title: hidesBarsOnTap - Navigation Bar hidden/displayed event?
body: <p>I would like to set the background color of a view to black when the navigation bar is hidden, and to white when the navigation bar is displayed.</p>
  
  <p>The property <code>hidesBarsOnTap</code> is set to <code>true</code> in <code>viewDidLoad</code>. This works fine:</p>
  
  <pre><code>navigationController?.hidesBarsOnTap = true
  </code></pre>
  
  <p>How can I be notified when the bars are hidden and displayed?</p>