Usage

Assuming you have following model classes

@interface Cart
@property (nonatomic, copy) NSSet *products;
- (void)addProduct:(Product *)product;
@end

@interface Product
@property (nonatomic, copy) NSString *name;
@end

Settings up observer

Cart *cart = ...

NKVOObserver *observer = [[NKVOObserver alloc] initWithObject:cart keyPathes:@[ @"products", @"name" ]]
observer.delegate = self;
[observer startObserving];

Product *product = [Product new];
product.name = @"T-shirt";
[cart addProduct:product];
product.name = @"Red Sweater";

observer:didObserveChange:object: will be called twice:

• when adding product, object is product with name set to "T-shirt"
• when name is changed to "Red Sweater"