KVC 与 KVO 理解

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。