#import <Foundation/Foundation.h>@interface Child : NSObject@property (nonatomic,assign) int age;-(id) initWithAge:(int) age;@end

#import "Child.h"@implementation Child-(id) initWithAge:(int) age{    self=[super init];    if(self!=nil){        _age=age;    }    return self;}@end

#import <Foundation/Foundation.h>@class Child;@interface Nurse : NSObject@property Child *child;-(id)initWithChild:(Child *) child;-(void) observeChild;-(void) removeObserver;@end

#import "Nurse.h"#import "Child.h"@implementation Nurse#import "Child.h"-(id)initWithChild:(Child *) child{    self=[super init];    if(self!=nil){        _child=child;//         [_child addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:@"孩子长大一岁了"];    }    return self;}-(void) observeChild{    [_child addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];        NSLog(@"观察者创建好了");}-(void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{    NSLog(@"孩子现在的年龄：%@",[change objectForKey:@"new"]);}-(void) removeObserver{[_child removeObserver:self forKeyPath:@"age"];     NSLog(@"观察者移除掉了");}@end

    //kvo--观察者        Child *child=[[Child alloc] initWithAge:23];        Nurse *nurse=[[Nurse alloc] initWithChild:child];        [nurse observeChild];        [child setAge:24];        [child setAge:25];        [nurse removeObserver];