--- /dev/null
+<?php
+
+namespace Drupal\Core\EventSubscriber;
+
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent;
+use Symfony\Component\HttpKernel\Exception\NotAcceptableHttpException;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Throws 406 if requesting non-HTML format and controller returns render array.
+ */
+class RenderArrayNonHtmlSubscriber implements EventSubscriberInterface {
+
+ /**
+ * Throws an HTTP 406 error if client requested a non-HTML format.
+ *
+ * @param \Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent $event
+ * The event to process.
+ */
+ public function onRespond(GetResponseForControllerResultEvent $event) {
+ $request = $event->getRequest();
+ $result = $event->getControllerResult();
+
+ // If this is a render array then we assume that the router went with the
+ // generic controller and not one with a format. If the format requested is
+ // not HTML though, we can also assume that the requested format is invalid
+ // so we provide a 406 response.
+ if (is_array($result) && $request->getRequestFormat() !== 'html') {
+ throw new NotAcceptableHttpException('Not acceptable format: ' . $request->getRequestFormat());
+ }
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public static function getSubscribedEvents() {
+ $events[KernelEvents::VIEW][] = ['onRespond', -10];
+ return $events;
+ }
+
+}