Using CAShapeLayer to draw a circle

UIViews are like Ogres, which are like onions – they have layers.

Source: Documentation

More accurately, UIView has a single layer property that can have infinite sublayers.

UIView’s have layers

Lets draw a circle.

First create a UIView and a CAShapeLayer:

let myView = UIView()

let myLayer = CAShapeLayer()

Layers have a path

UIBezierPath makes drawing shapes extremely smooth, lets say in the shape of a circle:

let circlePath = UIBezierPath(
  arcCenter: CGPoint(x: 100.0, y: 100.0),
  radius: 100,
  startAngle: 0 * CGFloat.pi / 180,
  endAngle: 360 * CGFloat.pi / 180,
  clockwise: true)

Great! Now we can add CAShapeLayer as a CGPath and spruce up our fill and stroke colors

myLayer.path = circlePath.cgPath
myLayer.strokeColor = UIColor.blue.cgColor
myLayer.fillColor = UIColor.yellow.cgColor

Last step! Add this layer to your UIView

myView.layer.addSublayer(myLayer)

Layers have attributes

strokeColor,fillColor, opacity, lineWidth and more!

Bonus: Layers can animate attributes

// create the animation
let opacityAnimation = CABasicAnimation(
  keyPath: #keyPath(CALayer.opacity))
opacityAnimation.fromValue = 0
opacityAnimation.toValue = 1
opacityAnimation.duration = 5.0

// set the final value; add the animation
myLayer.opacity = 1
myLayer.add(opacityAnimation, forKey: #keyPath(CALayer.opacity))