KVO和KVC的關係

KVC 與 KVO 是 Objective C 的關鍵概念，我的認爲必須理解的東西，下面是實例講解。

Key-Value Coding (KVC)

KVC，便是指 NSKeyValueCoding，一個非正式的 Protocol，提供一種機制來間接訪問對象的屬性。KVO 就是基於 KVC 實現的關鍵技術之一。

一個對象擁有某些屬性。好比說，一個 Person 對象有一個 name 和一個 address 屬性。以 KVC 說法，Person 對象分別有一個 value 對應他的 name 和 address 的 key。 key 只是一個字符串，它對應的值能夠是任意類型的對象。從最基礎的層次上看，KVC 有兩個方法：一個是設置 key 的值，另外一個是獲取 key 的值。以下面的例子：

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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);   }

如今，若是 Person 有另一個 key 配偶（spouse），spouse 的 key 值是另外一個 Person 對象，用 KVC 能夠這樣寫：

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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 與 key pat 要區分開來，key 能夠從一個對象中獲取值，而 key path 能夠將多個 key 用點號 「.」 分割鏈接起來，好比：

[p valueForKeyPath:@ "spouse.name" ];

至關於這樣……

[[p valueForKey:@ "spouse" ] valueForKey:@ "name" ];

好了，以上是 KVC 的基本知識，接着看看 KVO。

Key-Value Observing (KVO)

Key-Value Observing (KVO) 創建在 KVC 之上，它可以觀察一個對象的 KVC key path 值的變化。舉個例子，用代碼觀察一個 person 對象的 address 變化，如下是實現的三個方法：

• watchPersonForChangeOfAddress: 實現觀察

• observeValueForKeyPath:ofObject:change:context: 在被觀察的 key path 的值變化時調用。

• dealloc 中止觀察

 

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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

這就是 KVO 的做用，它經過 key path 觀察對象的值，當值發生變化的時候會收到通知。