override func drawRect(rect: CGRect) {
let π = M_PI
let context = UIGraphicsGetCurrentContext()
CGContextSetLineWidth(context, 20.0)
// Back circle
CGContextSetStrokeColorWithColor(context,UIColor.blueColor().colorWithAlphaComponent(0.2).CGColor)
CGContextAddArc(context, 150,150, 100, CGFloat(0), CGFloat(2 * π), 0)
CGContextStrokePath(context)
let radius = 100.0
// Create the circle layer
var circle = CAShapeLayer()
// Set the center of the circle to be the center of the view
let center = CGPointMake(150,150)
let fractionOfCircle = 3.0 / 4.0
let twoPi = 2.0 * Double(M_PI)
// The starting angle is given by the fraction of the circle that the point is at, divided by 2 * Pi and less
// We subtract M_PI_2 to rotate the circle 90 degrees to make it more intuitive (i.e. like a clock face with zero at the top, 1/4 at RHS, 1/2 at bottom, etc.)
let startAngle = Double(fractionOfCircle) / Double(twoPi) - Double(M_PI_2)
let endAngle = 0.0 - Double(M_PI_2)
let clockwise: Bool = true
// `clockwise` tells the circle whether to animate in a clockwise or anti clockwise direction
circle.path = UIBezierPath(arcCenter: center, radius: CGFloat(radius), startAngle: CGFloat(startAngle), endAngle: CGFloat(endAngle), clockwise: clockwise).CGPath
// Configure the circle
circle.fillColor = UIColor.clearColor().CGColor
circle.strokeColor = UIColor.blueColor().CGColor
circle.lineWidth = 20.0
circle.lineCap = kCALineCapRound
// When it gets to the end of its animation, leave it at 0% stroke filled
circle.strokeEnd = CGFloat(fractionOfCircle)
// Add the circle to the parent layer
self.layer.addSublayer(circle)
// Configure the animation
var drawAnimation = CABasicAnimation(keyPath: "strokeEnd")
drawAnimation.repeatCount = 1.0
// Animate from the full stroke being drawn to none of the stroke being drawn
drawAnimation.fromValue = NSNumber(float: 0.0)
drawAnimation.toValue = NSNumber(double: fractionOfCircle)
drawAnimation.duration = 0.8
drawAnimation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
// Add the animation to the circle
circle.addAnimation(drawAnimation, forKey: "drawCircleAnimation")
}