Symfony 2 - 提交后修改表单值

Question

如果表单无效，如何在提交后将 firstName 值更改为空字符串？

当我使用$form->get('firstName')->setData('');时，它会给我

  

您无法更改已提交表单的数据。

在SUBMIT中同样的错误，在PRE_SUBMIT中，该值尚未知晓。

Symfony 2.8.4。它是我的简化代码：

class TaskType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('firstName', TextType::class, [
                'required' => false,
                'attr' => ['maxlength' => 30,],
                'constraints' => [
                    new NotBlank(),
                    new Length(array('min' => 5, 'max' => 30)),
                ],
            ])
            ->add('Submit', SubmitType::class, ['label' => 'Submit first name'])
            ->addEventListener(FormEvents::POST_SUBMIT, function (FormEvent $event) {
                $formData = $event->getData();
                $form = $event->getForm();

                if (!$form->isValid()) {
                    //$form->get('firstName')->setData('');
                }
            });
    }
}

class DefaultController extends Controller
{
    public function newAction(Request $request)
    {
        $defaultData = ['firstName' => 'John'];

        $form = $this->createForm(TaskType::class, $defaultData);
        $form->handleRequest($request);

        if ($form->isSubmitted()) {
            if ($form->isValid()) {
                return $this->redirectToRoute('app_homepage');
            } else {
                //$form->get('firstName')->setData('');
            }
        }

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