iOS 怎么设置 UITabBarController 的第n个item为第一响应者?

UITabBarController 里面有个属性：selectedIndex

@property(nonatomic) NSUInteger selectedIndex;

 UITabBarController *VC = [[UITabBarController alloc]init];

 VC.selectedIndex = n-1;

[self.navigationController pushViewController:VC animated:YES];