NSURLSessionDownloadTask cancelByProducingResumeDa

问题:

I use NSURLSessionDownloadTask to download a file from server, it is run in a background session. I want to save resume data to resume the download when the app is terminated. However, the call of cancelByProducingResumeData always returns null. I cannot figure out the issue, cause I used to do a sample of resumable download task successful before. Does anybody get the same issue?

回答1:

Do you know if the request satisfies the criteria outlined in the documentation for the cancelByProducingResumeData method?

A download can be resumed only if the following conditions are met:

• The resource has not changed since you first requested it
• The task is an HTTP or HTTPS GET request
• The server provides either the ETag or Last-Modified header (or both) in its response
• The server supports byte-range requests
• The temporary file hasn’t been deleted by the system in response to disk space pressure