title: @IBInspectable in Swift 4.0
body: <p>While migrating to <code>Swift 4.0</code>, I am facing an issue with <code>@IBInspectable</code>,</p>
  
  <pre><code>open class SVContactBubbleView: UIView 
  {
     @IBInspectable open var dataSource: SVContactBubbleDataSource? //ERROR..!!
     @IBInspectable open var delegate: SVContactBubbleDelegate? //ERROR..!!
  }
  
  public protocol SVContactBubbleDataSource
  {
      //Methods here
  }
  
  public protocol SVContactBubbleDelegate
  {
      //Methods here
  }
  </code></pre>
  
  <p>The error that appears is:</p>
  
  <blockquote>
    <p>Property cannot be marked @IBInspectable because its type cannot be
    represented in Objective-C</p>
  </blockquote>
  
  <p>In <code>Swift 3</code>, it was working fine. I don't understand what went wrong in <code>Swift 4</code>. </p>
  
  <p>Also, the compiler is not showing any suggestion. It just shows an error message.</p>