やりたいこと

角丸にしたUIImageViewに、ボーダーをつけたい場合、以下のような関数をつくります。

extension UIImageView {
  func cropAsCircleWithBorder(borderColor : UIColor, strokeWidth: CGFloat) {
    var radius = min(self.bounds.width, self.bounds.height)
    var drawingRect : CGRect = self.bounds
    drawingRect.size.width = radius
    drawingRect.origin.x = (self.bounds.size.width - radius) / 2
    drawingRect.size.height = radius
    drawingRect.origin.y = (self.bounds.size.height - radius) / 2

    radius /= 2

    var path = UIBezierPath(roundedRect: CGRectInset(drawingRect, strokeWidth/2, strokeWidth/2), cornerRadius: radius)
    let border = CAShapeLayer()
    border.fillColor = UIColor.clearColor().CGColor
    border.path = path.CGPath
    border.strokeColor = borderColor.CGColor
    border.lineWidth = strokeWidth
    self.layer.addSublayer(border)

    path = UIBezierPath(roundedRect: drawingRect, cornerRadius: radius)
    let mask = CAShapeLayer()
    mask.path = path.CGPath
    self.layer.mask = mask
  }
}

つかいかた

imageView.cropAsCircleWithBorder(UIColor.whiteColor(), strokeWidth: 1)