title: Enable and disable a UIBarButtonItem
body: <p>I have a screen with 6 UITextFields and 1 UIBarButtonItem called "Done".  As soon as the user type anything on any of the 6 UITextFields I would like to enable my UIBarButtonItem called "Done", and also if the user delete the content I would like to return my UIBarByttonItem called "done" to disable mode. I would like to apply the same rule that Apple applied when you are adding a new contact in the iphone agenda. As soon as you type any information you are allowed to tap "done". Is there any smart way to that? </p>
  
  <p>Many thanks,
  Marcos. </p>