The systemwide output volume set by the user.
- iOS 6.0+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
This property returns a value in the range
0 representing the minimum volume, and
1 representing the maximum volume.
Only the user can directly set the system volume. Provide a volume control in your app, using
MPVolume, to provide the interface to adjust the system volume.
You can observe changes to the value of this property by using Key-value observing.