Strange behaviour when animating between two circular UIBezierPaths
The Problem
I am creating an exploding rings effect. I'm using multiple CAShapeLayer 's and creating a UIBezierPath for each layer. When the view is initialised, all the layers have a path with width 0 . When an action is triggered, the rings animate to a larger path.
As seen in the demo below, the right hand edge of each layer is slower to animate than the remainder of each circular layer.
Demo 
Code
Drawing the layers:
func draw(inView view: UIView) -> CAShapeLayer {
let shapeLayer = CAShapeLayer()
// size.width defaults to 0
let circlePath = UIBezierPath(arcCenter: view.center, radius: size.width / 2, startAngle: 0, endAngle: CGFloat(360.0).toRadians(), clockwise: true)
shapeLayer.path = circlePath.CGPath
shapeLayer.frame = view.frame
return shapeLayer
}
Updating the layers
func updateAnimated(updatedOpacity: CGFloat, updatedSize: CGSize, duration: CGFloat) {
let updatePath = UIBezierPath(arcCenter: view.center, radius: updatedSize.width / 2,
startAngle: 0, endAngle: CGFloat(360).toRadians(), clockwise: true)
let pathAnimation = CABasicAnimation(keyPath: "path")
pathAnimation.fromValue = layer.path // the current path (initially with width 0)
pathAnimation.toValue = updatePath.CGPath
let opacityAnimation = CABasicAnimation(keyPath: "opacity")
opacityAnimation.fromValue = self.opacity
opacityAnimation.toValue = updatedOpacity
let animationGroup = CAAnimationGroup()
animationGroup.animations = [pathAnimation, opacityAnimation]
animationGroup.duration = Double(duration)
animationGroup.fillMode = kCAFillModeForwards
animationGroup.removedOnCompletion = false
layer.addAnimation(animationGroup, forKey: "shape_update")
... update variables ...
}
Avoid starting with a path width of 0. It is very difficult to scale from 0 and tiny floating-point errors magnify. Values very close to zero are not very continuous in floating point (hmmm.... I guess that actually is a little like the real universe). Try starting with larger values and see how close to zero you can go safely.
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