Symfony2:如何从自定义验证器中的未映射字段(集合内)获取数据

use Symfony\Component\Validator\Constraints as Assert;

class MyFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('items', CollectionType::class, [
                'entry_type' => ItemType::class,
                'constraints' => [
                    new MyCustomValidator(),
                ],
            ]);
    }
}

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

class MyCustomValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint)
    {
        foreach ($value as $item) {
            // 从集合内的每个元素中获取数据
            $data = $item['fieldName'];

            // 进行数据验证逻辑
            // ...
        }
    }
}

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Validator\Validator\ValidatorInterface;

class MyController extends Controller
{
    public function submitForm(Request $request, ValidatorInterface $validator)
    {
        $form = $this->createForm(MyFormType::class);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            // 验证表单数据
            $errors = $validator->validate($form->getData());

            if (count($errors) > 0) {
                // 处理验证错误
                // ...
            } else {
                // 表单数据验证通过
                // ...
            }
        }

        // ...
    }
}