Objective-C Categories

Sometimes, you may find that you wish to extend an existing class by adding behavior that is useful only in certain situations. In order add such extension to existing classes, Objective-C provides categories and extensions.

If you need to add a method to an existing class, perhaps, to add functionapty to make it easier to do something in your own apppcation, the easiest way is to use a category.

The syntax to declare a category uses the @interface keyword, just pke a standard Objective-C class description, but does not indicate any inheritance from a subclass. Instead, it specifies the name of the category in parentheses, pke this −

@interface ClassName (CategoryName)

@end

Characteristics of Category

A category can be declared for any class, even if you don t have the original implementation source code.

Any methods that you declare in a category will be available to all instances of the original class, as well as any subclasses of the original class.

At runtime, there s no difference between a method added by a category and one that is implemented by the original class.

Now, let s look at a sample category implementation. Let s add a category to the Cocoa class NSString. This category will make it possible for us to add a new method getCopyRightString which helps us in returning the copyright string. It is shown below.

#import <Foundation/Foundation.h>

@interface NSString(MyAdditions)
+(NSString *)getCopyRightString;
@end

@implementation NSString(MyAdditions)

+(NSString *)getCopyRightString {
   return @"Copyright TutorialsPoint.com 2013";
}

@end

int main(int argc, const char * argv[]) {
   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
   NSString *copyrightString = [NSString getCopyRightString];
   NSLog(@"Accessing Category: %@",copyrightString);
   
   [pool drain];
   return 0;
}

2013-09-22 21:19:12.125 Categories[340:303] Accessing Category: Copyright TutorialsPoint.com 2013