swift-如何仅为左下角，右下角和左上角设置图层cornerRadius？

如何仅设置textview的左下角、右下角和左上角的角半径？

let rectShape = CAShapeLayer()
rectShape.backgroundColor = UIColor.redColor().CGColor
rectShape.bounds = messages.frame
rectShape.position = messages.center
rectShape.path = UIBezierPath(roundedRect: messages.bounds, byRoundingCorners: .BottomLeft | .TopRight, cornerRadii: CGSize(width: 20, height: 20)).CGPath

messages.layer.addSublayer(rectShape)

这段代码创建了两个矩形。我不知道为什么。