Informs the observed object that the value of a given property is about to change.
- iOS 2.0+
- macOS 10.3+
- tvOS 9.0+
- watchOS 2.0+
The name of the property that will change.
Use this method when implementing key-value observer compliance manually to inform the observed object that the value at
key is about to change.
The change type of this method is
You rarely need to override this method in subclasses, but if you do, be sure to call