Delegation and Core Location(Chapter 4 of iOS Programming: The Big Nerd Ranch Guide)

Delegation is an object-oriented approach to callbacks. A callback is a function that is supplied in advance of an event and is called every time the event occurs. Some objects need to make a callback for more than one event. For instance, the location manager wants to “callback” when it finds a new location and when it encounters an error.

However, there is no built-in way for two (or more) callback functions to coordinate and share information. This is the problem addressed by delegation – we supply a single delegate to receive all of the event messages for a particular object. This delegate object can then store, manipulate, act on, and relay the related information as it sees fit.

Before sending an optional method, the object first asks its delegate by sending another message, respondsToSelector:. Every object implements this method, which checks at runtime whether an object implements a given method. 

To avoid retain cycles, delegate properties use the assign attribute instead of retain or copy. We call this a “weak reference,” where an object has a pointer to another object but does not retain it.