KVC 與 KVO 是 Objective C 的關鍵概念,個人認爲必須理解的東西,下面是實例講解。
Key-Value Coding (KVC)
KVC,即是指 NSKeyValueCoding,一個非正式的 Protocol,提供一種機制來間接訪問對象的屬性。KVO 就是基於 KVC 實現的關鍵技術之一。
一個對象擁有某些屬性。比如說,一個 Person 對象有一個 name 和一個 address 屬性。以 KVC 說法,Person 對象分別有一個 value 對應他的 name 和 address 的 key。 key 只是一個字符串,它對應的值可以是任意類型的對象。從最基礎的層次上看,KVC 有兩個方法:一個是設置 key 的值,另一個是獲取 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);
}
現在,如果 Person 有另外一個 key 配偶(spouse),spouse 的 key 值是另一個 Person 對象,用 KVC 可以這樣寫:
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 要區分開來,key 可以從一個對象中獲取值,而 key path 可以將多個 key 用點號 “.” 分割連接起來,比如:
<pre name="code" class="objc">[p valueForKeyPath:@"spouse.name"];
相當於這樣……
[[p valueForKey:@"spouse"] valueForKey:@"name"];
好了,以上是 KVC 的基本知識,接着看看 KVO。
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。舉個例子,用代碼觀察一個 person 對象的 address 變化,以下是實現的三個方法:
- watchPersonForChangeOfAddress: 實現觀察
- observeValueForKeyPath:ofObject:change:context: 在被觀察的 key path 的值變化時調用。
- dealloc 停止觀察
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
這就是 KVO 的作用,它通過 key path 觀察對象的值,當值發生變化的時候會收到通知。