一，概述

KVO,即：Key-Value Observing，它提供一种机制，当指定的对象的属性被修改后，则对象就会接受到通知。简单的说就是每次指定的被观察的对象的属性被修改后，KVO就会自动通知相应的观察者了。

定义一个类A

@interface A : NSObject {
int age;
}
@end

2.定义此A为Controller的属性，实例化它，监听它的属性，并显示在当前的View里边

- (void)viewDidLoad
{
[super viewDidLoad];

a = [[A alloc] init];
a.age = 5;
[a addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:NULL];

myLabel = [[UILabel alloc]initWithFrame:CGRectMake(100, 100, 100, 30 )];
myLabel.textColor = [UIColor redColor];
myLabel.text = [NSString StringWithFormat:@"%d",a.age];
[self.view addSubview:myLabel];

UIButton * b = [UIButton buttonWithType:UIButtonTypeRoundedRect];
b.frame = CGRectMake(0, 0, 100, 30);
[b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
[self.view addSubview:b];

}

3.当点击button的时候，调用buttonAction方法，修改对象的属性

-(void) buttonAction
{
a.age += 5；
}

4.实现回调方法

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
if([keyPath isEqualToString:@"age"])
{
[NSString StringWithFormat:@"%d",a.age];
}
}

5.最后当页面退出时需remove观察者