[CollectionType] Get binded data in FormType

[cavasinf]

I don't know if it is an expected behavior,
but when using an Embed Collection of Forms,
when you try to access the bound data with $builder->getData(), the result is null.

// src/Form/TaskType.php

public function buildForm(FormBuilderInterface $builder, array $options): void
{
    $builder->add('tags', CollectionType::class, [
        'entry_type' => TagType::class,
    ]);
}

// src/Form/TagType.php

public function buildForm(FormBuilderInterface $builder, array $options): void
{
    $builder->getData(); // <-- Is null
}

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

You need to go through the PRE_SET_DATA eventListener to get the data with $event->getData().

// src/Form/TagType.php

public function buildForm(FormBuilderInterface $builder, array $options): void
{
    $builder->addEventListener(FormEvents::PRE_SET_DATA, function(FormEvent $event) {
         $event->getData(); // <-- Is OK
    });
}

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

Even worth, when the form (which call the CollectionType) sets the by_reference to false,
the entire form must to be inside a PRE_SET_DATA event.

// src/Form/TaskType.php

public function buildForm(FormBuilderInterface $builder, array $options): void
{
    $builder->add('tags', CollectionType::class, [
        'entry_type' => TagType::class,
        'by_reference' => false, // $event->getData() inside TagType::class will be null
    ]);
}

// src/Form/TaskType.php

public function buildForm(FormBuilderInterface $builder, array $options): void
{
   $builder->addEventListener(FormEvents::PRE_SET_DATA, function(FormEvent $event) {
       $form = $event->getForm();
       $form->add('tags', CollectionType::class, [
            'entry_type' => TagType::class,
            'by_reference' => false, // $event->getData() is OK
       ]);
   });
}

If it is a thing,
I think it is worth mentioning that somewhere in the doc.