Symfony -使用递归表在ChoiceType上创建组

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Component\Form\FormBuilderInterface;

class CountryCityType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('country', ChoiceType::class, [
                'choices' => [
                    'China' => 'CN',
                    'United States' => 'US',
                    'United Kingdom' => 'UK',
                ],
                'placeholder' => 'Select a country',
                'required' => true,
                'mapped' => false,
                'attr' => [
                    'class' => 'country-select',
                ],
            ])
            ->add('city', ChoiceType::class, [
                'choices' => [],
                'placeholder' => 'Select a city',
                'required' => true,
                'mapped' => false,
                'attr' => [
                    'class' => 'city-select',
                ],
            ]);
    }
}