KVO(Key Value Observing)

*KVO可以监听某个对象属性的改变
原理：只要给一个对象注册一个监听，那么在运行时， 系统就会自动给该对象生成一个子类对象，并且重写自动生成的子类对象的被监听属性的set方法，然后在set方法中通知监听者
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定义一个HSPerson类

////  HSPerson.h//  KVC////  Created by hans on 15/7/13.//  Copyright © 2015年 hans. All rights reserved.//#import <Foundation/Foundation.h>@class HSCar;@interface HSPerson : NSObject{@public    int _age;}/** 名字 */@property (nonatomic, copy) NSString *name;/** 年龄 */@property (nonatomic, assign) int age;@end

在ViewController实现

////  ViewController.m//  KVO////  Created by hans on 15/7/14.//  Copyright © 2015年 hans. All rights reserved.//#import "ViewController.h"#import "HSPerson.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    HSPerson *person = [HSPerson new];    person.name = @"hans";    person.age = 23;    /*     第一个参数: 告诉系统哪个对象监听     第二个参数: 监听当前对象的哪个属性     第三个参数: 监听到属性改变之后，传递什么值     第四个参数: 需要传递的参数 (这个参数不是传递给属性的)     */    [person addObserver:self forKeyPath:@"name" options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew context:nil];    person.name =@"汉斯哈哈哈";//    输出：//    2015-07-14 08:24:18.697 KVO[55716:952095] keyPath = name, object = <HSPerson: 0x7fdd00d2dbc0> , change = {//        kind = 1;//        new = "\U6c49\U65af\U54c8\U54c8\U54c8";//        old = hans;//    }, context = (null)    // 注意: KVO只能监听通过set方法修改的值    // 不会被监听到    person->_age = 100;    // 注意: 如果使用KVO监听某个对象的属性, 当对象释放之前一定要移除监听    //  reason: 'An instance 0x7f9483516610 of class Person was deallocated while key value observers were still registered with it.    // 从person对象上移除self对它的age属性的监听    [person removeObserver:self forKeyPath:@"name"];}// 只要监听到属性的改变就会调用// keyPath: 被监听的属性名称// object : 被监听的对象// context: 注册监听的时候传入的值- (void)observeValueForKeyPath:(nullable NSString *)keyPath ofObject:(nullable id)object change:(nullable NSDictionary *)change context:(nullable void *)context{    NSLog(@"keyPath = %@, object = %@ , change = %@, context = %@", keyPath, object, change, context);}- (void)didReceiveMemoryWarning {    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}@end