#import <Foundation/Foundation.h>

@interface Character : NSObject
@property (nonatomic, strong) NSString *characterName;
@property (nonatomic, strong) NSNumber *ownedClowCards;
- (void)hasLostClowCard;
- (void)hasGainedClowCard;
@end

@implementation Character

@synthesize ownedClowCards = _ownedClowCards;

- (NSNumber *)ownedClowCards
{
    NSLog(@"Priv getter");
    return _ownedClowCards;
}

- (void)setOwnedClowCards:(NSNumber *)newValue
{
    NSLog(@"Priv setter");
    _ownedClowCards = newValue;
}

- (void)hasLostClowCard
{
    NSLog(@"%@ has lost a card! Cards now: %@", self.characterName, self.ownedClowCards);
}

- (void)hasGainedClowCard
{
    NSLog(@"%@ has earned a card! Cards now: %@", self.characterName, self.ownedClowCards);
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog(@"Observer stuff working");

    if ([keyPath isEqualToString:@"ownedClowCards"]) {
        if (change[NSKeyValueChangeNewKey] < change [NSKeyValueChangeOldKey]) {
            [self hasLostClowCard];
        } else {
            [self hasGainedClowCard];
        }
    }
}

@end


int main()
{
    @autoreleasepool {

        Character *sakura;
        Character *shaoran;

        //Create and give the properties some values with KVC...

        sakura = [[Character alloc] init];
        sakura.characterName = @"Sakura Kinomoto";
        sakura.ownedClowCards = @20;

        shaoran = [[Character alloc] init];
        shaoran.characterName = @"Li Shaoran";
        shaoran.ownedClowCards = @21;

        NSLog(@"%@ has %@ Clow Cards", sakura.characterName, sakura.ownedClowCards);
        NSLog(@"%@ has %@ Clow Cards", shaoran.characterName, shaoran.ownedClowCards);

        [sakura addObserver:sakura
                 forKeyPath:@"ownedClowCards"
                    options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
                    context:nil];

        NSLog(@"------------  setter  ----------");
        sakura.ownedClowCards = @22;
        NSLog(@"------------ setValue ----------");
        [sakura setValue:[NSNumber numberWithInt:25] forKey:@"ownedClowCards"];

        [sakura removeObserver:sakura forKeyPath:@"ownedClowCards"];
    }
    return 0;
}









$ clang -framework Foundation main.m && ./a.out
2013-11-28 14:54:08.387 a.out[76538:507] Priv setter
2013-11-28 14:54:08.389 a.out[76538:507] Priv setter
2013-11-28 14:54:08.390 a.out[76538:507] Priv getter
2013-11-28 14:54:08.390 a.out[76538:507] Sakura Kinomoto has 20 Clow Cards
2013-11-28 14:54:08.391 a.out[76538:507] Priv getter
2013-11-28 14:54:08.391 a.out[76538:507] Li Shaoran has 21 Clow Cards
2013-11-28 14:54:08.392 a.out[76538:507] ------------  setter  ----------
2013-11-28 14:54:08.393 a.out[76538:507] Priv getter
2013-11-28 14:54:08.393 a.out[76538:507] Priv setter
2013-11-28 14:54:08.394 a.out[76538:507] Priv getter
2013-11-28 14:54:08.394 a.out[76538:507] Observer stuff working
2013-11-28 14:54:08.394 a.out[76538:507] Priv getter
2013-11-28 14:54:08.395 a.out[76538:507] Sakura Kinomoto has earned a card! Cards now: 22
2013-11-28 14:54:08.395 a.out[76538:507] ------------ setValue ----------
2013-11-28 14:54:08.396 a.out[76538:507] Priv getter
2013-11-28 14:54:08.396 a.out[76538:507] Priv setter
2013-11-28 14:54:08.397 a.out[76538:507] Priv getter
2013-11-28 14:54:08.397 a.out[76538:507] Observer stuff working
2013-11-28 14:54:08.398 a.out[76538:507] Priv getter
2013-11-28 14:54:08.398 a.out[76538:507] Sakura Kinomoto has earned a card! Cards now: 25