KVO

  self.stu=[[Student alloc]init];

    [self.stu setValue:@"小羊羔" forKey:@"name"];

    [self.stu addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld  context:nil];

    [self performSelector:@selector(toChangeValue) withObject:nil afterDelay:2.0];

-(void)toChangeValue

{

    [self.stu setValue:@"鱼儿飞" forKey:@"name"];

}

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context

{

    NSLog(@"%@",change);//改变的数据(包含该改变前的和改变后的数据)

    if([keyPath isEqualToString:@"name"])

    {

        NSLog(@"%@",[self.stu valueForKey:@"name"]);

    }

}