Ejemplo KVO

gistfile1.m

//Vamos a observar la propiedad cartUnits del objeto _product
[_product addObserver:self forKeyPath:@"cartUnits" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
 
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if([keyPath isEqualToString:@"cartUnits"])
    {
        //Acción
    }
}