Sylius\Bundle\PromotionBundle\Form\Type\Core\AbstractConfigurationCollectionType::buildView PHP Method

    public function buildView(FormView $view, FormInterface $form, array $options)
    {
        $view->vars['prototypes'] = [];
        foreach ($form->getConfig()->getAttribute('prototypes') as $type => $prototype) {
            /* @var FormInterface $prototype */
            $view->vars['prototypes'][$type] = $prototype->createView($view);
        }
    }