Symfony 3在每个字段下从@Assert获取表单错误

// 实体类
namespace AppBundle\Entity;

use Symfony\Component\Validator\Constraints as Assert;

class User
{
    /**
     * @Assert\NotBlank(message="用户名不能为空")
     */
    private $username;

    // 其他属性和方法...
}

// 控制器
namespace AppBundle\Controller;

use AppBundle\Entity\User;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;

class UserController extends Controller
{
    public function registerAction(Request $request)
    {
        $user = new User();
        $form = $this->createForm(UserType::class, $user);

        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            // 处理表单提交
        }

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

// 模板
{{ form_start(form) }}
    {{ form_row(form.username) }}
    {% if form.username.vars.errors|length > 0 %}
        <ul>
            {% for error in form.username.vars.errors %}
                <li>{{ error.message }}</li>
            {% endfor %}
        </ul>
    {% endif %}
{{ form_end(form) }}