The number of bytes that the task expects to send in the request body.
- iOS 7.0+
- macOS 10.9+
- tvOS 9.0+
- watchOS 2.0+
The URL loading system can determine the length of the upload data in three ways:
From the length of the data object provided as the upload body.
From the length of the file on disk provided as the upload body of an upload task (not a download task).
Content-Lengthin the request object, if you explicitly set it.
Otherwise, the value is
-1) if you provided a stream or body data object, or zero (
0) if you did not.