模态视图控制器子视图

我已经尝试将子视图的alpha设置为1,但它不会改变外观。

class GameOverViewController: UIViewController {
    private let restart = UIButton(type: .custom)
    private let mainmenu = UIButton(type: .custom)

    override func viewDidLoad() {
        //displays view controller modally
        super.viewDidLoad()
        self.view.backgroundColor = .white
        self.view.alpha = 0.6
        self.modalPresentationStyle = .overCurrentContext

        //add dialogue box
        let dialoguebox = UIView(frame: CGRect(origin: CGPoint(x: self.view.frame.width / 2, y: self.view.frame.height / 2), size: CGSize(width: self.view.frame.width / 2, height: self.view.frame.height / 2)))
        dialoguebox.backgroundColor = .red
        dialoguebox.center = self.view.center
        dialoguebox.alpha = 1
        self.view.addSubview(dialoguebox)
    }
}