+keyPathsForValuesAffectingValueForKey: vs +keyPathsForValuesAffecting<Key>.

gistfile1.m

//
//  main.m
//  KVO
//
//  Created by Damien DeVille on 10/26/13.
//  Copyright (c) 2013 Damien DeVille. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface Animal : NSObject

- (NSString *)identity;

@property (copy, nonatomic) NSString *name;

@end

@implementation Animal

- (NSString *)identity
{
	return [NSString stringWithFormat:@"My name is %@.", [self name]];
}

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{
	NSMutableSet *keyPaths = [NSMutableSet setWithSet:[super keyPathsForValuesAffectingValueForKey:key]];
	
	if ([key isEqualToString:@"identity"]) {
		[keyPaths addObject:@"name"];
	}
	
	return keyPaths;
}

@end

#pragma mark -

@interface Monkey : Animal

@property (assign, nonatomic) NSUInteger age;

@end

@implementation Monkey

- (NSString *)identity
{
	return [[super identity] stringByAppendingFormat:@" I am %lu.", [self age]];
}

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{
	NSMutableSet *keyPaths = [NSMutableSet setWithSet:[super keyPathsForValuesAffectingValueForKey:key]];
	
	if ([key isEqualToString:@"identity"]) {
		[keyPaths addObject:@"age"];
	}
	
	return keyPaths;
}

@end

#pragma mark -

@interface Animal (Funky)

@property (readonly, copy, nonatomic) NSString *funkyName;

@end

@implementation Animal (Funky)

- (NSString *)funkyName
{
	NSString *name = [self name];
	if (name == nil) {
		return nil;
	}
	return [@"Funky " stringByAppendingString:name];
}

+ (NSSet *)keyPathsForValuesAffectingFunkyName
{
	return [NSSet setWithObjects:@"name", nil];
}

@end

#pragma mark -

@interface Observer : NSObject

@end

@implementation Observer

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
	// nop
}

@end

#pragma mark -

int main(int argc, const char **argv)
{
	Monkey *monkey = [[Monkey alloc] init];
	
	Observer *observer = [[Observer alloc] init];
	[monkey addObserver:observer forKeyPath:@"identity" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:NULL];
	[monkey addObserver:observer forKeyPath:@"funkyName" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:NULL];
	
	[monkey setName:@"Bryan"];
	[monkey setAge:12];
	
	return 0;
}