How to link validation to action

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I have successfully added validation to my form, in that, an invalid entry is highlighted in the danger style.

However, I also want to take action if the entry is invalid, for example disable the 'update' button.

I'd be grateful if someone could point out how to I link some kind of action to the state of the entry.

I could check the state before allowing the update, but I'd rather disable the button and give an error message.

thanks