FormTestMiddleware.php

<?php

namespace Drupal\form_test\StackMiddleware;

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\HttpKernelInterface;

/**
 * Provides a test middleware which sets a custom response header.
 */
class FormTestMiddleware implements HttpKernelInterface {
    
    /**
     * The decorated kernel.
     *
     * @var \Symfony\Component\HttpKernel\HttpKernelInterface
     */
    protected $httpKernel;
    
    /**
     * Constructs a FormTestMiddleware object.
     *
     * @param \Symfony\Component\HttpKernel\HttpKernelInterface $http_kernel
     *   The decorated kernel.
     */
    public function __construct(HttpKernelInterface $http_kernel) {
        $this->httpKernel = $http_kernel;
    }
    
    /**
     * {@inheritdoc}
     */
    public function handle(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE) : Response {
        $response = $this->httpKernel
            ->handle($request, $type, $catch);
        $response->headers
            ->set('X-Form-Test-Stack-Middleware', 'invoked');
        return $response;
    }

}