TYPO3.Forms API and field conditions

Hi guys,
what’s the best way to introduce conditions between form fields in Neos? I wandered here around for a little but I only found a comment of @dfeyer about the possibility to do it with Form API.
Even looking into the Form package and FormBuilderBundle didn’t give me much clues.

  • Should it be done at layout/template level in fluid, implementing javascript functionallity accordingly?
  • How does it match with the Yaml configuration or can it be done only with php implementation?
  • Is there any package-provided functionality that I missed?

Thanks to anyone who can point me to the right direction.