On 2012 年 6 月 7 日, in iPhone, by donly
KVC 与 KVO 是 Objective C 的关键概念,个人认为必须理解的东西,下面是实例讲解。
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)
{
// 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:
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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 与 key pat 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) 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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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
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