title: How do I show/hide a UIBarButtonItem?
body: <p>I created a toolbar in IB with several buttons. I would like to be able to hide/show one of the buttons depending on the state of the data in the main window. </p>
  
  <p><code>UIBarButtonItem</code> doesn't have a hidden property, and any examples I've found so far for hiding them involve setting nav bar buttons to nil, which I don't think I want to do here because I may need to show the button again (not to mention that, if I connect my button to an IBOutlet, if I set that to nil I'm not sure how I'd get it back).</p>