怎么在一个线程结束后回到主线程？

我用一个for循环上传三张照片（用的是afnetworking框架）。
上传它就开了一个线程，上传成功后后台返回一个filename，把filename放进一个数组
我想等三张照片全都上传结束后，对获取到的三个filename进行操作
要怎么做？

     NSMutableArray *fileNameArr = [[NSMutableArray alloc]init];
     for (int i= 0; i<[self.imgArr count]; i++) {
        NSData *data = [self.imgArr objectAtIndex:i];

        [manager POST:url parameters:nil constructingBodyWithBlock:^(id<AFMultipartFormData> formData){
            [formData appendPartWithFileData:data name:@"pic" fileName:fileName mimeType:@"image/jpeg"];
        }success:^(AFHTTPRequestOperation *operation,id responseObject){
            NSLog(@"success:%@",responseObject);
            NSString *str = [[responseObject objectForKey:@"data"] objectForKey:@"fileName"];
            [fileNameArr addObject:str];
            NSString *ids = [[responseObject objectForKey: @"data"] objectForKey:@"ids"];          
        }
              failure:^(AFHTTPRequestOperation *operation,NSError *error){
                  NSLog(@"%@",error);
              }];
    }

 for (int i= 0; i<[self.imgArr count]; i++) {
    NSData *data = [self.imgArr objectAtIndex:i];

    [manager POST:url parameters:nil constructingBodyWithBlock:^(id<AFMultipartFormData> formData){
        [formData appendPartWithFileData:data name:@"pic" fileName:fileName mimeType:@"image/jpeg"];
    }success:^(AFHTTPRequestOperation *operation,id responseObject){
        NSLog(@"success:%@",responseObject);
        NSString *str = [[responseObject objectForKey:@"data"] objectForKey:@"fileName"];
        [fileNameArr addObject:str];
        NSString *ids = [[responseObject objectForKey: @"data"] objectForKey:@"ids"];        

dispatch_async(dispatch_get_main_queue(), ^{

// do something

})

    }
          failure:^(AFHTTPRequestOperation *operation,NSError *error){
              NSLog(@"%@",error);
          }];
}