title: Display activity indicator inside UIButton
body: <p>I would like to change the content of a UIButton to an ActivityIndicator after it is pressed.</p>
  
  <p>I know buttons have an imageView and a titleLabel, but I don't know how to put an activity indicator in any of them.</p>
  
  <p>This is how I create activity indicators:</p>
  
  <pre><code>let aiView = UIActivityIndicatorView(activityIndicatorStyle: .Gray)
  aiView.startAnimating()
  aiView.center = CGPointMake(0,0)
  aiView.hidesWhenStopped = false
  </code></pre>