How to implement self defined request attributes in TYPO3 extensions

TYPO3 12 and later use request attributes to store internal informations. How does it work to add self defined attributes in TYPO3 extensions? I could not find anything in the docs.

Request Attributes

TYPO3/typo3_src-13.2.1/typo3/sysext/core/Classes/Http/ServerRequest.php:

namespace TYPO3\CMS\Core\Http;

class ServerRequest extends Request implements ServerRequestInterface
{
    protected array $attributes = [];

You can use a PSR-15 Middleware to achive that.

All you need is a Middleware class like:

Classes/Middleware/RequestEnrichingMiddleware.php

<?php

declare(strict_types=1);

namespace VENDOR\ExtName\Middleware;

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

class RequestEnrichingMiddleware implements MiddlewareInterface
{

    public function process(
        ServerRequestInterface $request,
        RequestHandlerInterface $handler,
    ): ResponseInterface {

        $myCustomDataArray = [
            'Foo' => 'Bar',
            'Foooo' => 'Baar',
        ];

        $request = $request->withAttribute('myCustomDataArray', $myCustomDataArray);

        return $handler->handle($request);
    }
}

And then you must register this middleware in: Configuration/RequestMiddlewares.php

<?php

return [
    'frontend' => [
        'vendor/my-middleware-identifier' => [
            'target' => \VENDOR\ExtName\Middleware\RequestEnrichingMiddleware::class,
            'after' => [
                'typo3/cms-core/response-propagation',
            ],
        ],
    ],
];

For more information see:
https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/ApiOverview/RequestLifeCycle/Middlewares.html#enriching-the-request