The system wide output volume set by the user.
- iOS 6.0+
- tvOS 9.0+
- watchOS 2.0+
A value in the range
0 representing the minimum volume and
1 representing the maximum volume.
The system wide output volume can be set directly only by the user; to provide volume control in your app, use the
You can observe changes to the value of this property by using Key-value observing.