Objective-C code is all about objects sending messages to other objects. This communication usually occurs between two objects, but sometimes a bunch of objects are concerned with one object. They all want to know when this object does something interesting, and it’s not feasible for that object to send messages to every interested object.

Instead, an object can post notifications about what it is doing to a centralized notification center. Interested objects register to receive a message when a particular notification is posted or when a particular object posts. In this chapter, you will learn how to use a notification center to handle notifications. You’ll also learn about the autorotation behavior of UIViewController.