Beginning iOS 5 Application Development

CATEGORIES

A category in Objective-C enables you to add methods to an existing class without the need to subclass it. You can also use a category to override the implementation of an existing class.

NOTE In some languages (such as C#), a category is known as an extension method.

For example, imagine you want to test whether a string contains a valid e-mail address. You can add an isEmail method to the NSString class so that you can call the isEmail method on any NSString instance, like this:

NSString *email = @“[e-mail]”;
if ([email isEmail]) {
    //...
}

To do so, simply create a new class file and code it as follows:

//---Utils.h---
#import <Foundation/Foundation.h>

//---NSString is the class you are extending---
@interface NSString (Utilities)

//---the method you are adding to the NSString class---
-(BOOL) isEmail;

@end

Basically, it looks the same as declaring a new class except that it does not inherit from any other class. The stringUtils is a name that identifies the category you are adding, and you can use any name you want.

Next, you need to implement the method(s) you are adding:

//---Utils.m---
#import “Utils.h”

@implementation NSString (Utilities)

- (BOOL) isEmail {

NSString *emailRegEx =
   @“(?:[a-z0-9!#$%\\&‘*+/=?\\^_`{|}~-]+(?:\\.[a-z0-9!#$%\\&’*+/=?\\^_`{|}”
   @“~-]+)*|\”(?:[\\x01-\\x08\\x0b\\x0c\\x0e-\\x1f\\x21\\x23-\\x5b\\x5d-\\“
   @”x7f]|\\\\[\\x01-\\x09\\x0b\\x0c\\x0e-\\x7f])*\“)@(?:(?:[a-z0-9](?:[a-” ...