Facebook SDK获得了被邀请的朋友数量

Question

我正在使用此代码邀请朋友：

[FBWebDialogs presentRequestsDialogModallyWithSession:]

如何获得被邀请加入应用的朋友数量？像这样：

    [FBWebDialogs presentRequestsDialogModallyWithSession:nil
                                                  message:@"Hello"
                                                    title:@"My app"
                                               parameters:params
                                                  handler:^(FBWebDialogResult result, NSURL *resultURL, NSError *error) {
                                                      if (error) {
//Something bad happened
                                                      } else {
                                                          if (result == FBWebDialogResultDialogNotCompleted) {
//canceled
                                                          } else {
//request sended. How many friends were invited?????
                                                          }
                                                      }}];