Unable to use Callback assert with a form without data_class - symfony

Question

I'm creating a custom FormType called IntervalType. My IntervalType will have two fields, start and end and will be of type integer. This custom FormType will always be used without data_class.
I want to add a constraint to guarantee that start is lower than end.
How do I use the Symfony\Component\Validator\Constraints\Callback directly in a FormType without data_class?
Here is my IntervalType, just for reference:
// src/AppBundle/Form/Type/IntervalType.php
namespace AppBundle\Form;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;
use Symfony\Component\Form\Extension\Core\Type\IntegerType;
use Symfony\Component\Validator\Constraints\NotBlank;
class TaskType extends AbstractType
{
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('start', IntegerType::class, array(
'constraints' => array(
new NotBlank(),
),
))
->add('end', IntegerType::class, array(
'constraints' => array(
new NotBlank(),
),
))
);
}
}

Answer 1

When the form won't be using any data_class the only option seems to be the Callback constraint.
namespace AppBundle\Form;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;
use Symfony\Component\Form\Extension\Core\Type\IntegerType;
use Symfony\Component\Validator\Constraints\Callback;
use Symfony\Component\Validator\Constraints\NotBlank;
use Symfony\Component\Validator\Context\ExecutionContextInterface;
class IntervalType extends AbstractType
{
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('start', IntegerType::class, array(
'constraints' => array(
new NotBlank(),
),
))
->add('end', IntegerType::class, array(
'constraints' => array(
new NotBlank(),
new Callback(array($this, 'validateInterval')),
),
))
->add('submit', SubmitType::class);
}
public function validateInterval($value, ExecutionContextInterface $context)
{
$form = $context->getRoot();
$data = $form->getData();
if ($data['start'] >= $value) {
$context
->buildViolation('The end value has to be higher than the start value')
->addViolation();
}
}
}