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。
Key-Value Coding (KVC)
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)
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：
- watchPersonForChangeOfAddress: 实现观察
- observeValueForKeyPath:ofObject:change:context: 在被观察的 key path 的值变化时调用。
- dealloc 停止观察
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。
其他例子
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