How to use Swift 4 KeyPath in CAShapeLayer animation

Yes, it is possible to use swift keypaths. Like:

extension UIView {

func animateLayer<Value>(_ keyPath: WritableKeyPath<CALayer, Value>, to value:Value, duration: CFTimeInterval) {

    let keyString = NSExpression(forKeyPath: keyPath).keyPath
    let animation = CABasicAnimation(keyPath: keyString)
    animation.fromValue = self.layer[keyPath: keyPath]
    animation.toValue = value
    animation.duration = duration
    self.layer.add(animation, forKey: animation.keyPath)
    var thelayer = layer
    thelayer[keyPath: keyPath] = value
}
}

Usage like:

animateLayer(\.shadowOffset, to: CGSize(width: 3, height: 3), duration:1)
animateLayer(\.shadowOpacity, to: 0.4, duration: 1)

It's not thoroughly tested. but worked for me.

As for now CABasicAnimation still uses the old String keyPaths so you should still use #keyPath(CAShapeLayer.path) even though you are using Swift 4.

Apple will probably update all it's APIs in the future to make use of these safer key path references. But for now you are stuck with "unsafe" Strings.