Informs the observed object that the value of a given property has changed.
- iOS 2.0+
- macOS 10.3+
- tvOS 9.0+
- watchOS 2.0+
The name of the property that changed.
Use this method when implementing key-value observer compliance manually to inform the observed object that the value at
key has just changed. Calls to this method are always paired with a matching call to
You rarely need to override this method in subclasses, but if you do, be sure to call