iOS13之后，xcode 11新建项目，大家会发现项目中多出了SceneDelegate这个类，而且新建项目之后，在模拟器上运行没问题，真机上低于iOS13系统的手机就会出现黑屏(解决方法就是在AppDelegate.h中按照以前的方法添加一个UIWindow *window的属性即可,因为iOS13以下根本没有UIScene这一层)，而后看了代码发现，

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在AppDelegate.h中少了@property (strong, nonatomic) UIWindow * window;

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AppDelegate.m中也少了一些生命周期方法

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反观之SceneDelegate中，window被放在了SceneDelegate.h中，而一些生命周期跑到了SceneDelegate.m中

#import "SceneDelegate.h"

@interface SceneDelegate ()

@end

@implementation SceneDelegate

- (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).

}

- (void)sceneDidDisconnect:(UIScene *)scene {

// Called as the scene is being released by the system.

// This occurs shortly after the scene enters the background, or when its session is discarded.

// Release any resources associated with this scene that can be re-created the next time the scene connects.

// The scene may re-connect later, as its session was not neccessarily discarded (see `application:didDiscardSceneSessions` instead).

}

- (void)sceneDidBecomeActive:(UIScene *)scene {

// Called when the scene has moved from an inactive state to an active state.

// Use this method to restart any tasks that were paused (or not yet started) when the scene was inactive.

}

- (void)sceneWillResignActive:(UIScene *)scene {

// Called when the scene will move from an active state to an inactive state.

// This may occur due to temporary interruptions (ex. an incoming phone call).

}

- (void)sceneWillEnterForeground:(UIScene *)scene {

// Called as the scene transitions from the background to the foreground.

// Use this method to undo the changes made on entering the background.

}

- (void)sceneDidEnterBackground:(UIScene *)scene {

// Called as the scene transitions from the foreground to the background.

// Use this method to save data, release shared resources, and store enough scene-specific state information

// to restore the scene back to its current state.

}

@end

查了资料，看了看Apple的文档才知道，原来生命周期与window跑到了UIScene去管理了，它的出现是为了iPadOS的多进程准备的(可以同时开启多个相同App)，现在的逻辑是用Appdelegate管理单个或多个UIScene，一个UIScene管理单个或多个UIWindow。