For the More Curious: Pros and Cons of Callback Options

A callback, as you may remember from Chapter 4, is a chunk of code you supply in advance of an event occurring. When that event goes down, the chunk of code gets executed. While blocks have many more uses, you have seen in this chapter that they can be used as another approach to callbacks. Other approaches to callbacks you have seen are delegation, target-action pairs, and notifications. Each one has benefits and drawbacks compared to the others. This section will expand on these benefits and drawbacks so that you can pick the appropriate one for your own implementations.

First, let’s note that each of these approaches to callbacks are design patterns that transcend their implementations in Cocoa Touch. For example, the target-action pair design pattern is implemented by UIControl, but this does not mean you have to use UIControl to use the design pattern. You could create your own class that kept a pointer to a target object and a SEL for the message that object would be sent when some event occurred.