KVC / KVO

main.m

//
//  main.m
//  KVO
//
//  Created by Christopher Loessl on 7/14/13.
//  Copyright (c) 2013 Christopher Loessl. All rights reserved.
//

/*
 * The code is based on a tutorial from:
 * http://www.andyibanez.com/74-key-value-coding-key-value-observing-and-cocoa-bindings-a-basic-tutorial
 * but since it uses old Objective C style I updated it.
 * If you don't understand the code directly it might be a good idea to
 * have a look there since the explaination of how KVO and KVC works is good.
 */

/*
$ clang -framework Foundation main.m -o main
$ ./main
2013-07-14 23:13:15.189 main[26164:707] Sakura Kinomoto has 20 Clow Cards
2013-07-14 23:13:15.192 main[26164:707] Li Shaoran has 21 Clow Cards
2013-07-14 23:13:15.193 main[26164:707] Sakura Kinomoto has earned a card! Cards now: 22
*/

#import <Foundation/Foundation.h>

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

@implementation Character

- (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);
}

//---------------------------------------------------------------------
// method that get's executed if one of the observing values changes
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    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);


        //---------------------------------------------------------------------
        // Here begins the KVO section.

        // register KVO
        // Sakura is observing her own Clow Cards.
        [sakura addObserver:sakura
                 forKeyPath:@"ownedClowCards"
                    options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
                    context:nil];

        // observing value changes so
        // observeValueForKeyPath:ofObject:change:context: gets executed
        sakura.ownedClowCards = @22;

        // unregister KVO
        [sakura removeObserver:sakura forKeyPath:@"ownedClowCards"];

    }
    return 0;
}