Using Symfony2 form events to prevent overwriting not submitted fields with empty values

UserType.php

<?php

namespace Acme\Bundle\UserBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;

class UserType extends AbstractType
{
    /**
     * @param FormBuilder $builder
     * @param array       $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('email', 'text', array('error_bubbling' => true))
            ->add('name', 'text', array('error_bubbling' => true));

        $callback = function(FormEvent $event) {
            if (null === $event->getData()) {
                $event->setData($event->getForm()->getData());
            }
        };

        $builder->get('email')->addEventListener(FormEvents::PRE_SUBMIT, $callback);
        $builder->get('name')->addEventListener(FormEvents::PRE_SUBMIT, $callback);
    }

   // [ ... ]
}

This is useful for dynamically embed form.