How to make a shape draw itself using strokeEnd

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

iOS makes it easy to draw shapes using CAShapeLayer, but you also get the ability to adjust how shapes are drawn. By manipulating the strokeStart and strokeEnd properties you can make shapes draw themselves on the screen: you can present a half-drawn star, or a three-quarters drawn circle, for example.

However, as clever as that is what’s really neat is being able to animate the drawing process. To try it out, first create a CAShapeLayer with a stroke that’s visible, like this one:

let layer = CAShapeLayer()
let bounds = CGRect(x: 50, y: 50, width: 250, height: 250)
layer.path = UIBezierPath(roundedRect: bounds, byRoundingCorners: .allCorners, cornerRadii: CGSize(width: 20, height: 20)).cgPath
layer.strokeColor = UIColor.black.cgColor
layer.fillColor = nil
layer.lineDashPattern = [8, 6]
view.layer.addSublayer(layer)

Now create and add a CABasicAnimation to adjust the strokeEnd property:

let animation = CABasicAnimation(keyPath: "strokeEnd")
animation.fromValue = 0
animation.toValue = 1
animation.duration = 2
animation.autoreverses = true
animation.repeatCount = .infinity
layer.add(animation, forKey: "line")

I made that animate from 0 (not drawn) to 1 (fully drawn) over two seconds, but also made it reverse at the end and repeat infinite times.