Symfony2 add field to form dynamically when onChange occurs - symfony

Question

I'm starting to investigate Symfony2 framework and it seems that I've messed up some things in my mind.
I am trying to do a relatively simple thing; Dynamically add an extra field to a form depending on the value of a select box. Let's assume for simplicity that I don't have my form attached to any entity, so what I eventually want to get back is an array of values.
So, as a simple example I would have this controller:
/**
* #Route("/", name="homepage")
*/
public function indexAction(Request $request)
{
$form = $this->createFormBuilder(null, array('allow_extra_fields' => true))
->add('test', 'Symfony\Component\Form\Extension\Core\Type\TextType')
->add('select', 'Symfony\Component\Form\Extension\Core\Type\ChoiceType', array(
'choices' => array(
'Maybe' => null,
'Yes' => true,
'No' => false,
),
'choices_as_values' => true,
))
->add('submit', 'Symfony\Component\Form\Extension\Core\Type\SubmitType')
->getForm();
$form->handleRequest($request);
if ($form->isValid()) {
dump($form->getData());
die;
}
return $this->render('default/index.html.twig', array(
'form' => $form->createView()
));
}
/**
* #Route("/test", name="test")
*/
public function AddFieldAction(Request $request)
{
return new JsonResponse($request->get('form'));
}
And the javascript of the view could be:
$('document').ready(function(){
$('#form_select').change(function(){
var form = $(this).closest('form');
var value = $(this).val();
var data = {};
data[$(this).attr('name')] = value;
$.ajax({
url: "{{ url("test") }}",
type: 'POST',
data: data,
dataType: 'json'
}).done(function (response) {
console.log(response);
if (response.select == 1) {
$('#form').append("<label> extra field </label> <input type='text' id='extra_field' name="form[extra_field]"></input>");
}
}).fail(function (jqXHR, textStatus) {
});
})
})
Now this would return the field correctly, as this would invoke the AddFieldAction and it would add the appropriate HTML in the form. When i would try to submit the form, in the $form->isValid part, I could get all the submitted data with the $form->getData() and $form->getExtraData() methods.
Question is, is this the optimal way to do it? Should (and could) I use Form Events instead? I was trying to use form events but it becomes a mess and I get lost easily in the hierarchy of PRE_SUBMIT or PRE_SET_DATA conficting each other.