Key-Value Codin(KVC)
- kvc即是指 NSKeyValueCoding耍目，一個非正式的 Protocol巫击，提供一種機(jī)制來間接訪問對象的屬性背镇。KVO 就是基于 KVC 實(shí)現(xiàn)的關(guān)鍵技術(shù)之一尘应。
- 一個對象擁有某些屬性上渴。比如說，一個Person對象有一個name和一個address屬性.以KVC說法元暴，Person 對象分別有一個 value 對應(yīng)他的 name 和 address 的 key篷扩。 key 只是一個字符串，它對應(yīng)的值可以是任意類型的對象昨寞。從最基礎(chǔ)的層次上看瞻惋，KVC 有兩個方法：一個是設(shè)置 key 的值，另一個是獲取 key 的值援岩。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）掏导，spouse 的 key 值是另一個 Person 對象享怀，用 KVC 可以這樣寫：
```objc
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趟咆，而 key path 可以將多個 key 用點(diǎn)號 “.” 分割連接起來添瓷，比如：
[p valueForKeyPath:@"spouse.name"];
相當(dāng)于這樣……
[[p valueForKey:@"spouse"] valueForKey:@"name"];
好了，以上是 KVC 的基本知識值纱，接著看看 KVO鳞贷。
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 值的變化虐唠。舉個例子搀愧，用代碼觀察一個 person 對象的 address 變化，以下是實(shí)現(xiàn)的三個方法：
- watchPersonForChangeOfAddress: 實(shí)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
-(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];
}
(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;
}
這就是 KVO 的作用疆偿，它通過 key path 觀察對象的值咱筛，當(dāng)值發(fā)生變化的時候會收到通知。
