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func kt_drawRectWithRoundedCorner(radius radius: CGFloat,       
                       borderWidth: CGFloat,
                                  backgroundColor: UIColor,
                                  borderColor: UIColor) -> UIImage {    
     UIGraphicsBeginImageContextWithOptions(sizeToFit, false, UIScreen.mainScreen().scale)
     let context = UIGraphicsGetCurrentContext()
     
     CGContextMoveToPoint(context, 开始位置);  // 开始坐标右边开始
     CGContextAddArcToPoint(context, x1, y1, x2, y2, radius);  // 这种类型的代码重复四次
   
     CGContextDrawPath(UIGraphicsGetCurrentContext(), .FillStroke)  
     let output = UIGraphicsGetImageFromCurrentImageContext();
     UIGraphicsEndImageContext();
     return output
}

extension UIView {
    func kt_addCorner(radius radius: CGFloat,
                      borderWidth: CGFloat,
                      backgroundColor: UIColor,
                      borderColor: UIColor) {
        let imageView = UIImageView(image: kt_drawRectWithRoundedCorner(radius: radius,
                                    borderWidth: borderWidth,
                                    backgroundColor: backgroundColor,
                                    borderColor: borderColor))
        self.insertSubview(imageView, atIndex: 0)
    }
}

extension UIImage {
    func kt_drawRectWithRoundedCorner(radius radius: CGFloat, _ sizetoFit: CGSize) -> UIImage {
        let rect = CGRect(origin: CGPoint(x: 0, y: 0), size: sizetoFit)
         
        UIGraphicsBeginImageContextWithOptions(rect.size, false, UIScreen.mainScreen().scale)
        CGContextAddPath(UIGraphicsGetCurrentContext(),
            UIBezierPath(roundedRect: rect, byRoundingCorners: UIRectCorner.AllCorners,
                cornerRadii: CGSize(width: radius, height: radius)).CGPath)
        CGContextClip(UIGraphicsGetCurrentContext())
         
        self.drawInRect(rect)
        CGContextDrawPath(UIGraphicsGetCurrentContext(), .FillStroke)
        let output = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
         
        return output
    }
}

extension UIImageView {
    /**
     / !!!只有当 imageView 不为nil 时，调用此方法才有效果
     :param: radius 圆角半径
     */
    override func kt_addCorner(radius radius: CGFloat) {
        self.image = self.image?.kt_drawRectWithRoundedCorner(radius: radius, self.bounds.size)
    }
}