title: Notification Center vs. Delegation in iOS SDK
body: <p>Why did Apple choose to use delegation for some communication amongst SDK objects and to post notifications to Notification Center for others?</p>
  
  <p>In particular, I'm thinking about the set of keyboard appearance notifications that come from UIWindow. </p>
  
  <p>Is it because the Notification Center system means that more than one object can use the keyboard appearance action as a trigger to change its state, whereas only one object would have been able to act with a delegate implementation?</p>