kvo深入浅出举例

Strock.h
@interface Strock : NSObject
 {
     NSString *_name;
     float _price;
 }
@end
Strock.m
-(NSString *)description
 {
     return [NSString stringWithFormat:@"name:%@,price:%f",_name,_price];
}
person.m
//回调方法
 -(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
 {
     NSLog(@"keyPath:%@,object:%@,change:%@",keyPath,object,change);
}
viewController.m
   self.person = [[Person alloc]init];
     self.strock = [[Strock alloc]init];
 
     //给属性赋值
     [self.strock setValue:@"baidu" forKey:@"_name"];
     [self.strock setValue:@155 forKey:@"_price"];
 
     //设置person为观察者
     [self.strock addObserver:self.person forKeyPath:@"_price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
 
 }
//拖进来一个点击 事件  当button点击的时候
 - (IBAction)priceClicked:(UIButton *)sender {
     NSInteger price = [[self.strock valueForKey:@"_price"]floatValue];
 
     [self.strock setValue:@(price+1) forKey:@"_price"];
 }
//移除观察者
 -(void)dealloc
 {
     [self.strock removeObserver:self.person forKeyPath:@"_price" context:nil];
}