KVC与KVO

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：

相当于这样……

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。