title: Ensure the user has filled in two UITextFields in a UIViewController before UIBarButton is Selectable in UINavigationBar
body: <p>I have a simple <code>UIViewController</code> that contains some <code>UITextField</code>. The navigation bar at the top has a <code>UIBarButtonItem</code> (created in Storyboard) which is called <em>Save</em> which ultimately allows the user to save the entry they are typing in and therefore dismiss this <code>UIViewController</code>. </p>
  
  <p>I want to enforce that the user has to at least enter in 2 or 3 text fields before the Save button becomes selectable, so it's there, but greyed out till the user fills in two specified text fields. </p>
  
  <p>How would I achieve something like this?</p>
  
  <p>I'm sorry for the small question, but there really isn't much more to it than this; grey out the <code>BarButtonItem</code> till the fields have been filled in and then allow the user to click on it. I've searched online and very surprisingly cannot find much information on this. </p>
  
  <p>Any guidance would be really appreciated. </p>
  
  <p><strong>Update</strong>. I have tried adding:</p>
  
  <p><code>self.navigationItem.rightBarButtonItem.enabled = NO;</code> to the <code>viewDidLoad</code> and <code>viewWillAppear</code> just to see if I can start disabling my bar button items and it doesn't get disabled at all.</p>