Periodically informs the delegate about the download’s progress.
- iOS 7.0+
- macOS 10.9+
- tvOS 9.0+
- watchOS 2.0+
The session containing the download task.
The download task.
The number of bytes transferred since the last time this delegate method was called.
The total number of bytes transferred so far.
The expected length of the file, as provided by the
Content-Lengthheader. If this header was not provided, the value is
Transfer Size Unknown