title: (NSNotificationCenter) How to add observers in different classes?
body: <p>Can someone explain how to use notification center if I want to add observers in different classes? For example: post a notification in classA. Then, add two observers, one in classB and another one in classC, both waiting for the same notification.</p>
  
  <p>I understand that I can use NSNotificationCenter to send and receive notifications like this. What do I need to add to each class in order to accomplish this?</p>