How to use CAReplicatorLayer to make activity indicator in iOS

Issue #230

CAReplicatorLayer is a layer that creates a specified number of sublayer copies with varying geometric, temporal, and color transformations

Here we use instanceTransform which applies transformation matrix around the center of the replicator layer

Below is how we use replicatorLayer to replicate lots of line and rotate them around the center.

let replicatorLayer = CAReplicatorLayer()
let animation = CABasicAnimation(keyPath: #keyPath(CALayer.opacity))

let line = CALayer()
let lineCount: Int = 12
let duration: TimeInterval = 1.0
let lineSize: CGSize = CGSize(width: 20, height: 6)
let lineColor: UIColor = UIColor.darkGray

let angle = CGFloat.pi * 2 / CGFloat(lineCount)
let rotation = CATransform3DMakeRotation(angle, 0, 0, 1.0)

replicatorLayer.instanceTransform = rotation
replicatorLayer.instanceCount = lineCount
replicatorLayer.instanceDelay = duration / TimeInterval(lineCount)

line.backgroundColor = lineColor.cgColor
line.frame.size = lineSize
line.cornerRadius = lineSize.height / 2

animation.fromValue = 1.0
animation.toValue = 0.0
animation.repeatCount = Float.greatestFiniteMagnitude
animation.timingFunction = CAMediaTimingFunction(name: .linear)
animation.duration = duration

replicatorLayer.addSublayer(line)
layer.addSublayer(replicatorLayer)

// x: 
// y: half the height, changing affects rotation of lines
line.position = CGPoint(x: 48, y: 75)

line.add(animation, forKey: nil)

Pay attention to position of the line. The larger the x, the closer to center. y should be half the height of the replicator layer size, changing it affects the skewness of the line.