Symfony表单生成器如何在选择选择事件上创建字段

It's possible on symfony in formbuilder to create a new field if I select a X choice from a select?

I have this:

    $builder
        ->add('tipo', 'choice', array(
            'label'=>'Tipo de material:',
            'choices' => array(
                'file'   => 'Archivo descargable',
                'link' => 'Link',
                'test' => 'Test de evaluación',
                'video' => 'Video',
            ),
            'multiple' => false,
            'required' => true,
        ));

And I want this: if the user select "test" option (before sumit) add new field:

->add('newfield')

Thanks a lot!