[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(contextChanged:) name:NSManagedObjectContextWillSaveNotification object:nil];

-(void)contextChanged:(NSNotification *)notification {
    for(NSManagedObject *object in [(NSManagedObjectContext *)notification.object updatedObjects]) {
        if([object isKindOfClass:[Product class]]) {
            id value = [object changedValues][@"value"];
            if (value){
                NSLog(@"%@",object);
            }
        }
    }
}