Ios SVProgressHUD在NSOperationQueue内显示警报

我有一个注册按钮，通过单击此按钮，我将向NSOperationQueue添加注册操作，但我的问题是，如果用户名字段为空，则不会出现警报。有人建议我向NSOperationQueue添加与注册相关的任务，方法是否正确

- (IBAction)Register:(UIButton *)sender {

  NSOperationQueue *queue = [NSOperationQueue new];
  NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self
                                                                    selector:@selector(loadDataWithOperation)
                                                                      object:nil];
 [queue addOperation:operation];

}

- (void) loadDataWithOperation {


    if(username.text.length==0)
    { NSLog(@"loadDataWithOperation");

    [SVProgressHUD showSuccessWithStatus:@"Enter User Name"];
    }

 else

    {
      //cose for registration

    }

}

使用此代码：

if ([username.text isEqualToString:@""])
        {
          UIAlertView *alert [[UIAlertView alloc] initWithTitle:@"Message" message:@"Please fill all the fields." delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];

          [alert show];
        }