[Messenger] Document the validation middleware

[ogizanagi]

Original PR: symfony/symfony#26648

An issue was already opened some times ago: #9641
and closed by 2099d48 but there is actually no mention to the ValidationMiddleware and how to use it (ValidationFailedException should also be explained, as it allows getting the violations).

Only the ValidationConfiguration is mentioned in https://symfony.com/doc/current/components/messenger.html#adding-metadata-to-messages-envelopes but I think this middleware deserves its own section/guide.

[Nek-]

This one looks critical to me. Can any maintainer say what kind of documentation you want?

Possibilities I see are:

• a new dedicated page
• a new section inside the middleware

For the record adding it to the middleware list is to do like this:

framework:
    messenger:
        buses:
            command_bus:
                middleware:
                    - messenger.middleware.validation

class CreateUser
{
    /**
     * @Assert\Uuid()
     * @Assert\NotBlank()
     */
    private $uuid;

    /**
     * @Assert\Lenght(min=3)
     * @Assert\NotBlank()
     */
    private $username;

    public function __construct(string $uuid, string $username)
    {
        $this->uuid = $uuid;
        $this->username = $username;
    }

    public function getUuid(): string
    {
        return $this->uuid;
    }

    public function getUsername(): string
    {
         return $this->username;
    }
}

The middleware will throw a ValidationFailedException that you may handle onKernelException and return formatted errors (in the case of an API for instance).

[carsonbot]

Thank you for this issue.
There has not been a lot of activity here for a while. Has this been resolved?

[Nek-]

no

[94noni]

@Nek- @ogizanagi I’ve opened a related PR
Feel free to discuss it :)