Use Input validation with json String, not with map

Hi,

good job on this package, I really like it!

However I have one problem: The json I use for the form will be sent from a remote server. I did not find a way to use validation (e.G. is numeric value between x and y?) when I depend on a json String, not on a dart map. Am I missing something?

Would be very cool if you guys could help me out.

Thanks a lot!
Daniel