PieProgressView.swift

public class PieProgressView: UIView {
    public var progress:Float {
        get {
            return _progress
        }
        
        set {
            _progress = newValue
            self.updatePath()
        }
    }
    
    var borderWidth:Float = 2.0 {
        didSet {
            self.updatePath()
        }
    }
    
    override public var frame:CGRect {
        didSet {
            self.updatePath()
        }
    }
    
    static private let animationDuration = 0.6
    
    public func setProgress(progress:Float, animated:Bool) {
        _progress = progress
        self.updatePath(animated)
    }
    
    private var _progress:Float = 0.0
    
    init() {
        super.init(frame: CGRectZero)
        self.initialize()
    }
    
    override public init(frame: CGRect) {
        super.init(frame:frame)
        self.initialize()
    }
    
    
    required public init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.initialize()
    }
    
    public var fillColor:UIColor = UIColor.lightGrayColor() {
        didSet {
            self.shapeLayer.fillColor = self.fillColor.CGColor
        }
    }
    
    public var backgroundFillColor:UIColor = UIColor(red: 250.0/255.0, green: 250.0/255.0, blue: 250.0/255.0, alpha: 0.6) {
        didSet {
            self.backgroundShapeLayer.fillColor = self.backgroundFillColor.CGColor
        }
    }
    
    private let shapeLayer = CAShapeLayer()
    private let backgroundShapeLayer = CAShapeLayer()
    
    private func initialize() {
        self.backgroundColor = UIColor.clearColor()
        
        self.backgroundShapeLayer.fillColor = self.backgroundFillColor.CGColor
        self.layer.addSublayer(self.backgroundShapeLayer)
        
        self.shapeLayer.fillColor = self.fillColor.CGColor
        //        self.shapeLayer.strokeColor = UIColor.redColor().CGColor
        
        self.layer.addSublayer(self.shapeLayer)
        self.updatePath()
    }
    
    private func updatePath(animated:Bool = false) {
        self.backgroundShapeLayer.path = UIBezierPath(ovalInRect: self.bounds).CGPath

        let center = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds))
        let radius = center.y - CGFloat(2 * self.borderWidth)
        let angle = DegreesToRadians(Double((360.0 * _progress) + -90.0))
        let points = [CGPointMake(center.x, CGFloat(self.borderWidth)),
            center,
            CGPointMake(center.x + radius * CGFloat(cosf(Float(angle))), center.y + radius * CGFloat(sinf(Float(angle))))]
        
        let path = UIBezierPath()
        
        path.moveToPoint(points[2])
        path.addLineToPoint(points[1])
        path.addLineToPoint(points[0])
        
        path.addArcWithCenter(center, radius: radius, startAngle: CGFloat(DegreesToRadians(-90)), endAngle: CGFloat(angle), clockwise: true)
        
        if animated {
            let animation = CABasicAnimation(keyPath: "path")
            animation.duration = PieProgressView.animationDuration
            animation.fillMode = kCAFillModeForwards
            animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
            animation.removedOnCompletion = true
            animation.fromValue = self.shapeLayer.path
            animation.toValue = path.CGPath
            self.shapeLayer.path = path.CGPath
            
            self.shapeLayer.addAnimation(animation, forKey: "path")
        } else {
            self.shapeLayer.path = path.CGPath
        }
    }
}

Wrong animation example