RACSignal：RACSignal代码未执行

是否有一个愿意帮助我解决这个问题的灵魂？

这是我的情况：

我需要将表单附件上传到服务器。所以这是步骤：

1. 使用Web服务从服务器检索附件标头。

2. 将附件标题与本地附件匹配。

3. 将附件上传到服务器。

问题：

使用RACSignal，我可以成功获取附件标头，但是当需要使用NSArray上传附件时，UploadFormItemAttachments方法的内部RACSignal将不会执行。

附加信息：

这是显示如何处理RACSignal的代码段。BatchSignal永远不会执行！：

[[[self getFormItemAttachmentHeaders:listName
                          topListItemID:form.topListItemID
                                  form:form
     ] map:^id(NSMutableArray* value) {
        NSArray* attachmentHeaders = [value copy];

        // the code of uploadFormItemAttachments is called but the inner signal does not execute. Why?
        return [self uploadFormItemAttachments:pendingAttachments
                                        attachmentHeaders:attachmentHeaders
                                                     form:form];
        }
      ] subscribeNext:^(id value) {
          // I was expecting the completion result form uploadFormItemAttachments here.
        }
     ];

该方法遍历附件数组并将其上传到服务器：

- (RACSignal *)uploadFormItemAttachments:(NSArray*)pendingAttachments attachmentHeaders:(NSArray*)attachmentHeaders form:(SEFSManagedForm*)form
{

  RACSignal* batchSignal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
    [attachmentHeaders enumerateObjectsUsingBlock:^(SEFSFormItemAttachmentHeader* attachmentHeader, NSUInteger idx, BOOL *stop)
     {
       // Look for the local attachment using attachment header from server

       NSPredicate* predicate = [NSPredicate predicateWithFormat:@"identifier = %@", attachmentHeader.document];
       NSArray* foundAttachment = [pendingAttachments filteredArrayUsingPredicate:predicate];

       SEFSManagedAttachment* fullAttachment = foundAttachment[0];

       RACSignal* uploadFormItemAttachmentSignal = [[self uploadFormItemAttachment:fullAttachment
                                                                  attachmentHeader:attachmentHeader                                                            ] map:^id(NSNumber* value) {
         NSMutableArray* valuesArray = [NSMutableArray array];
         [valuesArray addObject:value];
         [valuesArray addObject:attachmentHeader.document];
         RACTuple* tuple = [RACTuple tupleWithObjectsFromArray:valuesArray
                                            convertNullsToNils:YES];
         return tuple;
       }];


       [subscriber sendNext:uploadFormItemAttachmentSignal];
     }];

    [subscriber sendCompleted];
    return nil;
  }];

  return [batchSignal flatten:2];
}