FormTestEventSubscriber.php

<?php

namespace Drupal\form_test\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

/**
 * Test event subscriber to add new attributes to the request.
 */
class FormTestEventSubscriber implements EventSubscriberInterface {
    
    /**
     * Adds custom attributes to the request object.
     *
     * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
     *   The kernel request event.
     */
    public function onKernelRequest(GetResponseEvent $event) {
        $request = $event->getRequest();
        $request->attributes
            ->set('custom_attributes', 'custom_value');
        $request->attributes
            ->set('request_attribute', 'request_value');
    }
    
    /**
     * Adds custom headers to the response.
     *
     * @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
     *   The kernel request event.
     */
    public function onKernelResponse(FilterResponseEvent $event) {
        $response = $event->getResponse();
        $response->headers
            ->set('X-Form-Test-Response-Event', 'invoked');
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() {
        $events[KernelEvents::REQUEST][] = [
            'onKernelRequest',
        ];
        $events[KernelEvents::RESPONSE][] = [
            'onKernelResponse',
        ];
        return $events;
    }

}