title: Set attributed text to UIButton
body: <p>I am trying to set an attributed string to a button.</p>
  
  <p>The definition of this function on swift is this</p>
  
  <pre><code>func setAttributedTitle(_ title: NSAttributedString!,
                 forState state: UIControlState)
  </code></pre>
  
  <p>so I thought I would have to type it like</p>
  
  <pre><code>myButton.setAttributedTitle(title:attrString, forState: UIControlState.Normal)
  </code></pre>
  
  <p>but the correct is</p>
  
  <pre><code>myButton.setAttributedTitle(attrString, forState: UIControlState.Normal)
  </code></pre>
  
  <p>why it is necessary to put <code>forState:</code> but not <code>title</code>? I don't get that.</p>
  
  <p>And by the way what is the meaning of that underscore on the func definition?</p>