Last Updated: February 25, 2016

Symfony2.1: Adding (and/or Removing) fields using Event Subscribers

src/Bundle/Form/Type/NewsType.php

class NewsType extends AbstractType
{
 public function buildForm(FormBuilderInterface $builder, array $options)
 {
 $builder
 ->add('title')
 ->add('body')
 ;

 $subscriber = new AddExpiresAtFieldSubscriber($builder->getFormFactory());
 $builder->addEventSubscriber($subscriber);

 }

 // ...
}

src/Bundle/Form/EventListener/AddExpiresAtFieldSubscriber.php

class AddExpiresAtFieldSubscriber implements EventSubscriberInterface
{
 private $factory;

 public function __construct(FormFactoryInterface $factory)
 {
 $this->factory = $factory;
 }

 public static function getSubscribedEvents()
 {
 return array(FormEvents::PRE_SET_DATA => 'preSetData');
 }

 public function preSetData(FormEvent $event)
 {
 $data = $event->getData();
 $form = $event->getForm();

 if (null === $data) {
 return;
 }

 // if is not new (updating data) add the field
 if ($data->getId()) {
 $form->add($this->factory->createNamed('expires_at', 'datetime', null, array(
 'input' => 'datetime',
 'widget' => 'single_text',
 'format' => 'dd/MM/yyyy HH:mm'
 )));
 }
 }
}

And in case you having trouble adding or removing a field:
https://github.com/symfony/symfony/pull/4552#issuecomment-6809655