ATPDecorator.h

#import <Foundation/Foundation.h>
#import "ATPInfo.h"


@interface ATPDecorator : NSObject <ATPInfo>

@property (nonatomic, strong, readonly) NSString* fullName;

- (id)initWithInfo:(NSObject<ATPInfo>*)info;

@end

ATPDecorator.m

#import "ATPDecorator.h"


@interface ATPDecorator ()

@property (nonatomic, strong) NSObject<ATPInfo>* info;

@end


@implementation ATPDecorator

@dynamic firstName;
@dynamic lastName;

#pragma mark - Initialization

- (id)initWithInfo:(NSObject<ATPInfo>*)info
{
	if (self = [super init]) {
		_info = info;
	}
	
	return self;
}

#pragma mark - Properties

+ (NSSet*)keyPathsForValuesAffectingFullName
{
	return [NSSet setWithObjects:@"info.firstName", @"info.lastName", nil];
}

- (NSString*)fullName
{
	return [NSString stringWithFormat:@"%@ %@", self.info.firstName, self.info.lastName];
}

#pragma mark - Forwarding Methods

- (id)forwardingTargetForSelector:(SEL)selector
{
	return [self.info respondsToSelector:selector] ? self.info : self;
}

- (BOOL)respondsToSelector:(SEL)selector
{
	return [super respondsToSelector:selector] || [self.info respondsToSelector:selector];
}

- (BOOL)isKindOfClass:(Class)class
{
	return [super isKindOfClass:class] || [self.info isKindOfClass:class];
}

- (void)addObserver:(NSObject*)observer forKeyPath:(NSString*)keyPath options:(NSKeyValueObservingOptions)options context:(void*)context
{
	if ([self respondsToKeyPath:keyPath]) {
		[super addObserver:observer forKeyPath:keyPath options:options context:context];
	} else {
		[self.info addObserver:observer forKeyPath:keyPath options:options context:context];
	}
}

- (void)removeObserver:(NSObject*)observer forKeyPath:(NSString*)keyPath
{
	if ([self respondsToKeyPath:keyPath]) {
		[super removeObserver:observer forKeyPath:keyPath];
	} else {
		[self.info removeObserver:observer forKeyPath:keyPath];
	}
}

- (void)removeObserver:(NSObject*)observer forKeyPath:(NSString*)keyPath context:(void*)context
{
	if ([self respondsToKeyPath:keyPath]) {
		[super removeObserver:observer forKeyPath:keyPath context:context];
	} else {
		[self.info removeObserver:observer forKeyPath:keyPath context:context];
	}
}

- (BOOL)respondsToKeyPath:(NSString*)keyPath
{
	NSArray* keys = [keyPath componentsSeparatedByString:@"."];
	NSString* firstKey = keys[0];
	
	if (![super respondsToSelector:NSSelectorFromString(firstKey)]) {
		return NO;
	}
	
	NSUInteger keyCount = [keys count];
	
	if (keyCount > 1) {
		id currentObject = [self valueForKey:firstKey];
		
		for (NSUInteger index = 1; index < keyCount; index++) {
			NSString* currentKey = keys[index];
			
			if (![currentObject respondsToSelector:NSSelectorFromString(currentKey)]) {
				return NO;
			}
			
			currentObject = [currentObject valueForKey:currentKey];
		}
	}
	
	return YES;
}

@end

ATPInfo.h

#import <Foundation/Foundation.h>


@protocol ATPInfo <NSObject>

@property (nonatomic, strong) NSString* firstName;
@property (nonatomic, strong) NSString* lastName;

@end


@interface ATPInfo : NSObject <ATPInfo>

@end

ATPInfo.m

#import "ATPInfo.h"


@implementation ATPInfo

@synthesize firstName = _firstName;
@synthesize lastName = _lastName;

@end

Error

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Cannot update for observer <ATPAppDelegate 0x8a1f5f0> for the key path "fullName.length" from <ATPDecorator 0x8a53f70>, most likely because the value for the key "fullName" has changed without an appropriate KVO notification being sent. Check the KVO-compliance of the ATPDecorator class.'

Testing

ATPInfo* info = [ATPInfo new];
info.firstName = @"David";
info.lastName = @"Hart";
	
ATPDecorator* decorator = [[ATPDecorator alloc] initWithInfo:info];
[decorator addObserver:self forKeyPath:@"firstName" options:NSKeyValueObservingOptionNew context:kKVOContext];
[decorator addObserver:self forKeyPath:@"firstName.length" options:NSKeyValueObservingOptionNew context:kKVOContext];
[decorator addObserver:self forKeyPath:@"fullName" options:NSKeyValueObservingOptionNew context:kKVOContext];
[decorator addObserver:self forKeyPath:@"fullName.length" options:NSKeyValueObservingOptionNew context:kKVOContext];
	
info.firstName = @"Stephen";
decorator.firstName = @"Charlotte";