如何实现半透明的uiviewcontroller

目标：通过self.present弹出新窗口，达到半透明的效果；类似uiviewcontroller

错误的位置：

override func viewDidLoad() {
    super.viewDidLoad()
    self.modalPresentationStyle = .overCurrentContext
    view.backgroundColor = UIColor.lightGray.withAlphaComponent(0.5)
}

正确的位置：

override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
    super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
    
    self.modalPresentationStyle = .overCurrentContext
    view.backgroundColor = UIColor.lightGray.withAlphaComponent(0.5)
}

原因：
当新窗口弹出的动画结束后，底层的view其实已经不存在了，所以半透明就看不见他了：
After your modal view animates in, it is resized to be equal in size to its parent view. What you can do is inside your viewDidAppear:, take a picture of the parentController’s view, then insert a UIImageView containing the picture of the parent at the back of your own view’s list of subviews:

但如果在init中设置了alpha值，系统貌似帮我们做了这些事。