ios13以上SceneDelegate介绍及swiftUI应用_uiscenedelegate

那么，这个多出的类是做什么的呢？

简单来说，是ios13之后AppDelegate这个类的职责发生了改变！
在ios之前，Appdelegate的职责全权处理App生命周期和UI生命周期，如图：

而在ios13之后，Appdelegate的职责发生了相应的改变！

   1、处理 App 生命周期
   2、新的 Scene Session 生命周期
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而UI的生命周期就交给新增的SceneDelegate来处理，如图！

这样的话，我们在Xcode11新建iOS项目时,如果你跟往常一样在Appdelegate的- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions方法中创建根控制器，就会崩溃报错！那解决办法有2个！
1，把原先在didFinishLaunchingWithOptions写代码移到SceneDelegate的willConnectTo方法中，如图：

2，如果您所用到的应用程序并不需要这个功能，可以相应的删除以下代码，这样就跟xcode11之前的版本一样啦～


但就目前看来，适配SceneDelegate还是很有必要的，比如用到的swiftUI，小组件等功能，都用到了SceneDelegate的类，所以我们还是未雨绸缪的先了解一下～
我们先看一下在appdelegate做了什么

//会返回一个创建场景时需要的UISceneConfiguration对象
  func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
    print("__7__configurationForConnecting")
    // 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)
  }

  //当用户通过“应用切换器”关闭一个或多个场景时会被调用
  func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
    print("__8__didDiscardSceneSessions")
    // 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.
  }
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在以下图片中，当需要设置多场景的时候，mutiple windows属性应设置为Yes，在application session role中的configuration name应与代码中UIsceneConfiguration的配置名字相同，而delegate class name应与所在的类名相同！

好啦～再来看一下sceneDelegate中的代码

//当场景添加到app中时，函数会被调用，这里是配置场景的最理想地方，相当于application的didFinishLaunchingWithOptions！（一个delegate配置所有场景）
  func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    print("__1__willConnectTo")
    // 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 _ = (scene as? UIWindowScene) else { return }

    // Create the SwiftUI view that provides the window contents.
       let contentView = ContentView()
//    // Use a UIHostingController as window root view controller.
    if let windowScene = scene as? UIWindowScene {
        let window = UIWindow(windowScene: windowScene)
        window.rootViewController = UIHostingController(rootView: contentView)
        self.window = window
        window.makeKeyAndVisible()
    }
  }

  //当场景与app断开连接是调用（注意，以后它可能被重新连接）
  func sceneDidDisconnect(_ scene: UIScene) {
    print("__2__sceneDidDisconnect")
    // 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 necessarily discarded (see `application:didDiscardSceneSessions` instead).
  }

  //当用户开始与场景进行交互（例如从应用切换器中选择场景）时，会调用
  func sceneDidBecomeActive(_ scene: UIScene) {
    print("__3__sceneDidBecomeActive")
    // 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.
  }

  //当用户停止与场景交互（例如通过切换器切换到另一个场景）时调用
  func sceneWillResignActive(_ scene: UIScene) {
    print("__4__sceneWillResignActive")
    // 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).
  }

  //当场景变成活动窗口时调用，即从后台状态变成开始或恢复状态
  func sceneWillEnterForeground(_ scene: UIScene) {
    print("__5__sceneWillEnterForeground")
    // Called as the scene transitions from the background to the foreground.
    // Use this method to undo the changes made on entering the background.
  }

  //当场景进入后台时调用，即该应用已最小化但仍存活在后台中
  func sceneDidEnterBackground(_ scene: UIScene) {
    print("__6__sceneDidEnterBackground")
    // 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.
  }
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注：如果适配了SceneDelegate，在ios13以下的application的判断app状态的方法失效

官网支持多场景：
https://developer.apple.com/documentation/uikit/uiscenedelegate/supporting_multiple_windows_on_ipad

参考文章：
1，https://blog.csdn.net/potato512/article/details/106542809
2，https://www.jianshu.com/p/6d6573fbd60b