Extending Objects in Cocoa with Categories‎

Cocoa categories provide a great way to modify existing classes with additional functionality. Here is a good example of extensions in action:

NSDate+Accessors.h

//
//  NSDate+Accessors.h
//  Illustrate
//
//  Created by Kevin Sylvestre on 10-03-24.
//  Copyright 2010 Kevin Sylvestre. All rights reserved.
//

@interface NSDate (Accessors)

@property (nonatomic, retain, readonly) NSString *distance;

@end

NSDate+Accessors.m


//
//  NSDate+Accessors.m
//  Illustrate
//
//  Created by Kevin Sylvestre on 10-03-24.
//  Copyright 2010 Kevin Sylvestre. All rights reserved.
//

#import "NSDate+Accessors.h"

@implementation NSDate (Accessors)

# define SINGULAR (1)

- (NSString *)distance
{
  // Load interval.
  NSTimeInterval interval = [self timeIntervalSinceNow];

  // Calculate interval.
  NSInteger seconds = round(abs(interval));
  NSInteger minutes = round(abs(interval) / 60);
  NSInteger hours   = round(abs(interval) / 3600);
  NSInteger days    = round(abs(interval) / 86400);
  NSInteger weeks   = round(abs(interval) / 604800);
  NSInteger months  = round(abs(interval) / 2629743);
  NSInteger years   = round(abs(interval) / 31556926);

  // Calculate direction.
  NSString *direction;
  if (interval > 0) direction = @"until";
  if (interval < 0) direction = @"ago";

  // Create unit and amount.
  NSString *unit = nil;
  NSInteger amount;

  // Set unit and amount.
  if (interval == 0);
  else if (years)   { amount = years;   unit = amount == SINGULAR ? @"year"   : @"years";   }
  else if (months)  { amount = months;  unit = amount == SINGULAR ? @"month"  : @"months";  }
  else if (weeks)   { amount = weeks;   unit = amount == SINGULAR ? @"week"   : @"weeks";   }
  else if (days)    { amount = days;    unit = amount == SINGULAR ? @"day"    : @"days";    }
  else if (hours)   { amount = hours;   unit = amount == SINGULAR ? @"hour"   : @"hours";   }
  else if (minutes) { amount = minutes; unit = amount == SINGULAR ? @"minute" : @"minutes"; }
  else if (seconds) { amount = seconds; unit = amount == SINGULAR ? @"second" : @"seconds"; }
  else return @"now";

  // Format string and return.
  return [NSString stringWithFormat:@"%i %@ %@", amount, unit, direction];
}

@end

Accessing extensions is simple: just import the header file. In the case above, easily readable dates are now at the finger tips!