根据按钮点击状态切换背景颜色

  ```
    let btn = UIButton.init(frame: CGRect.init(x: 0, y: 100, width: 300, height: 100))
    btn.backgroundColor = UIColor.gray
    /// 按下去
    btn.addTarget(self, action: #selector(btnSel(sender:)), for: UIControlEvents.touchUpInside)
    /// 离开
    btn.addTarget(self, action: #selector(btnSel2(sender:)), for: UIControlEvents.touchDown)
    view.addSubview(btn)

@objc private func btnSel(sender : UIButton) {
sender.backgroundColor = UIColor.gray
print(sender.state)
}

@objc private func btnSel2(sender : UIButton) {
    sender.backgroundColor = UIColor.yellow
    print(sender.state)
}