用sb来推出页面

        UIStoryboard*sb=[UIStoryboard storyboardWithName:@"Main" bundle:nil];
        LoginVC*loginVC=[sb instantiateViewControllerWithIdentifier:@"Login"];
        UINavigationController*loginNav=[[UINavigationController alloc]initWithRootViewController:loginVC];
        [self presentViewController:loginNav animated:NO completion:nil];