Xcode 11 新建工程中的SceneDelegate

Xcode 11 建新工程默认会创建通过 UIScene 管理多个 UIWindow 的应用，工程中除了 AppDelegate 外还会有一个 SceneDelegate，这是为了实现iPadOS支持多窗口的结果。AppDelegate.h不再有window属性，window属性被定义在了SceneDelegate.h中，AppDelegate中有新增的关于scene的代理方法，SceneDelegate中也有相应的代理方法。因此，当我们用Xcode11针对不同版本的iOS开发应用时，就要做一些适配。

创建好一个工程后，可以看到相关Xcode开发界面变成了下面这样：

工程配置general

info.plist文件

AppDelegate.h中多了两个默认的代理方法：

#pragma mark - UISceneSession lifecycle

/*
关于Scene的配置已默认在info.plist中进行， 如果已在info.plist中配置过，不用实现该方法也没有关系。如果没有配置就需要实现这个方法并返回一个UISceneConfiguration对象。
配置参数中Application Session Role 是个数组，每一项有三个参数:
Configuration Name:   当前配置的名字;
Delegate Class Name:  与哪个Scene代理对象关联;
StoryBoard name: 这个Scene使用的哪个storyboard。
注意：代理方法中调用的是配置名为Default Configuration的Scene，则系统就会自动去调用SceneDelegate这个类。这样SceneDelegate和AppDelegate产生了关联。
*/
- (UISceneConfiguration *)application:(UIApplication *)application configurationForConnectingSceneSession:(UISceneSession *)connectingSceneSession options:(UISceneConnectionOptions *)options {
    // Called when a new scene session is being created.
    // Use this method to select a configuration to create the new scene with.
    return [[UISceneConfiguration alloc] initWithName:@"Default Configuration" sessionRole:connectingSceneSession.role];
}

// 在分屏中关闭其中一个或多个scene时候回调用
- (void)application:(UIApplication *)application didDiscardSceneSessions:(NSSet *)sceneSessions {
    // 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.
}

SceneDelegate.m中的默认代理方法如下：

- (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.
}

1. 不需要多窗口（multiple windows）

如果需要支持iOS 13 及之前多个版本的iOS，且又不需要多个窗口的功能，可以删除项目info.plist文件中的Application Scene Manifest的配置数据，AppDelegate中关于Scene的代理方法、SceneDelegate的类是否删除都可以。
如果使用纯代码来实现显示界面，需要在AppDelegate.h中手动添加window属性，添加以下代码即可：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  
    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    [self.window setBackgroundColor:[UIColor whiteColor]];
    
    ViewController *con = [[ViewController alloc] init];
    UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:con];
    [self.window setRootViewController:nav];
    [self.window makeKeyAndVisible];
    return YES;
}

2. 支持多窗口

iOS 13项目info.plist中的配置项Application Scene Manifest是针对ipad multiple windows功能推出的。在保留Application Scene Manifest配置项不予删除时（其中，项目是否支持多窗口功能是个可勾选项），原先AppDelegate的生命周期方法不再起作用，需要在SceneDelegate中使用UIScene提供的生命周期方法，并且需要针对ios13需要在Scene中配置和ios13以下在AppDelegate中做两套配置。
下面是纯代码实现界面显示的代码：

//// AppDelegate.m中

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    
    if (@available(iOS 13.0, *)) {

    } else {
        self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
        [self.window setBackgroundColor:[UIColor whiteColor]];
        
        ViewController *con = [[ViewController alloc] init];
        UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:con];
        [self.window setRootViewController:nav];
        [self.window makeKeyAndVisible];
    }
    return YES;
}

//// SceneDelegate.m中

- (void)scene:(UIScene *)scene willConnectToSession:(UISceneSession *)session options:(UISceneConnectionOptions *)connectionOptions {
    
    //在这里手动创建新的window
    if (@available(iOS 13.0, *)) {
        UIWindowScene *windowScene = (UIWindowScene *)scene;
        self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
        [self.window setWindowScene:windowScene];
        [self.window setBackgroundColor:[UIColor whiteColor]];
        
        ViewController *con = [[ViewController alloc] init];
        UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:con];
        [self.window setRootViewController:nav];
        [self.window makeKeyAndVisible];
    }
}

注意：如果不使用storyboard，需要将配置中的storyboard项删除：

general

info.plist