Learning the iOS 4 SDK for JavaScript Programmers

Constant Values

Following close behind the notion of a variable is that of the constant. Both entities reference values, but only a variable’s value can change during its lifetime. Once a constant has a value assigned to it, the value becomes read-only.

JavaScript lets you define a true constant as follows:

const constantName = foreverValue;

The C language offers the identical declaration, although you must also include the data type:

const dataType constantName = foreverValue;

You have an alternative technique available to you in C, and one that is very popular. The technique employs the compiler—via the #define directive—to substitute C values and expressions for occurrences of macro names within a file. It is commonly used for numeric values that permeate a class or project. If you need to change that value during the development of your app, you have to do it only once in the directive. For example, if you design an app that displays lots of moving graphic images on the screen, you might want to keep those images all the same size when they are created. You define the macro names and values like the following:

#define kSpriteHeight 24
#define kSpriteWidth 24

Although these statements can go anywhere in a file, they are traditionally positioned at the top (just after #import directives). Also notice that these macro expressions do not end with a semicolon.

With these macros defined, when you want to employ the values anywhere in the file, insert the macro name. The following statement ...