Raises an exception.
- iOS 2.0+
- macOS 10.4+
- tvOS 9.0+
- watchOS 2.0+
The object to remove as an observer.
A key-path, relative to the set, for which
observeris registered to receive KVO change notifications. This value must not be
NSSet objects are not observable, so this method raises an exception when invoked on an
NSSet object. Instead of observing a set, observe the unordered to-many relationship for which the set is the collection of related objects.