如何为NSButtonCell指定alpha?(对于禁用状态)

drawBezel 绘制具有特定形状的按钮。现在我想要的是在禁用时将单元格的alpha设置为50%。我知道我必须用 isEnabled ,但如何为整个单元格(包括标题)指定alpha?

class FunkyCell: NSButtonCell
{
    ...
    override func drawTitle(_ title: NSAttributedString, withFrame frame: NSRect, in controlView: NSView) -> NSRect {
        return super.drawTitle(title, withFrame: frame.offsetBy(dx: 0, dy: 0), in: controlView)
    }

    func redraw() {
        self.backgroundColor = self.backgroundColor
    }

    override func drawBezel(withFrame frame: NSRect, in controlView: NSView) {
        let path = NSBezierPath(bound: frame.insetBy(dx: 2, dy: 2), withCorners: corners, withRadius: CGFloat(NumKeypadConstants.cornerRadius), flip: flipIt)
        path.lineWidth = NumKeypadConstants.borderWidth
        if(isHighlighted)
        {
            let fillColor: NSColor = NumKeypadConstants.buttonHighlightColor
            let strokeColor: NSColor = NumKeypadConstants.buttonBorderColor
            fillColor.setFill()
            strokeColor.setStroke()
            path.fill()
            path.stroke()
        }
        else
        {
            let fillColor: NSColor = NumKeypadConstants.buttonBackgroundColor
            let strokeColor: NSColor = NumKeypadConstants.buttonBorderColor
            fillColor.setFill()
            strokeColor.setStroke()
            path.fill()
            path.stroke()
        }
    }
}