iPhone:保存后导航的验证

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?In my iPhone application I have navigation cont