The progress object returned by the class.
- iOS 9.0+
- macOS 10.11+
- UIKit for Mac 13.0+Beta
- tvOS 9.0+
- watchOS 2.0+
The progress object is usually setup at class initialization time and updated as work is completed. The
progress property is set only once. If another progress object is needed the caller should create a new instance of the custom class to represent the work.
progress property is only set once.