title: How to fix "'@IBInspectable' attribute is meaningless on a property that cannot be represented in Objective-C" warning
body: <p>In Xcode 9 and Swift 4 I always get this warning for some <code>IBInspectable</code> properties:</p>
  
  <pre><code>    @IBDesignable public class CircularIndicator: UIView {
          // this has a warning
          @IBInspectable var backgroundIndicatorLineWidth: CGFloat? {  // &lt;-- warning here
              didSet {
                  backgroundIndicator.lineWidth = backgroundIndicatorLineWidth!
              }
          }
  
      // this doesn't have a warning
      @IBInspectable var topIndicatorFillColor: UIColor? {
          didSet {
              topIndicator.fillColor = topIndicatorFillColor?.cgColor
          }
      }
  }
  </code></pre>
  
  <p>Is there a way to get rid of it ?</p>