Symfony 5 dynamic form conditional default logic - forms

Question

I've an use case where i need some default conditional logic on my dynamic form build in Symfony 5.
Let me try to explain what my use case is and my problem with a simple form.
For example i've a form Product with two fields:
Part (choiceType => left, right)
Length (numberType)
On change all fields (:input) are being submitted through an Ajax request.
I've two controller methods one for visiting the page (form is being build), the other
is being called for rendering the form through the ajax request (handle conditional logic).
For the conditional logic part the following needs te be done
When part is left, default length needs to be 50
When part is right, default length needs to be 100
user could change default data
Setting the default data on length based on left or right is not the problem.
When left is selected, default length becomes 50. When changing the value to 55 (form is being submitted through every change) it becomes 50 again.
This behaviour is logic, but how could the default data been overwritten?
Above situation could also been described as give user default data with option to change it
form type
<?php
// ... namespace, use statments
class ProductType extends AbstractType
{
/**
* {#inheritDoc}
*/
public function buildForm(FormBuilderInterface $builder, array $options): void
{
$builder->add('part', ChoiceType::class, array(
'choices' => array(
'Left' => 'left',
'Right' => 'right',
)
));
$builder->add('length', NumberType::class);
$builder->addEventListener(FormEvents::POST_SET_DATA, function(FormEvent $event) use ($options)
{
$form = $event->getForm();
if(null === $product = $event->getData()) {
return;
}
switch($product->getPart()) {
case 'left': $defaultLength = 50; break;
case 'right': $defaultLength = 100; break;
default: $defaultLength = 0;
}
$form->get('length')->setData($defaultLength);
});
}
/**
* {#inheritDoc}
*/
public function getName(): string
{
return 'product';
}
/**
* {#inheritDoc}
*/
public function configureOptions(OptionsResolver $resolver): void
{
$resolver->setDefaults(array(
'data_class' => Product::class,
'translation_domain' => 'forms',
));
}
}
controller
// src/Controller/ProductController.php
// ... namespace, use statments
namespace App\Controller;
class ProductController extends AbstractController
{
public function productAction(Request $request): Response
{
$product = new Product();
$form = $this->createForm(ProductType::class, $product);
$form->handleRequest($request);
if ($form->isSubmitted() && $form->isValid()) {
$product = $form->getData();
dd($product);
}
return $this->render('product_view.html.twig', array(
'form' => $form->createView()
));
}
public function productConfigureAjaxAction(Request $request): Response
{
$product = new Product();
$part = $request->request->get('product')['part'] ?? null;
$product->setPart($part);
$form = $this->createForm(ProductType::class, $product);
$form->handleRequest($request);
// product_form.html.twig is an separated file and included in product_view.html.twig
// by making the form separated is could been used for an ajax response
return $this->render('product_form.html.twig', array(
'form' => $form->createView()
));
}
}