How can you give design-time layout support to a custom UIView?

Question

Just playing around to see if something can be done. I know I can give design-time support to certain properties by adding @IBInspectable to them, and I'm also aware of making the view itself support design-time rendering and display by marking the entire view with @IBDesignable. That all works. What I can't get to work is custom layout support.

Consider this illustrative UIView subclass...

@IBDesignable
public class SimpleStackView : UIView {

    public var spacing:CGFloat = 10 {
        didSet{
            setNeedsLayout()
        }
    }

    @IBInspectable
    public var insets = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10) {
        didSet{
            setNeedsLayout()
        }
    }

    public override func layoutSubviews() {

        var offset:CGFloat = insets.top

        let visibleSubviews = subviews.filter({ $0.isVisible })
        for view in visibleSubviews{

            let desiredSize = view.intrinsicContentSize

            view.frame = CGRect(
                x      : insets.left,
                y      : offset,
                width  : bounds.width - insets.left - insets.right,
                height : desiredSize.height)

            offset += desiredSize.height + spacing
        }
    }
}

Note: This isn't full-featured--it doesn't calculate its own intrinsic height as an example--but it shows the basics

Adding the above SimpleStackView to a ViewController's main view, pinning all four edges to its container with a spacing of 20, as well as adding three subviews with intrinsic heights of 40, 120 and 80 respectively, you get this as expected...

Again, at runtime, this works as expected. What I'm wondering is how I can make this work at design-time (if at all) because this is how it looks at present...

Note: The on-screen positions are out of order to the ordinals of the views themselves (i.e. blue is in the middle at runtime, but the bottom at design-time because the design-time isn't properly (or at all!) ordering/laying them out.

I know the normal StackView supports this, but I'm wondering if it's because of the following:

1. Being designed by Apple, there are specific concessions made in Xcode/IB for it, or...
2. Being that it's in a separate library (dynamic I believe) and thus is compiled separately, that allows it to run differently.

Both of the above are guesses, but I'd be more inclined to think it's #1.

So can design-time layout support be implemented? If so, how?