如何绘制透明的NSScroller

import Cocoa

class CustomScroller: NSScroller {
    override func drawKnob() {
        let knobRect = self.rect(for: .knob)
        let knobColor = NSColor.red.withAlphaComponent(0.5) // 设置透明度为0.5的红色滑块
        knobColor.set()
        NSBezierPath(rect: knobRect).fill()
    }
    
    override func drawRect(dirtyRect: NSRect) {
        let backgroundRect = self.rect(for: .track)
        let backgroundColor = NSColor.blue.withAlphaComponent(0.2) // 设置透明度为0.2的蓝色背景
        backgroundColor.set()
        NSBezierPath(rect: backgroundRect).fill()
    }
}

let scrollView = NSScrollView(frame: frame)
let customScroller = CustomScroller()
scrollView.verticalScroller = customScroller