前言

xcode11创建项目新增SceneDelegate文件，AppDelegate文件结构也发生变化，在AppDelegate.h文件中没有了window属性，而是在SceneDelegate.h中，可见AppDelegate不管理window而是交给SceneDelegate

AppDelegate和SceneDelegate这是iPadOS带来的新的多窗口支持的结果，并且有效地将应用程序委托的工作分成两部分

变化
iOS13之前：在app状态发生改变时，UIKit 框架会发送通知到UIApplicationDelegate 对象上；

iOS13及其以后，在app状态发生改变时，UIKit 会优先发送通知到 SceneDelegate对象上；

• UIApplicationDelegate对象上只剩下三个函数：

加载完成：didFinishLaunchingWithOptions
配置将要连接的场景：configurationForConnectingSceneSession
丢弃场景对象之后：didDiscardSceneSessions

• SceneDelegate接管了所有的 app 过渡函数和场景的连接：

将要连接场景：willConnectToSession
已连接场景：sceneDidDisconnect
场景已经处于活动状态：sceneDidBecomeActive
场景将要处于非活动状态：sceneWillResignActive
场景将要进入前台：sceneWillEnterForeground
场景已经进入后台：sceneDidEnterBackground

注意
iOS13 之后不代表一定有分屏，也不代表 iPhone 可以使用分屏功能，当前仍然只有 iPad 可以使用分屏功能；
开启分屏需要勾选Supports multiple windows并进行配置和支持，iPad 开发在此不讨论，所以不会展开分屏功能的开发细节；

适配
方式一
如果我们不开发iPadOS多窗口APP，SceneDelegate窗口管理我们可以不需要直接删掉就好了

1. 删除掉info.plist中Application Scene Manifest选项
2. 注释AppDelegate中相关Scene方法

- (UISceneConfiguration *)application:(UIApplication *)application configurationForConnectingSceneSession:(UISceneSession *)connectingSceneSession options:(UISceneConnectionOptions *)options

- (void)application:(UIApplication *)application didDiscardSceneSessions:(NSSet<UISceneSession *> *)sceneSessions

3. 在AppDelegate 中添加window 属性，并在didFinishLaunchingWithOptions中创建window

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    self.window.rootViewController = [ViewController new];
    [self.window makeKeyAndVisible];
    return YES;
}

4.Appdelegate中增加APP状态的回调方法

-(void)applicationWillResignActive:(UIApplication *)application{
    NSLog(@"applicationWillResignActive");
}

- (void)applicationDidBecomeActive:(UIApplication *)application{
    NSLog(@"applicationDidBecomeActive");
}

- (void)applicationWillEnterForeground:(UIApplication *)application {
    NSLog(@"applicationWillEnterForeground");
}

- (void)applicationDidEnterBackground:(UIApplication *)application {
    NSLog(@"applicationDidEnterBackground");
}

5. SceneDelegate中的方法注释掉，或删除SceneDelegate文件

方式二
即要用iOS 13中新的SceneDelegate，又可以在iOS 13一下的设备中完美运行。那就添加版本判断，利用@available和__IPHONE_OS_VERSION_MIN_REQUIRED

1. Appdelegate didFinishLaunchingWithOptions中判断版本

    if (@available(iOS 13.0, *)) {
        
    }else{
        self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
        self.window.rootViewController = [ViewController new];
        [self.window makeKeyAndVisible];
    }
    return YES;

2.Appdelegate Scene相关方法适配

#if __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_13_0
- (UISceneConfiguration *)application:(UIApplication *)application configurationForConnectingSceneSession:(UISceneSession *)connectingSceneSession options:(UISceneConnectionOptions *)options  API_AVAILABLE(ios(13.0)){
    if (@available(iOS 13.0, *)) {
        return [[UISceneConfiguration alloc] initWithName:@"Default Configuration" sessionRole:connectingSceneSession.role];
    } else {
        return nil;
        // Fallback on earlier versions
    }
}

- (void)application:(UIApplication *)application didDiscardSceneSessions:(NSSet<UISceneSession *> *)sceneSessions  API_AVAILABLE(ios(13.0)){

}
#endif

3. SceneDelegate willConnectToSession适配

    if (@available(iOS 13.0, *)) {
        
    }else{
        UIWindowScene *windowScene = (UIWindowScene *)scene;
        self.window = [[UIWindow alloc] initWithWindowScene:windowScene];
        self.window.frame = windowScene.coordinateSpace.bounds;
        self.window.rootViewController = [ViewController new];
        [self.window makeKeyWindow];
        [self.window makeKeyAndVisible];
    }

参考文章
https://blog.csdn.net/weixin_44060108/article/details/103629392
https://blog.csdn.net/qq_36724920/article/details/103887518