创建一个UIViewController，将其view添加到需要显示UIAlertController的view上，然后用UIViewController显示

UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"温馨提示" message:@"设置别名不得超过三个字" preferredStyle:UIAlertControllerStyleAlert];
    UIAlertAction *confirmAction = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:nil];
    [alertController addAction:confirmAction];
    //创建用于显示alertController的UIViewController
    UIViewController *alertVC = [[UIViewController alloc]init];
    [self addSubview:alertVC.view];
    [alertVC presentViewController:alertController animated:YES completion:^{
      //移除用于显示alertController的UIViewController
      [alertVC.view removeFromSuperview];
    }];