenderArray',
    ];
    return $events;
  }

}