NSURLSessionDownloadTask - 下载但以错误结束

I am trying to download a pdf file. earlier when i used the completion handler block i was able to see the file in the tmp location. I then wanted to show download progress so i implemented the delegate methods. But i can now see the progress bar working and the file being downloaded. but once the download is complete (bytes written/total bytes = 1) the error delegate is called and there is no file in the tmp location. what am i missing ? below is my code. I have uploaded the project at I am trying to download a pdf file. earlier whe