图层动画没有效果，为什么？

Question

我只为任意视图设置了一个图层动画，但它没有任何效果。任何人都知道为什么它不起作用？

static func pulseView(v: UIView) {
    let animation = CABasicAnimation(keyPath: "shadowColor")
    animation.fromValue = UIColor.yellow.cgColor
    animation.toValue = UIColor.green.cgColor
    let animation2 = CABasicAnimation(keyPath: "shadowOpacity")
    animation2.fromValue = 0.5
    animation2.toValue = 1
    let agr = CAAnimationGroup()
    agr.duration = 0.5
    agr.repeatCount = 2
    agr.autoreverses = true
    agr.animations = [animation, animation2]
    v.layer.borderWidth = 5
    v.layer.add(agr, forKey: "shadowOpacity")
}