KVO

aBook = [[Book alloc] initWithBookNO:@"NO.21342413" bookName:@"清朝秘史" author:@"张家州" price:25.50];
[aBook addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:nil];

// 观察到变化（收到来自被观察者的消息通知）
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if([keyPath isEqual:@"price"]) {
        NSLog(@">>>>>>>price is changed");
        NSLog(@"old price is %@",[change objectForKey:@"old"]);
        NSLog(@"new price is %@",[change objectForKey:@"new"]);
    }
}

[aBook removeObserver:self forKeyPath:@"price"];