How to use form events to dynamically modify fields when i choice a specific value

Question

I have some problems in my forum's project.

I have one field named "Category" with choice list and i want to when i check the value "Characters" in this list, a new field with entity "subCategory" appear to display a list of these characters.

This field must be added only if the value "Characters" is selected.

I tried something like :

->add('category', EntityType::class, [
            'class' => Category::class,          
            'choice_label' => 'title',
            ])
        ->addEventListener(FormEvents::PRE_SUBMIT, function(FormEvent $event) {
        $sub = $event->getData();
        $form = $event->getForm();

        if (isset($sub['title']) && $sub['title'] == 'Personnages') {
        $form->add('subCategory', EntityType::class, [
        'class' => SubCategory::class,
        'choice_label' => 'name']);
        }
            
 })        
        ->add('subCategory', EntityType::class, [
        'class' => SubCategory::class,
        'choice_label' => 'name',
        ])
    

That means "If isset a title in Category and her value is Characters, add a field which add list of these characters"

I know that i have to use form events but i don't know the way to resolve it.

Thanks for support

Answer 1

I'm afraid you can't do this with Form Events. I hope I don't say you bullsh*t but it seems the event types proposed by this feature are not designed for "real-time" modification of the form. As stated by the documentation:

1. Pre-populating the Form (FormEvents::PRE_SET_DATA and FormEvents::POST_SET_DATA)
2. Submitting a Form (FormEvents::PRE_SUBMIT, FormEvents::SUBMIT and FormEvents::POST_SUBMIT)

Nothing about dynamic detection of field values.

My best suggestion here would be to do with another way.