TestMiddleware.php

<?php

namespace Drupal\http_kernel_test\HttpKernel;

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

/**
 * Provides a test middleware.
 */
class TestMiddleware implements HttpKernelInterface {
    
    /**
     * The decorated kernel.
     *
     * @var \Symfony\Component\HttpKernel\HttpKernelInterface
     */
    protected $kernel;
    
    /**
     * An optional argument.
     *
     * @var mixed
     */
    protected $optionalArgument;
    
    /**
     * Constructs a new TestMiddleware object.
     *
     * @param \Symfony\Component\HttpKernel\HttpKernelInterface $kernel
     *   The decorated kernel.
     * @param mixed $optional_argument
     *   (optional) An optional argument.
     */
    public function __construct(HttpKernelInterface $kernel, $optional_argument = NULL) {
        $this->kernel = $kernel;
        $this->optionalArgument = $optional_argument;
    }
    
    /**
     * {@inheritdoc}
     */
    public function handle(Request $request, $type = self::MAIN_REQUEST, $catch = TRUE) : Response {
        $request->attributes
            ->set('_hello', 'world');
        if ($request->attributes
            ->has('_optional_argument')) {
            $request->attributes
                ->set('_previous_optional_argument', $request->attributes
                ->get('_optional_argument'));
        }
        elseif (isset($this->optionalArgument)) {
            $request->attributes
                ->set('_optional_argument', $this->optionalArgument);
        }
        return $this->kernel
            ->handle($request, $type, $catch);
    }

}