Symfony 4 forms CollectionType: make FileType element required for new rows only - forms

Question

I have an array of images that I want to be able to add to/update/delete from in a Symfony 4 form.
To create a form for these images, I'm using a custom form with a FileType in it:
public function buildForm(FormBuilderInterface $builder, array $options) {
$builder
->add('image', FileType::class, array(
'data_class' => null
))
;
}
I am then using a CollectionType filled with instances of the form described above to render a form for each of the images in the array, with 'allow_add' and 'allow_delete' so I can add/remove rows via JavaScript.
public function buildForm(FormBuilderInterface $builder, array $options) {
$builder->add('imagesets', CollectionType::class, array(
'entry_type' => ImageType::class,
'entry_options' => array('label' => false),
'allow_add' => true,
'allow_delete' => true
));
}
This works fine for adding new images, but when updating existing images, the FileType element shouldn't be required, it should only be required for the new rows.
Question: How can I make the FileType NOT required for existing images, yet required for all the new rows?
(Note, I will be passing plain arrays to these form objects, not Doctrine entities.)

Answer 1

You should add an EventListener to your ImageType form and modify the required attribute if the object is not new (or not null). Have in mind that adding the second element with the same name as the previous to the form, replaces it.
$builder
->add('image', FileType::class, array(
'data_class' => null,
'required' => true,
))
;
$builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) {
// get the form object
$form = $event->getForm();
// get the entity/object data
$image = $event->getData();
// if it is new, it will be null
if(null !== $image) {
// modify the input
$form->add('image', FileType::class, array(
'data_class' => null,
'required' => false,
))
;
});
}