这是因为AppDelegate不再具有window属性。现在，您必须使用SceneDelegate的scene(_:willConnectTo:options:)方法来更改根视图控制器。如本例所示：
func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
guard let scene = (scene as? UIWindowScene) else { return }
// Instantiate UIWindow with scene
let window = UIWindow(windowScene: scene)
// Assign window to SceneDelegate window property
self.window = window
// Set initial view controller from Main storyboard as root view controller of UIWindow
self.window?.rootViewController = UIStoryboard(name: "Main", bundle: nil).instantiateInitialViewController()
// Present window to screen
self.window?.makeKeyAndVisible()
}
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