对KVC和KVO的理解

`kvc` `kvo`

KVC

KVC是KeyValueCoding的简称，它是一种可以直接通过字符串的名字(key)来访问类属性的机制。而不是通过调用Setter、Getter方法访问。

KVC实例

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为例，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keypath要区分开来，key 可以从一个对象中获取值，而 keyPath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"];

KVO

KVO是KeyValueObserving的简称，它提供一种机制，当指定的对象的属性被修改后，则对象就会接受到通知。简单的说就是每次指定的被观察的对象的属性被修改后，KVO就会自动通知相应的观察者了。
KVO实例

KeyValueObserving(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：
- watchPersonForChangeOfAddress: 实现观察
- observeValueForKeyPath:ofObject:change:context: 在被观察的 key path 的值变化时调用。
- 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;
}