Protocols

Chapter 1, “Introducing the iOS SDK,” introduced the notion of delegates. Delegates implement details that cannot be determined when a class is first defined. For example, a table knows how to display rows of cells, but it can’t know what to do when a cell is tapped. The meaning of a tapped row changes with whatever application implements that table. A tap might open another screen, send a message to a web server, or perform any other imaginable result. Delegation lets the table communicate with a smart object that is responsible for handling those taps but whose behavior is written at a completely separate time from when the table class itself is created.

Delegation basically provides a language that mediates contact between an object and its handler. A table tells its delegate “I have been tapped,” “I have scrolled,” and other status messages. The delegate then decides how to respond to these messages, producing updates based on its particular application semantics.