OC文法之KVC與KVO，ockvckvo

OC文法之KVC與KVO，ockvckvo

1、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：一個是設定 key 的值，另一個是擷取 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 可以從一個對象中擷取值，而 key path 可以將多個 key 用點號 “.” 分割串連起來，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。