The length, in bytes, of the data requested.
- iOS 7.0+
- macOS 10.9+
- Mac Catalyst 13.0+Beta
- tvOS 9.0+
If the content length of the resource is unknown, the sum of the
requested properties may be greater than the actual content length. When this situation occurs, an application must attempt to provide as much of the requested data beginning at the
requested property as the resource contains. The application must then invoke either the
finish method upon success, or the
finish method if an error is encountered during the loading.