Disabling a form at PRE_SET_DATA symfony2 - php

Question

I'm working on a form extension that can disable some fields in form types when the underlying data is not new.I do this so I don't have to create seperate forms or event listeners for update/creation forms.i.e: I create the entity creation form and the extension disables the fields signified by an option if the underlying entity is not new.I bound an event listener to the PRE_SET_DATA event as such:
class RemoveFieldsExtension extends AbstractTypeExtension{
public function buildForm(FormBuilderInterface $builder, array $options)
{
if (!$options['disable_on_update']) {
return;
}
$isNewCallback = $options['disable_on_update_is_new'];
$em = $this->em;
$builder->addEventListener(FormEvents::PRE_SET_DATA, function(FormEvent $event) use ($em, $isNewCallback, $builder){
if(is_bool($isNewCallback))
return $isNewCallback;
if(is_callable($isNewCallback)){
$isNew = call_user_func($isNewCallback, $event->getData(), $em);
if(!$isNew){//the check for resolving if this is an update form or creation form
// $builder->setDisabled(true);
$form = $event->getForm();
$form->getConfig()->setDisabled(true);
}
}
},
-200
);
}
}
the extension works fine the problem is that the above code will produce 'FormConfigBuilder methods cannot be accessed anymore once the builder is turned into a FormConfigInterface instance.'
it also didn't work when I tried $builder->setDisabled(true);.what can I do to accomplish this?