Xcode11之后，我们创建的工程中，会多出SceneDelegate.h 和 SceneDelegate.m文件。

在iOS13之前，App生命周期和UI生命周期是交由AppDelegate来管理的；

在iOS13之后，为了为了iPadOS的多窗口支持，交由AppDelegate和SceneDelegate来管理；

那么问题来了，我们不可能设置工程最低支持iOS13，如果才能保留SceneDelegate并且兼容之前的代码呢？

思路：添加支持版本定义 API_AVAILABLE(ios(13.0))

代码如下：

// 1. 在SceneDelegate.h中更改

#import <UIKit/UIKit.h>

UIKIT_EXTERN API_AVAILABLE(ios(13.0)) @interface SceneDelegate : UIResponder <UIWindowSceneDelegate>

@property (strong, nonatomic) UIWindow * window;

@end

// 2. 在AppDelegate.m中更改
#import "AppDelegate.h"

@interface AppDelegate ()

@end

@implementation AppDelegate


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    return YES;
}

#pragma mark - UISceneSession lifecycle


- (UISceneConfiguration *)application:(UIApplication *)application configurationForConnectingSceneSession:(UISceneSession *)connectingSceneSession options:(UISceneConnectionOptions *)options API_AVAILABLE(ios(13.0)) {
    // 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];
}


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

@end