IOS如何在任意位置拿到当前的导航控制器

代码：

// 取出当前的导航控制器
    UITabBarController *tabBarVc = (UITabBarController *)[UIApplication sharedApplication].keyWindow.rootViewController;
    UINavigationController *nav = (UINavigationController *)tabBarVc.selectedViewController;
    [nav pushViewController:loginController animated:YES];