middleware outgoing data flow

RenderingMiddleware.php

<?php

declare(strict_types=1);

namespace Project\Infrastrucuture\Middleware;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;

final class RenderingMiddleware implements MiddlewareInterface
{
    /** @var \Twig\Environment */
    private $twig;

    public function __construct(\Twig\Environment $twig)
    {
        $this->twig = $twig;
    }

    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        $response = $handler->handle($request);

        $template = $response->getAttribute('Twig-Template', null);
        $variables = $response->getAttribute('Twig-Variables', []);

        if (null === $template) {
            return $response;
        }

        $response->getBody()->write($this->twig->render($template, $variables));

        return $response->withHeader('Content-Type', 'text/html');
    }
}

SampleRequestHandler.php

<?php

declare(strict_types=1);

namespace Project\Infrastructure\Action;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Slim\Psr7\Response;

final class SampleRequestHandler implements RequestHandlerInterface
{
    public function handle(ServerRequestInterface $request): ResponseInterface
    {
        return new Response(200)
            ->withAttribute('Twig-Template', 'foo.twig')
            ->withAttribute('Twig-Variables', ['bar' => 'baz']);
    }
}