1. 填充颜色时设置混合模式

    func radiusMaskImage() -> UIImage? {
        let size = CGSize.init(width: 100, height: 100)
        let cornerRadius: CGFloat = 50
        UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale)
        UIColor.red.setFill()
        UIRectFill(CGRect.init(origin: CGPoint.zero, size: size))
        let path = UIBezierPath.init(roundedRect: CGRect.init(origin: CGPoint.zero, size: size), cornerRadius: cornerRadius)
        // 设置BlendMode为clear
        path.fill(with: CGBlendMode.clear, alpha: 1)
        var image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        let inset = cornerRadius + 1
        image = image?.resizableImage(withCapInsets: UIEdgeInsets.init(top: inset, left: inset, bottom: inset, right: inset), resizingMode: UIImageResizingMode.stretch)
        return image
    }

CGBlendMode： 绘制时使用的混合模式，设置为clear，在绘制时显示为透明

2. 设置fillRule

填充区域的计算规则有even-odd和non-zero。
even-odd：奇偶规则，从指定区域的一点绘制到路径外的任意一点，交点个数为偶数则认为这点在范围内，并填充相应的区域。为奇数则认为这点不在范围内，所在区域不会被填充。
non-zero: 非零规则，从指定区域的一点绘制到路径外的任意一点，与从左到右的路径相交，数值+1，与从右到左的的路径相交，数值-1，如果最后的总值非零认为这点在范围内，并填充相应的区域。为零则认为这点不在范围内，所在区域不会被填充。
在苹果官方文档有说明：https://developer.apple.com/documentation/uikit/uibezierpath/1624360-usesevenoddfillrule

一. 设置fillRule 为even-odd rule

    func radiusMaskImage() -> UIImage? {
        let size = CGSize.init(width: 100, height: 100)
        let cornerRadius: CGFloat = 50
        UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale)
        UIColor.red.setFill()
        let path = UIBezierPath.init(roundedRect: CGRect.init(origin: CGPoint.zero, size: size), cornerRadius: cornerRadius)
        path.append(UIBezierPath.init(rect: CGRect.init(origin: CGPoint.zero, size: size)))
        // 设置填充规则为even-odd
        path.usesEvenOddFillRule = true
        path.fill()
        var image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        let inset = cornerRadius + 1
        image = image?.resizableImage(withCapInsets: UIEdgeInsets.init(top: inset, left: inset, bottom: inset, right: inset), resizingMode: UIImageResizingMode.stretch)
        return image
    }

二. 使用non-zero rule

    func radiusMaskImage() -> UIImage? {
        let size = CGSize.init(width: 100, height: 100)
        let cornerRadius: CGFloat = 50
        UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale)
        UIColor.red.setFill()
        let path = UIBezierPath.init(rect: CGRect.init(origin: CGPoint.zero, size: size))
        // 添加一个逆时针的圆
        path.append(UIBezierPath.init(arcCenter: CGPoint.init(x: size.width / 2, y: size.height / 2), radius: cornerRadius, startAngle: 0, endAngle: 2 * CGFloat.pi, clockwise: false))
        path.fill()
        var image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        let inset = cornerRadius + 1
        image = image?.resizableImage(withCapInsets: UIEdgeInsets.init(top: inset, left: inset, bottom: inset, right: inset), resizingMode: UIImageResizingMode.stretch)
        return image
    }

最后上图：

效果图