Cocoa Programming for OS X: The Big Nerd Ranch Guide

NSNotification

Notification objects are very simple. A notification is like an envelope into which the poster places information for the observers. A notification has two important properties: name and object.

 ​ ​ ​ ​v​a​r​ ​n​a​m​e​:​ ​S​t​r​i​n​g​
 ​ ​ ​ ​v​a​r​ ​o​b​j​e​c​t​:​ ​A​n​y​O​b​j​e​c​t​?​

Nearly always, object is a reference to the object that posted the notification. (It is analogous to a return address.)