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KVC 与 KVO 是 Objective C 的关键概念，个人认为必须理解的东西，下面是实例讲解。
KVC，即是指 NSKeyValueCoding，一个非正式的 Protocol，提供一种机制来间接访问对象的属性。KVO 就是基于 KVC 实现的关键技术之一。
一个对象拥有某些属性。比如说，一个 Person 对象有一个 name 和一个 address 属性。以 KVC 说法，Person 对象分别有一个 value 对应他的 name 和 address 的 key。 key 只是一个字符串，它对应的值可以是任意类型的对象。从最基础的层次上看，KVC 有两个方法：一个是设置 key 的值，另一个是获取 key 的值。如下面的例子：
1 void changeName(Person *p, NSString *newName) 2 { 3 4 // using the KVC accessor (getter) method 5 NSString *originalName = [p valueForKey:@"name"]; 6 7 // using the KVC accessor (setter) method. 8 [p setValue:newName forKey:@"name"]; 9 10 NSLog(@"Changed %@‘s name to: %@", originalName, newName); 11 12 }
现在，如果 Person 有另外一个 key 配偶（spouse），spouse 的 key 值是另一个 Person 对象，用 KVC 可以这样写：
1 void logMarriage(Person *p) 2 { 3 4 // just using the accessor again, same as example above 5 NSString *personsName = [p valueForKey:@"name"]; 6 7 // this line is different, because it is using 8 // a "key path" instead of a normal "key" 9 NSString *spousesName = [p valueForKeyPath:@"spouse.name"]; 10 11 NSLog(@"%@ is happily married to %@", personsName, spousesName); 12 13 }
key 与 key pat 要区分开来，key 可以从一个对象中获取值，而 key path 可以将多个 key 用点号 “.” 分割连接起来，比如：
1 [p valueForKeyPath:@"spouse.name"];
相当于这样……
1 [[p valueForKey:@"spouse"] valueForKey:@"name"];
好了，以上是 KVC 的基本知识，接着看看 KVO。
Key-Value Observing (KVO) 建立在 KVC 之上，它能够观察一个对象的 KVC key path 值的变化。举个例子，用代码观察一个 person 对象的 address 变化，以下是实现的三个方法：
1 static NSString *const KVO_CONTEXT_ADDRESS_CHANGED = @"KVO_CONTEXT_ADDRESS_CHANGED" 2 3 @implementation PersonWatcher 4 5 -(void) watchPersonForChangeOfAddress:(Person *)p 6 { 7 8 // this begins the observing 9 [p addObserver:self 10 forKeyPath:@"address" 11 options:0 12 context:KVO_CONTEXT_ADDRESS_CHANGED]; 13 14 // keep a record of all the people being observed, 15 // because we need to stop observing them in dealloc 16 [m_observedPeople addObject:p]; 17 } 18 19 // whenever an observed key path changes, this method will be called 20 - (void)observeValueForKeyPath:(NSString *)keyPath 21 ofObject:(id)object 22 change:(NSDictionary *)change 23 context:(void *)context 24 25 { 26 // use the context to make sure this is a change in the address, 27 // because we may also be observing other things 28 if(context == KVO_CONTEXT_ADDRESS_CHANGED) { 29 NSString *name = [object valueForKey:@"name"]; 30 NSString *address = [object valueForKey:@"address"]; 31 NSLog(@"%@ has a new address: %@", name, address); 32 } 33 } 34 35 -(void) dealloc; 36 { 37 38 // must stop observing everything before this object is 39 // deallocated, otherwise it will cause crashes 40 for(Person *p in m_observedPeople){ 41 [p removeObserver:self forKeyPath:@"address"]; 42 } 43 44 [m_observedPeople release]; 45 m_observedPeople = nil; 46 47 [super dealloc]; 48 49 } 50 51 -(id) init; 52 { 53 if(self = [super init]){ 54 m_observedPeople = [NSMutableArray new]; 55 } 56 57 return self; 58 } 59 60 @end
这就是 KVO 的作用，它通过 key path 观察对象的值，当值发生变化的时候会收到通知。
假设一个场景,股票的价格显示在当前屏幕上，当股票价格更改的时候，实时显示更新其价格。
1.定义DataModel
1 @interface StockData : NSObject { 2 NSString * stockName; 3 float price; 4 } 5 @end 6 @implementation StockData 7 @end
2.定义此model为Controller的属性，实例化它，监听它的属性，并显示在当前的View里边。
1 - (void)viewDidLoad 2 { 3 [super viewDidLoad]; 4 5 stockForKVO = [[StockData alloc] init]; 6 [stockForKVO setValue:@"searph" forKey:@"stockName"]; 7 [stockForKVO setValue:@"10.0" forKey:@"price"]; 8 [stockForKVO addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:NULL]; 9 10 myLabel = [[UILabel alloc]initWithFrame:CGRectMake(100, 100, 100, 30 )]; 11 myLabel.textColor = [UIColor redColor]; 12 myLabel.text = [stockForKVO valueForKey:@"price"]; 13 [self.view addSubview:myLabel]; 14 15 UIButton * b = [UIButton buttonWithType:UIButtonTypeRoundedRect]; 16 b.frame = CGRectMake(0, 0, 100, 30); 17 [b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside]; 18 [self.view addSubview:b]; 19 20 }
3.当点击button的时候，调用buttonAction方法，修改对象的属性。
1 -(void) buttonAction 2 { 3 [stockForKVO setValue:@"20.0" forKey:@"price"]; 4 }
4. 实现回调方法
1 -(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context 2 { 3 if([keyPath isEqualToString:@"price"]) 4 { 5 myLabel.text =[NSString stringWithFormat:@"%@",[stockForKVO valueForKey:@"price"]] ;6 } 7 }
5.增加观察与取消观察是成对出现的，所以需要在最后的时候，移除观察者
1 - (void)dealloc 2 { 3 [super dealloc]; 4 [stockForKVO removeObserver:self forKeyPath:@"price"]; 5 [stockForKVO release]; 6 }
转载自：http://magicalboy.com/kvc_and_kvo/
http://blog.csdn.net/yuquan0821/article/details/6646400
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原文地址：http://www.cnblogs.com/novia/p/4501333.html
