Building Cocoa Applications: A Step by Step Guide

Adding the Unary Minus Function to the Controller Class

We want the unary minus function (the button with the “+/-” on it) to change the sign of the number currently displayed in our Calculator’s numeric display area. One way to implement this function is to handle it with another case in the switch statement in the enterOp: method — we could give the “+/-” key its own tag and have the enterOp: method intercept it and perform the appropriate function. The problem with this approach is that the unary minus function has little in common with the other arithmetic functions: it takes one argument instead of two, and it operates immediately on the displayed value. A far better way to implement this function is to implement a new action method in the Controller class.

Using IB’s Read Files Commandwith a New Action Method

Adding new action methods to existing classes is slightly more difficult than creating the initial class definition. Early versions of IB simply replaced the existing class files (Controller.h and Controller.m, here) with new versions, wiping out any source code that you might have added. Current versions of IB detect that you have made changes in the class files and allow you to merge the changes using the Merge Files application. Unfortunately, this can be a painful and error-prone process. The safest way to add new outlets and actions to files after they have been created and edited is to add these items directly to the Objective-C interface files in a text editor, ...