【iOS】设置 rootViewController

iOS 开发中，rootViewController 经常用到，示例代码如下：

self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
self.window.backgroundColor = [UIColor whiteColor];
    
// 设置相应的 ViewController
ViewController *vc = [[ViewController alloc] init];
self.window.rootViewController = vc;

[self.window makeKeyAndVisible];