http://www.verydemo.com/demo_c134_i5320.html1、什么是KVO?Key-Value-Observing(KVO，键值观察)：当指定的对象的属性被修改了，允许对象接受到通知的机制。每次指定的被观察对象的属性被修改的时候，KVO都会自动的去通知相应的观察者。2、KVO有什么优点？当有属性改变，KVO会提供自动的消息通知。3、KVO实现1）注册观察者（Registering as an Observer），指定被观察的属性。addObserver:forKeyPath:options:context:2）接收变化通知（Receiving Notification of a Change），实现回调方法。observeValueForKeyPath:ofObject:change:context:3）移除观察者（Removing an Object as an Observer）removeObserver:forKeyPath:4、简单示例1）创建一个简单的SingleView模板的iPhone应用，我取名为KVODemo。2）在界面添加一个label和一个button，并生成相应的outlet。下面ViewConroller.h和ViewConroller.mViewConroller.h源码清单：#import@interface ViewController : UIViewController@property (retain,nonatomic) IBOutlet UILabel *label;- (IBAction)buttonPressed:(UIButton *)sender;@endViewConroller.m源码清单：#import "ViewController.h"@interface ViewController (){ NSString *_str;}@property (nonatomic,retain) NSString *str;@end@implementation ViewController@synthesize label=_label;@synthesize str=_str;- (void)viewDidLoad{ [super viewDidLoad];// Do any additional setup after loading the view, typically from a nib. _str =@"sharon"; _label.text = _str; // 添加Observer 这样被注册的属性发生改变的时候Observer回调方法就被调用 [selfaddObserver: selfforKeyPath: @"str"options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];}- (IBAction)buttonPressed:(UIButton *)sender { static BOOL flag =YES; if (flag) { // 修改正在监听的属性值 [selfsetValue: @"joe"forKey: @"str"]; }else { // 修改正在监听的属性值 [selfsetValue: @"sharon"forKey: @"str"]; } flag = !flag; }// 实现Observer的回调方法- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context { // 如果改变的属性是"name" if ([keyPath isEqualToString:@"str"]) { _label.text = [self valueForKey:@"str"]; // 输出改变前的值 NSLog(@"old str is %@",[changeobjectForKey:@"old"]); // 输出改变后的值 NSLog(@"new str is %@",[changeobjectForKey:@"new"]); }}-(void)dealloc{ // 移除Observer [selfremoveObserver:selfforKeyPath:@"name"]; [super dealloc];}@end