title: Swift 3 unresolved identifier CGColorGetComponents
body: <p>How can I refactor this code in Swift 3?:</p>
  
  <pre><code>    extension UIColor {
          var hexString: String {
              // Produces "Use of unresolved identifier 'CGColorGetComponents'"
              let components = CGColorGetComponents(self.cgColor)
              ....
          }
      }
  </code></pre>