Corner radius not showing on button in storyboard with @IBDesigable/@IBInspectable

I have this custom class for a button.

import UIKit

@IBDesignable
class CustomButton: UIButton {

    @IBInspectable var cornerRadiusValue: CGFloat = 10.0 {
        didSet {
            setUpView()
        }
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.layer.cornerRadius = 10.0
    }

    override func awakeFromNib() {
        setUpView()
    }

    override func prepareForInterfaceBuilder() {
        super.prepareForInterfaceBuilder()
        setUpView()
    }

    func setUpView() {
        self.layer.cornerRadius = 10.0
    }

}

But the corner radius is not showing on the button in the storyboard.

I understand @IBInspectable just allows you to change the value in the inspector panel. I guess thats not what I am looking for.

I would like the corner radius to just show in storyboard when I create a button with that class. Which I thought that's what @IBDesignable does.

Your code is crashing in IB, so designability fails. Here is much simpler code that works:

@IBDesignable
class CustomButton: UIButton {
    @IBInspectable var cornerRadiusValue: CGFloat = 10.0 {
        didSet {
            setUpView()
        }
    }
    override func awakeFromNib() {
        super.awakeFromNib()
        setUpView()
    }
    override func prepareForInterfaceBuilder() {
        super.prepareForInterfaceBuilder()
        setUpView()
    }
    func setUpView() {
        self.layer.cornerRadius = self.cornerRadiusValue
        self.clipsToBounds = true
    }
}

Now the button is both inspectable and designable:

And it also works in the running app.