Username value object mapped to a form

1. UsernameType.php

<?php

namespace AppBundle\Form;

use AppBundle\Model\Username;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\DataMapperInterface;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormError;

class UsernameType extends AbstractType implements DataMapperInterface
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('username', 'text')
            ->setDataMapper($this);
    }

    public function mapDataToForms($data, $forms)
    {
        foreach ($forms as $form) {
            if ('username' !== $form->getName() || null === $data) {
                continue;
            }

            $form->setData($data->getUsername());
        }
    }

    public function mapFormsToData($forms, &$data)
    {
        $forms = iterator_to_array($forms);

        try {
            $data = new Username($forms['username']->getData());
        } catch (\InvalidArgumentException $e) {
            $error = new FormError($e->getMessage());
            $forms['username']->addError($error);
        }
    }

    public function getName()
    {
        return 'username';
    }
}

2. Username.php

<?php

namespace AppBundle\Model;

use Webmozart\Assert\Assert;

class Username
{
    const MIN_LENGTH = 5;
    const FORMAT     = '/^[a-zA-Z0-9_]+$/';

    /**
     * @var string
     */
    private $username;

    public function __construct($username)
    {
        Assert::notEmpty($username, 'The username should not be blank.');
        Assert::minLength($username, self::MIN_LENGTH, 'The username should contains at least %2$s characters.');
        Assert::regex($username, self::FORMAT, 'The username is invalid.');

        $this->username = $username;
    }

    /**
     * @return string
     */
    public function getUsername()
    {
        return $this->username;
    }
}

3. DefaultController.php

<?php

namespace AppBundle\Controller;

use AppBundle\Form\UsernameType;
use AppBundle\Model\Username;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;

class DefaultController extends Controller
{
    /**
     * @Route("/", name="creation")
     */
    public function creationAction(Request $request)
    {
        $form = $this->createForm(new UsernameType());
        $form->add('submit', 'submit');

        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            dump($form->getData()); exit;
        }

        return $this->render('default/index.html.twig', [
            'form' => $form->createView(),
        ]);
    }

    /**
     * @Route("/edition", name="edition")
     */
    public function editionAction(Request $request)
    {
        $username = new Username('jbarthe');

        $form = $this->createForm(new UsernameType(), $username);
        $form->add('submit', 'submit');

        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            dump($form->getData()); exit;
        }

        return $this->render('default/index.html.twig', [
            'form' => $form->createView(),
        ]);
    }
}