NSArrayController and +keyPathsForValuesAffectingValueForKey:

gistfile1.m

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

#import <Cocoa/Cocoa.h>

@interface Shop : NSObject

@property (readonly, strong, nonatomic) NSArray *products;

- (void)addProduct:(NSString *)product;
- (void)removeProduct:(NSString *)product;

@end

@interface Shop (/* Private */)

@property (strong, nonatomic) NSArrayController *productsController;
@property (readwrite, strong, nonatomic) NSArray *products;

@end

@implementation Shop

static NSString * _ShopProductsControllerArrangedObjectsObservationContext = @"_ShopProductsControllerArrangedObjectsObservationContext";

- (id)init
{
	self = [super init];
	if (self == nil) {
		return nil;
	}
	
	NSMutableArray *products = [NSMutableArray arrayWithObjects:@"Cheetah", @"Puma", @"Jaguar", @"Panther", @"Tiger", @"Leopard", @"Snow Leopard", @"Lion", @"Mountain Lion", nil];
	_productsController = [[NSArrayController alloc] initWithContent:products];
	
	[_productsController addObserver:self forKeyPath:@"arrangedObjects" options:(NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:&_ShopProductsControllerArrangedObjectsObservationContext];
	
	return self;
}

- (void)dealloc
{
	[_productsController removeObserver:self forKeyPath:@"arrangedObjects" context:&_ShopProductsControllerArrangedObjectsObservationContext];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
	if (context == &_ShopProductsControllerArrangedObjectsObservationContext) {
		NSArray *products = [NSArray arrayWithArray:[[self productsController] arrangedObjects]];
		[self setProducts:products];
	}
	else {
		[super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
	}
}

- (void)addProduct:(NSString *)product
{
	[[self productsController] addObject:product];
}

- (void)removeProduct:(NSString *)product
{
	[[self productsController] removeObject:product];
}

@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)
{
	Shop *shop = [[Shop alloc] init];
	
	Observer *observer = [[Observer alloc] init];
	[shop addObserver:observer forKeyPath:@"products" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:NULL];
	
	[shop addProduct:@"Mavericks"];
	
	[shop removeObserver:observer forKeyPath:@"products" context:NULL];
	
	return 0;
}