The documentation http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSNotificationCenter_Class/Reference/Reference.html talks of the sender and notification name but where does it mention whom to post this notification to?
You don't post a notification directly to someone. The name of the notification, and sender determine who gets the notification.
Interested objects can subscribe to a notification. When you post a notification, all subscribers who are listening to a notification by that name will get notified. Actually Cocoa notifications can be tweaked at two levels:
The class documentation illustrates this clearly.
Here's a little ASCII table from the docs showing who gets notified depending on what notification name and sender were used when created:
Notification name | Notification sender | Notification set specified -------------------------------------------------------------------- Specified | Specified | Notifications with a particular name from a specific sender. Specified | Unspecified | Notifications with a particular name by any sender. Unspecified | Specified | Notifications posted by a specific sender. Unspecified | Unspecified | All notifications.
Unspecified means a nil value was supplied for that field.
Notifications allows for a loosely coupled design as objects are not tied together in their implementations and can work independently off each other.