kvo
kvo(键值观察)
kvo是一个典型的观察者模式，观察者在键值改变时会得到通知。

stockForKVO = [[StockData alloc] init];  
[stockForKVO setValue:@"searph" forKey:@"stockName"];
[stockForKVO setValue:@"10.0" forKey:@"price"];        
[stockForKVO addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:NULL];  
  
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context  
{  
    if([keyPath isEqualToString:@"price"])  
    {  
        myLabel.text = [stockForKVO valueForKey:@"price"];  
    }  
}  


通知中心

通知中心就是以NSNotificationCenter为中心，观察者往Center中注册对某个主题对象的变化感兴趣，主体对象通过NSNotificationCenter进行变化广播，所有的变化和监听行为都向同一个中心进行注册，所有对象的变化也通过同一个中心对外广播。

NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
//注册成为观察者 观察对象是Observer 
[notificationCenter addObserver:self selector:@selector(doAction:) name:@"Observer" object:nil];

NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];

[notificationCenter postNotificationName:@"Observer" object:self userInfo:info];

代理机制，协议与代理
A
@protocol XDelegate <NSObject>
- (void)xx;
@end

@interface X : NSObject
@property (nonatomic, weak) id <XDelegate> delegate;
@end

 if ([self.delegate respondsToSelector:@selector(xx)]) {
    [self.delegate xx];
}

B
@interface Y ()<XDelegate>

- (void)xx{

}