Building Cocoa Applications: A Step by Step Guide

Building the Calculator’s Controller Class

It’s time to start thinking about the Objective-C object that will control our Calculator — that is, respond to button clicks, calculate the values that the user wants, and display the results. By convention, this kind of object, which performs behind-the-scenes work and communicates with the user interface, is called a Controller .

Controllers generally don’t have main event loops; instead, they perform actions in response to events that are received and interpreted by other objects. A good rule of thumb is to place as little code in your Controller as is necessary for it to do its job. If it is possible to create a second Controller that is used only for a clear, particular purpose, do so — the less complicated you make your application’s objects, the easier they are to debug. In addition to controlling the overall flow of the application, Calculator’s Controller will contain the code to perform the basic arithmetic and thus can be thought of as the computational engine or back end (albeit a very simple one) of the application.

Designing the Controller Class

Cocoa doesn’t provide you with a Controller class — it’s up to you to write one for your application. (IB and the AppKit are fabulous, but they can’t do everything for you — at least not yet!)

Before you start coding, it’s a good idea to sit down and think about your problem. What does the Controller have to do? What kind of messages will it need to respond to? What kind of ...