# 258-Architecting Your App for Multiple Windows

# Changes to app lifecycle

iOS 13 支持多 window，为了支持多 window，引入 Scene Delegate，将 UI Lifecycle 从 App Delegate 移动到 Scene Delegate。现在 Scene Delegate 负责 UI Lifecycle，App Delegate 负责 Process Lifecycle (App Launched, App Terminated, ...) 和 Session Lifecycle。

iOS 13 以前，App 有一个进程和一个用户界面，而 iOS 13 App 多个用户界面（scenes）共享一个进程。

# Lifecycle

# Configuring new session

创建 scene configurations 有两种方式，一个是在 info.plist 进行配置，一个是编码

# info.plist

现在用 Xcode 11 新建一个工程，默认就是这个方式

UISceneConfigurations

The default configuration details for UIKit to use when creating new scenes. Two keys:

UIWindowSceneSessionRoleApplication

Scenes that you use to display content on the device's main screen and respond to user interactions.

UIWindowSceneSessionRoleExternalDisplay

Scenes that you use to display content on an externally connected display.

# Programming

class AppDelegate: UIResponder, UIApplicationDelegate {
  /// Returns the configuration data for UIKit to use when creating a new scene.
  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.
    let config = UISceneConfiguration()
    config.sceneClass = UIWindowScene.self
    config.delegateClass = SceneDelegate.self
    config.storyboard = UIStoryboard(name: "Main", bundle: nil)
    return config
  }
}

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# Without storyboard

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window: UIWindow?
    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 }
        
        window = UIWindow(windowScene: windowScene)
        let viewController = ...
        window?.rootViewController = viewController
        window?.backgroundColor = UIColor.white
        window?.makeKeyAndVisible()
    }
  }

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# State restoration

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
  // 保存
  func stateRestorationActivity(for scene: UIScene) -> NSUserActivity? {
    let currentActivity = fetchCurrentUserActivity(for: self.window)
    return currentActivity
  }
  // 恢复
  func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options: UIScene.ConnectionOptions)
    if let restorationActivity = session.stateRestorationActivity {
      self.configure(window: window, with: restorationActivity)
  }
}

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# Keeping scenes in sync

为了同步 scenes, 现在不能只更新一个 view controller, 需要更新 model, 然后由 model 更新所有相关的 view controller，可以使用 delegates, notifications, 新增的 Combine Framework等

# Demo

class ChatViewController: UIViewController {
  @objc func didEnterMessage(sender: UITextField) {
    let message = Message(text: sender.text)
    // Update the model
    ChatModelController.shared.add(message: message)
  }
  
  override func viewDidLoad() {
    // 接收通知，更新 UI
    NotificationCenter.default.addObserver(selector: ..., name: .NewMessageNotification)
  }
}

class ChatModelController {
  static let shared = ChatModelController()
  func add(message: Message) {
    saveToDisk(message)
    let event = UpdateEvent.NewMessage(message: message)
    event.post()
  }
}

enum UpdateEvent {
  case NewMessage(message: Message)
  static let NewMessageNotification = Notification.Name(rawValue: "NewMessage")
  func post() {
    // Notify subscribers
    switch self {
      case .NewMessage(message: _):
        NotificationCenter.default.post(name: UpdateEvent.NewMessageNotification, object: self)
    }
  }
}

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# Classes and Protocols

UISceneConfiguration

Information about the scene, scene delegate and storyboard for UIKit to use when creating a particular scene.

UISceneSession

An object containing information about one of your app's scenes.

UIScene -> UIWindowScene

An object that represents one instance of your app's user interface.

UISceneDelegate -> UIWindowSceneDelegate

The core methods you use to respond to life-cycle events occurring within a scene.

UIScene.ConnectionOptions

A data object containing information about the reasons why UIKit created the scene.

UISceneActivationConditions

The set of conditions that define when UIKit activates the current scene.