带有圆角和阴影的UIToolbar

Question

我正在尝试自定义带有圆角并阴影的UIToolbar。

我要添加一个子层来添加阴影，但是阴影层不会显示。

代码：

override func viewDidLoad() {
    super.viewDidLoad()

    //Round corners
    toolbar.barTintColor = UIColor.systemBlue
    toolbar.tintColor = UIColor.white
    toolbar.layer.cornerRadius = 30
    toolbar.clipsToBounds = true


    //Shadow
    let subLayer = CALayer()
    subLayer.frame = CGRect(origin: CGPoint(x: 0,y :0), size: toolbar.frame.size)
    subLayer.shadowPath = UIBezierPath(roundedRect: toolbar.frame, cornerRadius: 30).cgPath
    subLayer.shadowColor = UIColor.red.cgColor
    subLayer.shadowOpacity = 1
    subLayer.shadowRadius = 2
    subLayer.shadowOffset = CGSize(width: 0, height: 2)
    toolbar.layer.insertSublayer(subLayer, at: 0)

}

有什么主意为什么隐藏/不显示子层？