问Symfony2:嵌入式表单的动态生成

EN

$builder->get('contacts')->addEventListener(
  FormEvents::POST_SET_DATA
  function(FormEvent $event) {
    $data = $event->getData(); //$data will contain embedded form object - not the data object!
  }
);

$builder->get('contacts')->addEventListener(
  FormEvents::POST_SUBMIT,
  function(FormEvent $event) {
    $data = $event->getData(); //$data will contain filled data object - everything is ok
    $form = $event->getForm(); //form will be ok
    if ($data->getSomeValue()) {
      $form->add(...); //Error: "You cannot add children to a submitted form"
    }
  }
);