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Key-Value Coding (KVC)
KVC,即是指 NSKeyValueCoding,一个非正式的 Protocol,提供一种机制来间接访问对象的属性。KVO 就是基于 KVC 实现的关键技术之一。
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 说法,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:
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void changeName(Person *p, NSString *newName)
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{
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// using the KVC accessor (getter) method
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NSString *originalName = [p valueForKey:@"name"];
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// using the KVC accessor (setter) method.
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[p setValue:newName forKey:@"name"];
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NSLog(@"Changed %@'s name to: %@", originalName, newName);
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}
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:
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void logMarriage(Person *p)
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{
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// just using the accessor again, same as example above
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NSString *personsName = [p valueForKey:@"name"];
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// this line is different, because it is using
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// a "key path" instead of a normal "key"
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NSString *spousesName = [p valueForKeyPath:@"spouse.name"];
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NSLog(@"%@ is happily married to %@", personsName, spousesName);
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}
key 与 key path 要区分开来,key 可以从一个对象中获取值,而 key path 可以将多个 key 用点号 “.” 分割连接起来,比如:
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[p valueForKeyPath:@"spouse.name"];
相当于这样……
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[[p valueForKey:@"spouse"] valueForKey:@"name"];
好了,以上是 KVC 的基本知识,接着看看 KVO。
Key-Value Observing (KVO)
Key-Value Observing (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:
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watchPersonForChangeOfAddress: 实现观察
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observeValueForKeyPath:ofObject:change:context: 在被观察的 key path 的值变化时调用。
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dealloc 停止观察
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static NSString *const KVO_CONTEXT_ADDRESS_CHANGED = @"KVO_CONTEXT_ADDRESS_CHANGED"
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@implementation PersonWatcher
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-(void) watchPersonForChangeOfAddress:(Person *)p
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{
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// this begins the observing
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[p addObserver:self
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forKeyPath:@"address"
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options:0
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context:KVO_CONTEXT_ADDRESS_CHANGED];
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// keep a record of all the people being observed,
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// because we need to stop observing them in dealloc
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[m_observedPeople addObject:p];
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}
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// whenever an observed key path changes, this method will be called
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- (void)observeValueForKeyPath:(NSString *)keyPath
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ofObject:(id)object
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change:(NSDictionary *)change
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context:(void *)context
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{
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// use the context to make sure this is a change in the address,
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// because we may also be observing other things
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if(context == KVO_CONTEXT_ADDRESS_CHANGED) {
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NSString *name = [object valueForKey:@"name"];
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NSString *address = [object valueForKey:@"address"];
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NSLog(@"%@ has a new address: %@", name, address);
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}
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}
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-(void) dealloc;
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{
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// must stop observing everything before this object is
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// deallocated, otherwise it will cause crashes
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for(Person *p in m_observedPeople){
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[p removeObserver:self forKeyPath:@"address"];
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}
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[m_observedPeople release];
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m_observedPeople = nil;
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[super dealloc];
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}
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-(id) init;
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{
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if(self = [super init]){
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m_observedPeople = [NSMutableArray new];
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}
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return self;
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}
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@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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