title: Property @IBInspectable cannot be marked
body: <p>how to fix this problem, after installing xcode 9 show me this</p>
  
  <p>"Property cannot be marked @IBInspectable because its type cannot be representing in Objective-c"</p>
  
  <pre><code>/// The mode of the gradient. The default is `.Linear`.
  @IBInspectable open var mode: Mode = .linear {
      didSet {
          setNeedsDisplay()
      }
  }
  
  /// The direction of the gradient. Only valid for the `Mode.Linear` mode. The default is `.Vertical`.
  @IBInspectable open var direction: Direction = .vertical {
      didSet {
          setNeedsDisplay()
      }
  }
  </code></pre>