title: How to hide a bar button item for certain users
body: <p>I have a settings bar button item (set as left bar button item). I only want to display it if the user is logged in.</p>
  
  <p>I thought I could use the following for anonymous users</p>
  
  <pre><code>navigationItem.leftBarButtonItem = nil
  </code></pre>
  
  <p>But then how would I show it as soon as they logged in?</p>