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(xiàn)的關(guān)鍵技術(shù)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(chǔ)的層次上看，KVC 有兩個方法：一個是設(shè)置 key 的值，另一個是獲取 key 的值。如下面的例子。

 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(xiàn)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：

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(qū)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"];

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(xiàn)的三個方法：
watchPersonForChangeOfAddress: 實現(xiàn)觀察
observeValueForKeyPath:ofObject:change:context: 在被觀察的 key path 的值變化時調(diào)用。
dealloc 停止觀察。

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(fā)生變化的時候會收到通知。

本文摘自：http://magicalboy.com/kvc_and_kvo/