(Symfony 4表单类型)如何根据表单上的<select>值使整个子表单成为必需/不必需？

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Component\Form\Extension\Core\Type\FormType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\OptionsResolver\OptionsResolver;

class ParentFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('selectField', ChoiceType::class, [
                'choices' => [
                    'Option 1' => 'option1',
                    'Option 2' => 'option2',
                ],
                'placeholder' => 'Select an option',
                'required' => true,
            ])
            ->addEventListener(FormEvents::PRE_SUBMIT, function (FormEvent $event) {
                $form = $event->getForm();
                $data = $event->getData();

                // 根据<select>字段的值决定子表单是否必需
                if ($data['selectField'] === 'option1') {
                    $form->add('childForm', ChildFormType::class, [
                        'constraints' => [
                            new NotBlank(),
                        ],
                    ]);
                } else {
                    $form->add('childForm', ChildFormType::class);
                }
            });
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => ParentFormData::class,
        ]);
    }
}

class ChildFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        // 添加子表单字段
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => ChildFormData::class,
        ]);
    }
}