iOS13中appdelegate的职责发现了改变：
    iOS13之前，Appdelegate的职责全权处理App生命周期和UI生命周期；
    iOS13之后，Appdelegate的职责是：
    1、处理 App 生命周期
    2、新的 Scene Session 生命周期
    那UI的生命周期交给新增的Scene Delegate处理

    1. - (void)scene:(UIScene *)scene willConnectToSession:(UISceneSession *)session options:(UISceneConnectionOptions *)connectionOptions {
    2.     // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
    3.     // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
    4.     // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).
    5.     UIWindowScene *windowScene = (UIWindowScene *)scene;
    6.     self.window = [[UIWindow alloc] initWithWindowScene:windowScene];
    7.     self.window.frame = windowScene.coordinateSpace.bounds;
    9.     UITabBarController *tabbarController = [[UITabBarController alloc] init];
    11.     UIViewController *controller1 = [[UIViewController alloc] init];
    12.     controller1.view.backgroundColor = [UIColor redColor];
    13.     controller1.tabBarItem.title = @"新闻";
    15.     UIViewController *controller2 = [[UIViewController alloc] init];
    16.     controller2.view.backgroundColor = [UIColor yellowColor];
    17.     controller2.tabBarItem.title = @"视频";
    19.     UIViewController *controller3 = [[UIViewController alloc] init];
    20.     controller3.view.backgroundColor = [UIColor blueColor];
    21.     controller3.tabBarItem.title = @"推荐";
    23.     UIViewController *controller4 = [[UIViewController alloc] init];
    24.     controller4.view.backgroundColor = [UIColor greenColor];
    25.     controller4.tabBarItem.title = @"我的";
    27.     // 将四个页面的 UIViewController 加入到 UITabBarController 之中
    28.     [tabbarController setViewControllers: @[controller1, controller2, controller3, controller4]];
    30.     self.window.rootViewController = tabbarController;
    31.     [self.window makeKeyAndVisible];
    32. }