Swift - UIKit ：繪製圓角Border

class B: UIView

 override func draw(_ rect: CGRect) {
        borderColor.setStroke()
        borderColor.withAlphaComponent(backgroundOpacity).setFill()
        let backgroundPath = UIBezierPath(roundedRect: bounds, cornerRadius: borderCornerRadius)
        backgroundPath.fill()
        let borderPath: UIBezierPath
        let borderRect = bounds.insetBy(dx: borderWidth / 2, dy: borderWidth / 2)
        if borderCornerSize == 0 {
            borderPath = UIBezierPath(roundedRect: borderRect, cornerRadius: borderCornerRadius)
        } else {
            var cornerSizeH = borderCornerSize
            if cornerSizeH > borderRect.width / 2 - borderCornerRadius {
                cornerSizeH = max(borderRect.width / 2 - borderCornerRadius, 0)
            }
            var cornerSizeV = borderCornerSize
            if cornerSizeV > borderRect.height / 2 - borderCornerRadius {
                cornerSizeV = max(borderRect.height / 2 - borderCornerRadius, 0)
            }
            let cornerSize = CGSize(width: cornerSizeH, height: cornerSizeV)
            borderPath = UIBezierPath(cornersOfRect: borderRect, cornerSize: cornerSize, cornerRadius: borderCornerRadius)
        }
        borderPath.lineWidth = borderWidth
        borderPath.stroke()
    }

extension UIBezierPath

extension UIBezierPath {
    convenience init(cornersOfRect borderRect: CGRect, cornerSize: CGSize, cornerRadius: CGFloat) {
    //conrnerSize: 圓角邊長
        self.init()
        let cornerSizeH = cornerSize.width
        let cornerSizeV = cornerSize.height
        // top-left
        move(to: CGPoint(x: borderRect.minX, y: borderRect.minY + cornerSizeV + cornerRadius))
        addLine(to: CGPoint(x: borderRect.minX, y: borderRect.minY + cornerRadius))
        addArc(withCenter: CGPoint(x: borderRect.minX + cornerRadius, y: borderRect.minY + cornerRadius),
               radius: cornerRadius,
               startAngle: CGFloat.pi,
               endAngle: -CGFloat.pi / 2,
               clockwise: true)
        addLine(to: CGPoint(x: borderRect.minX + cornerSizeH + cornerRadius, y: borderRect.minY))
        // top-right
        move(to: CGPoint(x: borderRect.maxX - cornerSizeH - cornerRadius, y: borderRect.minY))
        addLine(to: CGPoint(x: borderRect.maxX - cornerRadius, y: borderRect.minY))
        addArc(withCenter: CGPoint(x: borderRect.maxX - cornerRadius, y: borderRect.minY + cornerRadius),
               radius: cornerRadius,
               startAngle: -CGFloat.pi / 2,
               endAngle: 0,
               clockwise: true)
        addLine(to: CGPoint(x: borderRect.maxX, y: borderRect.minY + cornerSizeV + cornerRadius))
        // bottom-right
        move(to: CGPoint(x: borderRect.maxX, y: borderRect.maxY - cornerSizeV - cornerRadius))
        addLine(to: CGPoint(x: borderRect.maxX, y: borderRect.maxY - cornerRadius))
        addArc(withCenter: CGPoint(x: borderRect.maxX - cornerRadius, y: borderRect.maxY - cornerRadius),
               radius: cornerRadius,
               startAngle: 0,
               endAngle: CGFloat.pi / 2,
               clockwise: true)
        addLine(to: CGPoint(x: borderRect.maxX - cornerSizeH - cornerRadius, y: borderRect.maxY))
        // bottom-left
        move(to: CGPoint(x: borderRect.minX + cornerSizeH + cornerRadius, y: borderRect.maxY))
        addLine(to: CGPoint(x: borderRect.minX + cornerRadius, y: borderRect.maxY))
        addArc(withCenter: CGPoint(x: borderRect.minX + cornerRadius,
                                   y: borderRect.maxY - cornerRadius),
               radius: cornerRadius,
               startAngle: CGFloat.pi / 2,
               endAngle: CGFloat.pi,
               clockwise: true)
        addLine(to: CGPoint(x: borderRect.minX, y: borderRect.maxY - cornerSizeV - cornerRadius))
    }
}