[常用代码]AppDelegate设置主界面

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    
    ViewController *mainVC = [[ViewController alloc] init];
    
    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    self.window.rootViewController = mainVC;
    [self.window makeKeyAndVisible];
    
    return YES;
}