Swift：押した時に画像が変わるNSButton

let btn = NSButton(frame: NSRect(x: 5, y: 5, width: 60, height: 40))

override func awakeFromNib() {
    super.awakeFromNib()
    btn.title = ""
    btn.image = NSImage(named: "image.png")
    btn.alternateImage = NSImage(named: "highlighted_image.png")
    btn.imageScaling = NSImageScaling.scaleProportionallyDown
    //重要なのはこれ↓
    btn.setButtonType(NSButton.ButtonType.momentaryChange)

    // btn.wantsLayer = true
    // btn.layer?.cornerRadius = 12
    // btn.isBordered = false

    btn.target = self
    btn.action = #selector(push)
    addSubview(btn)
}                

@objc func push() {
    print("hey")
}