title: add mask to UITextFile in Swift
body: <p>I would like to know how can I implement a mask in Swift to format the input in a UITextField in real time as a Telephone number. I was looking for but I've only found complex code, I think it would be more simple. Correct me if I'm wrong.
  The desired mask I describe below.</p>
  
  <pre><code>000-00-00-00
  </code></pre>