NSKeyValueObserverRegistration.h

@interface NSObject(NSKeyValueObserverRegistration)

/* Register or deregister as an observer of the value at a key path relative to the receiver. 
   The options determine what is included in observer notifications and when they're sent, as described above, 
   and the context is passed in observer notifications as described above. You should use 
   -removeObserver:forKeyPath:context: instead of -removeObserver:forKeyPath: whenever possible because it allows 
   you to more precisely specify your intent. When the same observer is registered for the same key path multiple times, 
   but with different context pointers each time, -removeObserver:forKeyPath: has to guess at the context pointer when 
   deciding what exactly to remove, and it can guess wrong.
*/
- (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(nullable void *)context;
- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath context:(nullable void *)context API_AVAILABLE(macos(10.7), ios(5.0), watchos(2.0), tvos(9.0));
- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;

@end