RenderArrayNonHtmlSubscriber.php

<?php

namespace Drupal\Core\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\ViewEvent;
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\ViewEvent $event
     *   The event to process.
     */
    public function onRespond(ViewEvent $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() : array {
        $events[KernelEvents::VIEW][] = [
            'onRespond',
            -10,
        ];
        return $events;
    }

}