The notification mechanism of
Cocoa implements one-to-many broadcast of messages based on the
Observer pattern. Objects in a program add themselves or other
objects to a list of observers of one or more notifications, each of
which is identified by a global string (the notification name). The
object that wants to notify other objects—the observed
object—creates a notification object and posts it to a notification
center. The notification center determines the observers of a
particular notification and sends the notification to them via a
message. The methods invoked by the notification message must conform
to a certain single-parameter signature. The parameter of the method
is the notification object, which contains the notification name, the
observed object, and a dictionary containing any supplemental
information.Posting a notification is a synchronous procedure. The
posting object doesn't regain control until the notification center
has broadcast the notification to all observers. For asynchronous
behavior, you can put the notification in a notification queue;
control returns immediately to the posting object and the
notification center broadcasts the notification when it reaches the
top of the queue.Regular notifications—that is, those broadcast by
the notification center—are intraprocess only. If you want to
broadcast notifications to other processes, you can use the
istributed notification center and its related API.
![notification-in-iphone.jpeg]()