Cocoa in a Nutshell

Categories

Inheritance is not the only way to add functionality to a class. With an Objective-C language construct called a category , you can add methods to an existing class, thereby extending its functionality—and the functionality of its subclasses.

A category interface declaration looks like a class interface declaration, with one exception: the category name is listed in parentheses after the class name, and the superclass is not mentioned. For example, if you wanted to add a rot13 method to the NSString class to get the rot13 version of any string, the category interface would be defined as shown in Example 1-11.

Example 1-11. Defining a category interface

#import "NSString.h"

@interface NSString (Obfuscation)
 
- (NSString *)rot13;

@end

The category’s implementation looks like the implementation of a class itself. Example 1-12 shows an interface implementation.

Example 1-12. Implementation of a category

#import "Obfuscation.h"

@implementation NSString (Obfuscation)

- (NSString *)rot13 {
    NSString * rot13string;

    // Perform logic to shift each character by 13 
    return rot13string;
}

@end

Remember that a category can’t declare new instance variables for a class; it can only add methods to an existing class.

Warning

A category is not a substitute for a subclass. You should not redefine methods already in a class or a class’s superclass—add only new methods to the class.

Protocols

Class and category interfaces define the methods that belong to a particular class. However, you might want many ...