本文转载自:
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:
-
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);
-
-
}
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:
-
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 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:
-
[p valueForKeyPath:@"spouse.name"];
相当于这样……
-
[[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:
-
watchPersonForChangeOfAddress: 实现观察
-
observeValueForKeyPath:ofObject:change:context: 在被观察的 key path 的值变化时调用。
-
dealloc 停止观察
-
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
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。
阅读(973) | 评论(0) | 转发(0) |