如何使用iPhone SDK清除圆角矩形外的角落？

let maskPath = UIBezierPath(roundedRect: self.bounds, 
                            byRoundingCorners: [.allCorners], 
                            cornerRadii: CGSize(width: 5.0, height: 5.0))

self.layer.mask = maskPath.cgPath

let bounds = CGRect(x: 0, y: 0, width: 100, height: 100)
let path = UIBezierPath(rect: bounds)
path.append(UIBezierPath(rect: CGRect(x: bounds.midX - 5, y: bounds.midY - 5, width: 10, height: 10)))
path.append(UIBezierPath(rect: CGRect(x: bounds.midX + 5, y: bounds.midY + 5, width: 10, height: 10)))

let shape = UIBezierPath(rect: bounds)

let cgPath = CGContext.createPath(using: path)
CGContext.setFillColor(cgPath!, with: CGColor(red: 1.0, green: 0.0, blue: 0.0, alpha: 0.5))
CGContext.fill(bounds)

class RoundedView: UIView {
    override func draw(_ rect: CGRect) {
        let maskPath = UIBezierPath(roundedRect: rect, 
                                    byRoundingCorners: .allCorners, 
                                    cornerRadii: CGSize(width: 5.0, height: 5.0))
        
        self.layer.mask = maskPath.cgPath
        
        let bounds = CGRect(x: 0, y: 0, width: 100, height: 100)
        let path = UIBezierPath(rect: bounds)
        path.append(UIBezierPath(rect: CGRect(x: bounds.midX - 5, y: bounds.midY - 5, width: 10, height: 10)))
        path.append(UIBezierPath(rect: CGRect(x: bounds.midX + 5, y: bounds.midY + 5, width: 10, height: 10)))

        let shape = UIBezierPath(rect: bounds)

        let cgPath = CGContext.createPath(using: path)
        CGContext.setFillColor(cgPath!, with: CGColor(red: 1.0, green: 0.0, blue: 0.0, alpha: 0.5))
        CGContext.fill(bounds)
    }
}