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对KVC和KVO的理解

kvckvo

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：

 
 
 
xxxxxxxxxx
 
 
 
 
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：

 
 
 
x
 
 
 
 
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"];

注意

关于KVC valueForKey:key的调用顺序

|- 先调用相关方法 先后顺序是：

|- getter方法：getKey -> key -> isKey

|- NSArray方法：countOfKey 和 objectInKeyAtIndex

|- 如果没有相关方法：看+(Bool)accessInstanceVariablesDirectly返回值（默认为YES）

|- YES 找成员变量，先后顺序是 _key > _isKey > key > isKey

|- NO 异常 valueForUndefinedKey

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; 
}
​
@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。

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