Basic KVC/KVO Implementation

main.m

#import <Foundation/Foundation.h>

static void* theContext = &theContext;

/// KVC/KVO 호환 클래스.
@interface Foo: NSObject
@property (copy, nonatomic) NSString* moo;
-(void)doubleBar;
@end

@implementation Foo

- (NSString *)moo
{
    if(!_moo) {
        _moo = @"hello";
    }
    return _moo;
}

/// 문자열을 두번 반복하도록 확장하는 메소드.
/// 수동으로 KVO 호환이 되도록 한다.
-(void)doubleBar
{
    [self willChangeValueForKey:@"bar"];
    _moo = [NSString
                stringWithFormat:@"%@%@", _moo, _moo];
    [self didChangeValueForKey:@"bar"];
    
}

@end

/// 옵저버 클래스
@interface Bar: NSObject
@end

@implementation Bar
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    if(context == theContext) {
        NSLog(@"the foo's bar has changed from:%@ to:%@",
              [change objectForKey:NSKeyValueChangeOldKey],
              [change objectForKey:NSKeyValueChangeNewKey]
              );
    } else {
        [super observeValueForKeyPath:keyPath
                             ofObject:object
                               change:change
                              context:context
         ];
    }
}
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        
        Foo* foo = [[Foo alloc] init];
        NSLog(@"foo's bar is set for initial time.");
        foo.moo = @"one";
        
        Bar* x = [[Bar alloc] init];
        [foo addObserver:x
            forKeyPath:@"bar"
               options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew
               context:theContext];
        
        foo.moo = @"two";
        [foo doubleBar];
        
        
        
        [foo removeObserver:x forKeyPath:@"bar"];
    }
    return 0;
}