gistfile1.php

<?php
use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Form\FormBuilderInterface;
class CollectionPrototypeDataTypeExtension extends AbstractTypeExtension {
	public function buildForm(FormBuilderInterface $builder, array $options) {
		parent::buildForm($builder, $options);
		if(($prototypeFormBuilder = $builder->getAttribute('prototype')) && isset($options['prototype_data'])) {
			$prototypeFormBuilder->setData($options['prototype_data']);
		}
	}
	public function setDefaultOptions(OptionsResolverInterface $resolver) {
		$resolver->setOptional(array('prototype_data'));
	}
	public function getExtendedType() {
		return 'collection';
	}
}