转载

iOS-面试助攻(二)

详细理解KVC与KVO

在面试的时候,KVC与KVO有些时候还是会问到的,并且他们都是Objective C的关键概念,在这里我们先做一个简单地介绍:

(一)KVC:

KVC即指: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sex的key;这里key只是一个字符串,他对应的值可以使任意类型的对象;从最基础的从此上看,KVC有两个方法:一个是设置key的值,另一个是获取key的值;(示例代码如下:)

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(示例代码如下:)

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"];

(二)KVO:

KVO即指:Key-Value Observing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:

(1)watchPersonForChangeOfAddress: 实现观察;

(2)observeValueForKeyPath:ofObject:change:context: 在被观察的 key path 的值变化时调用;

(3)dealloc 停止观察。

(示例代码如下:)

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。

(以上便是KVO和KVC的相关理解,还希望大家相互补充共同进步)

正文到此结束
Loading...