iOS13 SceneDelegate

iOS13更新之后增加了SceneDelegate文件，AppDelegate不再负责UI的生命周期，交给了SceneDelegate来处理，因此，我们创建根控制器的时候，就要在SceneDelegate文件中创建了，如下：
swift:

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
        // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
        // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).
        guard let windowScene = (scene as? UIWindowScene) else { return }
        
        let vc = MainViewController()
        let nav = UINavigationController(rootViewController: vc)
        nav.view.backgroundColor = UIColor.white
        // Create the window. Be sure to use this initializer and not the frame one.
        window = UIWindow(windowScene: windowScene)
        window?.rootViewController = nav
        window?.makeKeyAndVisible()
    }

OC:

- (void)scene:(UIScene *)scene willConnectToSession:(UISceneSession *)session options:(UISceneConnectionOptions *)connectionOptions {
    
    // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
    // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
    // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).
    
    UIWindowScene *windowScene = (UIWindowScene *)scene;
    self.window = [[UIWindow alloc] initWithWindowScene:windowScene];
    self.window.frame = windowScene.coordinateSpace.bounds;
    UINavigationController * nav = [[UINavigationController alloc] initWithRootViewController:[[MainViewController alloc] init]];
    nav.view.backgroundColor = UIColor.whiteColor;
    self.window.rootViewController = nav;
    [self.window makeKeyAndVisible];
}

以上是保留scene多场景的情况，如果你的APP不需要多场景，直接在AppDelegate.swift里创建window，在didFinishLaunchingWithOptions里创建启动控制器、注释掉下面的两个与scene有关的代理方法，SceneDelegate.swift可以删除，也可以不管他，info.plist文件中删除Application Scene Manifest选项，做完以上操作，就可以和之前一样了。
AppDelegate.swift文件就会如下，变成我们熟悉的样子：

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        
        self.window = UIWindow(frame: UIScreen.main.bounds)
        self.window?.backgroundColor = UIColor.white
        self.window?.rootViewController = UINavigationController(rootViewController: MainViewController())
        
        self.window?.makeKeyAndVisible()
  
        return true
    }

    // MARK: UISceneSession Lifecycle

//    @available(iOS 13.0, *)
//    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
//        // Called when a new scene session is being created.
//        // Use this method to select a configuration to create the new scene with.
//        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
//    }
//
//    @available(iOS 13.0, *)
//    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
//        // Called when the user discards a scene session.
//        // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
//        // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
//    }


}