在didFinishLaunchingWithOptions中访问ViewController

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // 创建窗口
    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    
    // 创建主视图控制器
    UIViewController *viewController = [[UIViewController alloc] init];
    
    // 设置主视图控制器为窗口的根视图控制器
    self.window.rootViewController = viewController;
    
    // 显示窗口
    [self.window makeKeyAndVisible];
    
    // 在这里可以访问并对主视图控制器进行操作
    
    return YES;
}