Circle view that progressively fills its border

gistfile1.txt

import UIKit
import XCPlayground

let viewWidth: CGFloat = 300
var end: CGFloat = 0
let lineWidth: CGFloat = 20
let circleView = UIView(frame: CGRectMake(0, 0, viewWidth, viewWidth))
XCPlaygroundPage.currentPage.liveView = circleView
let circleLayer = CAShapeLayer()
let width = circleView.frame.size.width;
let height = circleView.frame.size.height;
circleLayer.bounds = CGRectMake(-lineWidth/2, -lineWidth/2, width, height)
circleLayer.position = CGPointMake(width/2, height/2)
var c = circleView.frame
c.size.width -= lineWidth
c.size.height -= lineWidth
let path = UIBezierPath(roundedRect: c, cornerRadius: viewWidth/2)
circleLayer.path = path.CGPath
circleLayer.strokeEnd = end
circleLayer.lineCap = kCALineCapRound
circleLayer.fillColor = UIColor.clearColor().CGColor
circleLayer.strokeColor = UIColor.blueColor().CGColor
circleLayer.lineWidth = lineWidth
circleView.layer.addSublayer(circleLayer)

class Responder : NSObject {
  func tick() {
    end += 0.1
    circleLayer.strokeEnd = end
  }
}

let responder = Responder()

NSTimer.scheduledTimerWithTimeInterval(1.0, target: responder, selector: #selector(Responder.tick), userInfo: nil, repeats: true)

Preview