Notifications provide a mechanism for objects that have no other way to communicate with each other. The model used is a multicast model, in which client objects register themselves with a notification center to be notified in response to a certain event, which is encapsulated in an NSNotification object. NSNotification represents both notifications to the notification center, and the notifications that are sent out to the observers of a particular notification.

Clients generally interact with notifications as receivers; that is, they don't create notifications, but extract key bits of information out of received notifications. To obtain information about a notification we use the three methods name, userInfo, and object, which return the notification name, userInfo dictionary, and the notification's associated object, respectively.

@interface NSNotification : NSObject <NSCoding, NSCopying>
// Convenience Constructors
+ (id)notificationWithName:(NSString *)aName object:(id)anObject;
+ (id)notificationWithName:(NSString *)aName object:(id)anObject userInfo:(NSDictionary *)aUserInfo;
// Instance Methods
- (NSString *)name;
- (id)object;
- (NSDictionary *)userInfo;
// Methods Implementing NSCoding
- (void)encodeWithCoder:(NSCoder *)aCoder;
- (id)initWithCoder:(NSCoder *)aDecoder;
// Methods Implementing NSCopying
- (id)copyWithZone:(NSZone *)zone;