Answer 1

在您的app delegate中实施以下方法。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{    
    [window makeKeyAndVisible];
    if(condition) {
        [window addSubview:[mainViewControllerA view]];
    } else {
        [window addSubview:[mainViewControllerB view]];
    }
}

您可以根据自己的情况选择要加载的视图。

Answer 2

Answer 3