Objective-C Programming: The Big Nerd Ranch Guide

37 Categories

Categories let a programmer add methods to any existing class. For example, Apple gave us the class NSString. Apple does not share the source code to that class, but you can use a category to add new methods to it.

Create a new Foundation Command Line Tool called VowelCounter. Then create a new file that is an Objective-C category. Name the category BNRVowelCounting and make it a category on NSString.

Now open NSString+BNRVowelCounting.h and declare a method that you want to add to the NSString class:

#import <Foundation/Foundation.h>

@interface NSString (BNRVowelCounting)
- (int)bnr_vowelCount;

@end

Now implement the method in NSString+BNRVowelCount.m:

#import "NSString+BNRVowelCounting.h" @implementation ...