Protéger un champ de formulaire Symfony2 avec un droit

config.yml

services:
    jdecool_forum_bundle.security_type_extension:
        class: JDecool\Bundle\ForumBundle\Form\Extension\SecurityTypeExtension
        arguments:
            - @security.context
        tags:
            - { name: form.type_extension, alias: form }

SecurityTypeExtension.php

<?php

namespace JDecool\Bundle\ForumBundle\Form\Extension;

use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Security\Core\SecurityContextInterface;

class SecurityTypeExtension extends AbstractTypeExtension
{
    /**
     * The security context
     * @var SecurityContextInterface
     */
    private $securityContext;

    /**
     * Object constructor
     */
    public function __construct(SecurityContextInterface $securityContext)
    {
        $this->securityContext = $securityContext;
    }

    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $grant = $options['is_granted'];
        if (null === $grant || $this->securityContext->isGranted($grant)) {
            return;
        }

        $builder->addEventListener(FormEvents::PRE_SET_DATA, function (FormEvent $event) {
            $form = $event->getForm();
            if ($form->isRoot()) {
                return;
            }

            $form->getParent()->remove($form->getName());
        });
    }

    /**
     * {@inheritdoc}
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array('is_granted' => null));
    }

    /**
     * {@inheritdoc}
     */
    public function getExtendedType()
    {
        return 'form';
    }

}

ThreadType.php

<?php

namespace JDecool\Bundle\ForumBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class ThreadType extends AbstractType
{
     /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('title')
            ->add('content', 'textarea')
            ->add('solved', 'checkbox', [
                'required'   => false,
                'is_granted' => 'ROLE_ADMIN',
            ])
        ;
    }

    /**
     * @param OptionsResolverInterface $resolver
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'JDecool\Bundle\ForumBundle\Entity\Thread'
        ));
    }

    /**
     * @return string
     */
    public function getName()
    {
        return 'forum_thread';
    }
}