[转]KVC 与 KVO 理解自己动手实现KVO

本文主要是介绍[转]KVC 与 KVO 理解自己动手实现KVO，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

文章转自：http://magicalboy.com/kvc_and_kvo/

KVC 与 KVO 是 Objective C 的关键概念，个人认为必须理解的东西，下面是实例讲解。

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) methodNSString *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 aboveNSString *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"];

好了，以上是 KVC 的基本知识，接着看看 KVO。

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：

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:selfforKeyPath:@"address"options:0context: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 *)keyPathofObject:(id)objectchange:(NSDictionary *)changecontext:(void *)context
{// use the context to make sure this is a change in the address,// because we may also be observing other thingsif(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 crashesfor(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