CALayer路径动画不工作

为什么此路径动画不起作用?

let frame = CGRect(x: 20, y: 20, width: 10, height: 10)
let frame2 = CGRect(x: 20, y: 20, width: 100, height: 100)

let path = UIBezierPath(rect: frame)
let path2 = UIBezierPath(rect: frame2)

let animation = CABasicAnimation(keyPath: "path")
animation.fromValue = path
animation.toValue = path2
animation.duration = 3
animation.repeatCount = Float.greatestFiniteMagnitude

let l = CAShapeLayer()
l.path = path.cgPath
l.fillColor = UIColor.black.cgColor
self.view.layer.addSublayer(l)

l.add(animation, forKey: "pathAnimation")

完整操场:

import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white
        self.view = view

        let frame = CGRect(x: 20, y: 20, width: 10, height: 10)
        let frame2 = CGRect(x: 20, y: 20, width: 100, height: 100)

        let path = UIBezierPath(rect: frame)
        let path2 = UIBezierPath(rect: frame2)

        let animation = CABasicAnimation(keyPath: "path")
        animation.fromValue = path
        animation.toValue = path2
        animation.duration = 3
        animation.repeatCount = Float.greatestFiniteMagnitude

        let l = CAShapeLayer()
        l.path = path.cgPath
        l.fillColor = UIColor.black.cgColor
        self.view.layer.addSublayer(l)

        l.add(animation, forKey: "pathAnimation")
    }
}

PlaygroundPage.current.liveView = MyViewController()