如果在iOS13之前，你是這樣使用彈框

let window = UIWindow(frame: UIScreen.main.bounds)
let viewController = UIViewController()
viewController.view.backgroundColor = .clear
window.rootViewController = viewController
window.windowLevel = UIWindow.Level.statusBar + 1
window.makeKeyAndVisible()

那么在iOS13時(shí)，你會(huì)發(fā)現(xiàn)UI界面并沒有任何反應(yīng)，之所以無效，是因?yàn)槟闶褂昧薸OS 13 的SceneDelegate，

此時(shí)你需要使用下面這種方式來創(chuàng)建window

let windowScene = UIApplication.shared
                .connectedScenes
                .filter { $0.activationState == .foregroundActive }
                .first
if let windowScene = windowScene as? UIWindowScene {
    // 該window是全局變量
    window = UIWindow(windowScene: windowScene)
    window?.frame = UIScreen.main.bounds
    window?.backgroundColor = .clear
}

之后展示的時(shí)候

window?.windowLevel = UIWindow.Level.statusBar + 1
window?.rootViewController = viewController
window?.makeKeyAndVisible()

此時(shí)你會(huì)發(fā)現(xiàn)makeKeyAndVisible又起作用了。。。

另：
在iOS13中，keyWindow已被廢棄，若使用了SceneDelegate, 則在iOS13上獲取會(huì)得到nil，可以使用

UIApplication.shared.windows.first

或

    var window: UIWindow?
    if #available(iOS 13.0, *) {
          window = (UIApplication.shared.connectedScenes.first?.delegate as? SceneDelegate)?.window
    } else {
          window = UIApplication.shared.keyWindow
    }