KVC 与 KVO

1、Key-Value Coding (KVC)
KVC，即是指 NSKeyValueCoding，一个非正式的 Protocol，提供一种机制来间接访问对象的属性。KVO 就是基于 KVC 实现的关键技术之一。

一个对象拥有某些属性。比如说，一个 Person 对象有一个 name 和一个 address 属性。以 KVC 说法，Person 对象分别有一个 value 对应他的 name 和 address 的 key。 key 只是一个字符串，它对应的值可以是任意类型的对象。从最基础的层次上看，KVC 有两个方法：一个是设置 key 的值，另一个是获取 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);

}

现在，如果 Person 有另外一个 key 配偶（spouse），spouse 的 key 值是另一个 Person 对象，用 KVC 可以这样写：

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 与 key path 要区分开来，key 可以从一个对象中获取值，而 key path 可以将多个 key 用点号 "." 分割连接起来，比如：
[p valueForKeyPath:@"spouse.name"];

相当于这样……
[[p valueForKey:@"spouse"] valueForKey:@"name"];

2、Key-Value Observing (KVO)
Key-Value Observing (KVO) 建立在 KVC 之上，它能够观察一个对象的 KVC key path 值的变化。举个例子，用代码观察一个 person 对象的 address 变化，以下是实现的三个方法：

①、watchPersonForChangeOfAddress: 实现观察
②、observeValueForKeyPath:ofObject:change:context: 在被观察的 key path 的值变化时调用。
③、dealloc 停止观察
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

这就是 KVO 的作用，它通过 key path 观察对象的值，当值发生变化的时候会收到通知。