Categories

When programmer wants to add some more functionality to the class, typically extend the class. But this is not a right way everywhere, so like ruby Objective-C also provides categories to achieve this. Categories allows programmer to add functionality to already existing classes without extending them.

In the example given below we have a class BaseClass that has some methods and the second class SubClass that is used to add a method to the BaseClass. In the main, we have created object of base class and use the method defined in the sub class.

Example:
This is code of primary class.

#import<Foundation/NSObject.h>

@interface BaseClass : NSObject {
    int num1, num2;
  }
  -(void)set :(int) x and: (int) y;
  -(int)add;
  -(int)sub;
@end

This is code of sub class that is used to add method in the primary class.

#import"BaseClass.h"

@interface BaseClass(Category)
  -(void)show:(int)x;
@end

#import"SubClass.h"

@implementation BaseClass(BaseClass)

  -(void)show:(int)x {
      printf("Result is : %d \n",x);
  }
@end

main.m

#import"BaseClass.m"
#import"SubClass.m"
#import<stdio.h>

int main(){
   BaseClass *obj = [[BaseClass alloc] init];
   [obj set:10 and:8];
   [obj show:[obj add]];
   [obj show:[obj sub]];

   [obj release];
   return 0;
}

Output:

Result is : 18 
Result is : 2 

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