category

1. add new method to existing class

2. can not add new instance variable to existing class

3. if the method defined in category is the same as existing class,

    it will replace original method of existing class

ex:

add category NumberConvenience to NSString

@interface NSString (NumberConvenience) 

- (NSNumber *) lengthAsNumber; 

@end // NumberConvenience 

@implementation NSString (NumberConvenience) 

- (NSNumber *) lengthAsNumber 

{ 

  unsigned int length = [self length]; 

  return ([NSNumber numberWithUnsignedInt: length]); 

} // lengthAsNumber 

@end // NumberConvenience

4. splitting a class's implementation across multiple files or multiple frameworks

ex:

CategoryThing.h

#import  

@interface CategoryThing : NSObject { 

  int thing1; 

  int thing2; 

  int thing3; 

} 

@end // CategoryThing 

@interface CategoryThing (Thing1) 

- (void) setThing1: (int) thing1; 

- (int) thing1; 

@end // CategoryThing (Thing1) 

@interface CategoryThing (Thing2) 

- (void) setThing2: (int) thing2; 

- (int) thing2; 

@end // CategoryThing (Thing2) 

@interface CategoryThing (Thing3) 

- (void) setThing3: (int) thing3; 

- (int) thing3; 

@end // CategoryThing (Thing3) 

categoryThing.m

#import "CategoryThing.h" 

@implementation CategoryThing 

- (NSString *) description 

{ 

  NSString *desc; 

  desc = [NSString stringWithFormat: @"%d %d %d", 

           thing1, thing2, thing3]; 

  return (desc); 

} // description 

@end // CategoryThing 

thing1.m

#import "CategoryThing.h" 

@implementation CategoryThing (Thing1) 

- (void) setThing1: (int) t1 

{ 

  thing1 = t1; 

} // setThing1 

 

- (int) thing1 

{ 

  return (thing1); 

} // thing1 

@end // CategoryThing 

thing2.m & thing3.m are like thing1.m