iOS 在AppDelegate中添加UIWindow

iOS13以后AppDelegate.h文件中没有了window属性，而是在SceneDelegate.h中，
iOS13中appdelegate的职责发现了改变：

iOS13之前，Appdelegate的职责全权处理App生命周期和UI生命周期；
iOS13之后，Appdelegate的职责是：
1、处理 App 生命周期
2、新的 Scene Session 生命周期

iOS13之后，Appdelegate不在负责UI生命周期，所有UI生命周期交给SceneDelegate处理。

在AppDelegate中添加window的方法：

1，在AppDelegate.h中添加

@property (nonatomic, strong) UIWindow *window;

2，在AppDelegate.m中添加

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

    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    ViewController *vc = [[ViewController alloc] init];
    self.window.rootViewController = vc;
    [self.window makeKeyAndVisible];

    return YES;
}

3，删除AppDelegate.m中以下代码

#pragma mark - UISceneSession lifecycle


- (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];
}


- (void)application:(UIApplication *)application didDiscardSceneSessions:(NSSet<UISceneSession *> *)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.
}

4，删除Info.plist中以下参数 (Main storyboard file base name, Application Scene Manifest)

<key>UISceneConfigurations</key>
        <dict>
            <key>UIWindowSceneSessionRoleApplication</key>
            <array>
                <dict>
                    <key>UISceneConfigurationName</key>
                    <string>Default Configuration</string>
                    <key>UISceneDelegateClassName</key>
                    <string>SceneDelegate</string>
                    <key>UISceneStoryboardFile</key>
                    <string>Main</string>
                </dict>
            </array>
        </dict>
<key>UIMainStoryboardFile</key>
    <string>Main</string>

注意：window需要设置根视图，
ViewController上的view需要设置背景色，不然是黑色的。