[转] iOS (OC) 中 KVC 与 KVO 理解

时间:2021-12-10 08:17:14

转自: http://magicalboy.com/kvc_and_kvo/  

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。

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 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)

  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:
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。

如果 使用ReactiveCocoa 监控某个对象的属性的时候, 当对象的值发生了改变而没有触发对应事件的时候, 那么可能是你没有通过KVC来操作对象的值!