Informs the observing object when the value at the specified key path relative to the observed object has changed.
- iOS 2.0+
- macOS 10.3+
- tvOS 9.0+
- watchOS 2.0+
The key path, relative to
object, to the value that has changed.
The source object of the key path
A dictionary that describes the changes that have been made to the value of the property at the key path
object. Entries are described in Change Dictionary Keys.
The value that was provided when the observer was registered to receive key-value observation notifications.
object to begin sending change notification messages for the value at
key, you send it an
add message, naming the observing object that should receive the messages. When you are done observing, and in particular before the observing object is deallocated, you send the observed object a
remove message to unregister the observer, and stop sending change notification messages.