KVC 與 KVO 是 Objective C 的關鍵概念,個人認爲必須理解的東西,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 就是基於 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) 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。