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:
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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:
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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:
[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
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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。
轉自:http://magicalboy.com/kvc_and_kvo/