The number of bytes that the task has received from the server in the response body.
- iOS 7.0+
- macOS 10.9+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
To be notified when this value changes, implement the
URLSession: delegate method (for data and upload tasks) or the
URLSession: method (for download tasks).