In my iPhone application I have navigation controller, main screen and some edit screens. On edit screen user does some input that has to be validated before I can save it. Ideally I would like to update data automatically on back navigation without additional "Done" button. Can I do some validation and save on back navigation (i.e. when user taps on standard back button) in a way that allows my to stop navigation and show some error message if something is wrong?

I see several other possibilities:

• Create my custom left button and make it looks like standard back. (Why Apple didn't put this button style into public API?)
• Add "Done" button and save data only if user taps it

but both these choices I like much less. So if there is a way to achieve what I want, I'd like to use it.