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：
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void changeName(Person *p, NSString *newName){ // using the KVC accessor (getter) method NSString *originalName = [p valueForKey:@"name"]; // using the KVC accessor (setter) method. [p setValue:newName forKey:@"name"]; NSLog(@"Changed %@'s name to: %@", originalName, newName);} |
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：
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void logMarriage(Person *p){ // just using the accessor again, same as example above NSString *personsName = [p valueForKey:@"name"]; // this line is different, because it is using // a "key path" instead of a normal "key" NSString *spousesName = [p valueForKeyPath:@"spouse.name"]; NSLog(@"%@ is happily married to %@", personsName, spousesName);} |
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：
[p valueForKeyPath:@"spouse.name"]; |
相当于这样……
[[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
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static NSString *const KVO_CONTEXT_ADDRESS_CHANGED = @"KVO_CONTEXT_ADDRESS_CHANGED"@implementation PersonWatcher-(void) watchPersonForChangeOfAddress:(Person *)p{ // this begins the observing [p addObserver:self forKeyPath:@"address" options:0 context:KVO_CONTEXT_ADDRESS_CHANGED]; // keep a record of all the people being observed, // because we need to stop observing them in dealloc [m_observedPeople addObject:p];}// whenever an observed key path changes, this method will be called- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{ // use the context to make sure this is a change in the address, // because we may also be observing other things if(context == KVO_CONTEXT_ADDRESS_CHANGED) { NSString *name = [object valueForKey:@"name"]; NSString *address = [object valueForKey:@"address"]; NSLog(@"%@ has a new address: %@", name, address); }}-(void) dealloc;{ // must stop observing everything before this object is // deallocated, otherwise it will cause crashes for(Person *p in m_observedPeople){ [p removeObserver:self forKeyPath:@"address"]; } [m_observedPeople release]; m_observedPeople = nil; [super dealloc];}-(id) init;{ if(self = [super init]){ m_observedPeople = [NSMutableArray new]; } return self;}@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。