ios13多窗口（UIWindowScene）学习笔记

ios13引入了UIWindowScene类、UIWindowSceneDelegate协议以便支持多窗口功能，但其适用于ipad，不适用于iphone，因为iphone不支持多窗口功能。注意，这里说的窗口不是UIWindow，而是UIWindowScene。

ios13前后的app的UI架构变化

ios13之前的app的UI架构
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ios13及以后的app的UI架构

由上图可知，UIWindowScene和UIWindow是一对多的关系。

下图所示的是UIWindowScene的定义，它用于管理1个或者多个UIWindow。
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第一个app demo

如下面两张图所示，当我们在xcode11及以后的xcode版本中创建一个ios app工程时，xcode会自动为我们创建“一个app运行起来所需”的文件。当创建一个新的app工程时，相比于xcode10及其之前的版本，xcode11之后会自动为我们新增3处内容（如下图的红框所示）：①SceneDelegate类；②info.plist里面的Application Scene Manifest配置选项；③AppDelegate类里面的

application:configurationForConnectingSceneSession:options:方法和
application:didDiscardSceneSessions:方法。

SceneDelegate类

我们先来看看SceneDelegate类里面的内容，如下代码所示。SceneDelegate有个window属性，说明ios13之后的window由SceneDelegate管理。当看到方法里面的“DidBecomeActive”、“WillResignActive”、“WillEnterForeground”、“DidEnterBackground”时，我们便可以猜测出这是app进入和退出前后台时的回调。

#import <UIKit/UIKit.h>
@interface SceneDelegate : UIResponder <UIWindowSceneDelegate>@property (strong, nonatomic) UIWindow * window;@end@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 necessarily 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

info.plist里面的Application Scene Manifest配置选项

如下第1张图所示，当我们给SceneDelegate类的方法添加断点，然后运行app时，发现断点生效了。那么，SceneDelegate类是在哪初始化的呢？如下第2张图所示，如果你在工程中搜索“SceneDelegate”关键字，就会发现并没有任何代码来创建SceneDelegate实例，此时你可以看到info.plist文件里面有关于SceneDelegate的配置项。我们便可以猜测出SceneDelegate实例是系统默认帮我们创建的。
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第1张图

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第2张图

app demo运行

当我们在ipad上运行前面刚刚创建的app时，结果如下图所示，发现屏幕的顶部中间多了个3点按钮（下图的红框所示）。你可以点击3点按钮来使用里面的功能（请看下面的录屏）。
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录屏demo链接

UIApplication、UIApplicationDelegate、UIWindowScene、UIWindowSceneDelegate、UISceneSession的职责

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xcode11版本之后的xcode创建的ios app如何适配ios13以下的系统

在AppDelegate.h中添加window属性。

#import <UIKit/UIKit.h>@interface AppDelegate : UIResponder <UIApplicationDelegate>@property (strong, nonatomic) UIWindow * window;@end

在AppDelegate.m中初始化ios13系统以下的情况下的window。

#import "AppDelegate.h"
#import "ViewController.h"@implementation AppDelegate- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {if (@available(iOS 13.0, *)) {} else {//适配ios13以下的系统self.window = [[UIWindow alloc] initWithFrame:UIScreen.mainScreen.bounds];ViewController *vc = [ViewController new];self.window.rootViewController = vc;[self.window makeKeyAndVisible];}return YES;
}#pragma mark - UISceneSession lifecycle- (UISceneConfiguration *)application:(UIApplication *)application configurationForConnectingSceneSession:(UISceneSession *)connectingSceneSession options:(UISceneConnectionOptions *)options {return [[UISceneConfiguration alloc] initWithName:@"Default Configuration" sessionRole:connectingSceneSession.role];
}- (void)application:(UIApplication *)application didDiscardSceneSessions:(NSSet<UISceneSession *> *)sceneSessions {
}@end

在SceneDelegate.m里面初始化window。

#import "SceneDelegate.h"
#import "ViewController.h"@implementation SceneDelegate- (void)scene:(UIScene *)scene willConnectToSession:(UISceneSession *)session options:(UISceneConnectionOptions *)connectionOptions {if (@available(iOS 13.0, *)) {UIWindowScene *windowScene = (UIWindowScene *)scene;self.window = [[UIWindow alloc] initWithFrame:UIScreen.mainScreen.bounds];[self.window setWindowScene:windowScene];ViewController *vc = [ViewController new];UINavigationController *nvc = [[UINavigationController alloc] initWithRootViewController:vc];self.window.rootViewController = nvc;[self.window makeKeyAndVisible];}
}@end