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：

? 1 2 3 4 5 6 7 8 9 10 11 12 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：

? 1 2 3 4 5 6 7 8 9 10 11 12 13 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

 

? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 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。