How to Draw a Circle Using Core Animation in Swift

override func drawRect(rect: CGRect)
{
  var startAngle: Float = Float(2 * M_PI)
  var endAngle: Float = 0.0

  // Drawing code
  // Set the radius
  let strokeWidth = 1.0
  let radius = CGFloat((CGFloat(self.frame.size.width) - CGFloat(strokeWidth)) / 2)

  // Get the context
  var context = UIGraphicsGetCurrentContext()

  // Find the middle of the circle
  let center = CGPointMake(self.frame.size.width / 2, self.frame.size.height / 2)

  // Set the stroke color
  CGContextSetStrokeColorWithColor(context, Colors.primaryColor().CGColor)

  // Set the line width
  CGContextSetLineWidth(context, CGFloat(strokeWidth))

  // Set the fill color (if you are filling the circle)
  CGContextSetFillColorWithColor(context, UIColor.clearColor().CGColor)

  // Rotate the angles so that the inputted angles are intuitive like the clock face: the top is 0 (or 2π), the right is π/2, the bottom is π and the left is 3π/2.
  // In essence, this appears like a unit circle rotated π/2 anti clockwise.
  startAngle = startAngle - Float(M_PI_2)
  endAngle = endAngle - Float(M_PI_2)

  // Draw the arc around the circle
  CGContextAddArc(context, center.x, center.y, CGFloat(radius), CGFloat(startAngle), CGFloat(endAngle), 0)

  // Draw the arc
  CGContextDrawPath(context, kCGPathStroke) // or kCGPathFillStroke to fill and stroke the circle

}

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(CGRectGetMidX(self.frame) - radius, CGRectGetMidY(self.frame) - radius)

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: radius, startAngle: CGFloat(startAngle), endAngle: CGFloat(endAngle), clockwise: clockwise).CGPath

// Configure the circle
circle.fillColor = UIColor.blackColor().CGColor
circle.strokeColor = UIColor.redColor().CGColor
circle.lineWidth = 5

// When it gets to the end of its animation, leave it at 0% stroke filled
circle.strokeEnd = 0.0

// 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(double: fractionOfCircle)
drawAnimation.toValue = NSNumber(float: 0.0)

drawAnimation.duration = 30.0

drawAnimation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionLinear)

// Add the animation to the circle
circle.addAnimation(drawAnimation, forKey: "drawCircleAnimation")