dynamically change the form presentation

In this article, we discussed how to do it:
http://symfony.com/doc/current/cookbook/form/dynamic_form_modification.html
I also give my example :

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
 
     $factory=$builder->getFormFactory();
     $builder->addEventListener(FormEvents::PRE_SET_DATA, function(FormEvent $event) use ($factory){
      // ... add a choice list of friends of the current application user
      
      $address = $event->getData();
      $form = $event->getForm();
      if ($address instanceof Address && $address->getWebSite()!=""){
       $url=$address->getWebSite();
      }else{
       $url= 'http://www.';
      }
   $form->add($factory->createNamed('webSite','url',$url,array(
        'required'=> false,
        'attr' => array('class' => 'defaultInvalid'),
        'label' => 'form.address.label.webSite'
        ))
   );
     });
     
    }