OC語法之KVC與KVO

2019-11-14 18:41:23

字體：大中小

來源：轉載

供稿：網友

1、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 就是基于 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：

[p valueForKeyPath:@"spouse.name"];

相當于這樣……

[[p valueForKey:@"spouse"] valueForKey:@"name"];

好了，以上是 KVC 的基本知識，接著看看 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