Auto-disabling a trigger if the model is invalid

Hello;

I'm hoping someone can shed some light on the best way (or the right way) to
achieving a particular forms usability feature with XForms. 

What I'd like, is to automatically disable a trigger if any part of the
XForms model is invalid. 

For example, if the user has not entered in all required fields, I'd like to
disable the "Submit" button. Or in a "Wizard" like form, I'd like to disable
the "Next" button if the user hasn't correctly filled in the fields properly
(e.g. Didn't fill in required fields, didn't pass constraint check, or
didn't pass XML Schema validation etc.). 

I'm sure there's an easy and obvious way to do this that I'm just
overlooking. The only mechanism I've figured to implement this seems
horribly (overly) complex.

Thanks in advance for any/all responses,

Brian.