The correct way to react to changes on observed objects.

gistfile1.m

//
//  main.m
//  KVO
//
//  Created by Damien DeVille on 10/22/13.
//  Copyright (c) 2013 Damien DeVille. All rights reserved.
//

#import <Foundation/Foundation.h>

#define CORRECT_KVO	0

@interface Stalked : NSObject

@property (assign, nonatomic) NSInteger tag;

@end

@implementation Stalked

@end

#pragma mark -

@interface Parent : NSObject
{
 @protected
	Stalked *_stalked;
}

@property (strong, nonatomic) Stalked *stalked;

@end

@implementation Parent

static NSString * _ParentStalkedTagContext = @"_ParentStalkedTagContext";

- (id)init
{
	self = [super init];
	if (self == nil) {
		return nil;
	}
	
	_stalked = [[Stalked alloc] init];
	
	[_stalked addObserver:self forKeyPath:@"tag" options:(NSKeyValueObservingOptions)0 context:&_ParentStalkedTagContext];
	
	return self;
}

- (void)dealloc
{
	[_stalked removeObserver:self forKeyPath:@"tag" context:&_ParentStalkedTagContext];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
#if CORRECT_KVO
	if (context == &_ParentStalkedTagContext) {
		NSLog(@"Parent");
	}
#else
	if ([keyPath isEqualToString:@"tag"]) {
		NSLog(@"Parent");
	}
#endif /* CORRECT_KVO */
	else {
		[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
	}
}

@end

#pragma mark -

@interface Child : Parent

@end

@implementation Child

static NSString * _ChildStalkedTagContext = @"_ChildStalkedTagContext";

- (id)init
{
	self = [super init];
	if (self == nil) {
		return nil;
	}
	
	[_stalked addObserver:self forKeyPath:@"tag" options:(NSKeyValueObservingOptions)0 context:&_ChildStalkedTagContext];
	
	return self;
}

- (void)dealloc
{
	[_stalked removeObserver:self forKeyPath:@"tag" context:&_ChildStalkedTagContext];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
#if CORRECT_KVO
	if (context == &_ChildStalkedTagContext) {
		NSLog(@"Child");
	}
#else
	if ([keyPath isEqualToString:@"tag"]) {
		NSLog(@"Child");
	}
#endif /* CORRECT_KVO */
	else {
		[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
	}
}

@end

#pragma mark -

int main(int argc, const char **argv)
{
	Child *child = [[Child alloc] init];
	
	[[child stalked] setTag:2];
	
	return 0;
}