http://magicalboy.com/kvc_and_kvo/

KVC 與 KVO 是 Objective C 的關鍵概念，個人認為必須理解的東西，下面是實例講解。

Key-Value Coding (KVC)

KVC，即是指?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 就是基于 KVC 實現的關鍵技術之一。

一個對象擁有某些屬性。比如說，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，它對應的值可以是任意類型的對象。從最基礎的層次上看，KVC 有兩個方法：一個是設置 key 的值，另一個是獲取 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); }

現在，如果 Person 有另外一個 key 配偶（spouse），spouse 的 key 值是另一個 Person 對象，用 KVC 可以這樣寫：

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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 可以從一個對象中獲取值，而 key path 可以將多個 key 用點號 “.” 分割連接起來，比如：

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

相當于這樣……

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

好了，以上是 KVC 的基本知識，接著看看 KVO。

Key-Value Observing (KVO)

Key-Value Observing (KVO) 建立在 KVC 之上，它能夠觀察一個對象的 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，以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 停止觀察

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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 的作用，它通過 key path 觀察對象的值，當值發生變化的時候會收到通知。

轉載于:https://www.cnblogs.com/hl666/p/3655266.html

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