KVO与KVC理解

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，NSObject的一个category，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。

其他例子

假设一个场景,股票的价格显示在当前屏幕上，当股票价格更改的时候，实时显示更新其价格。

1.定义DataModel

@interface StockData : NSObject {
    NSString * stockName;
    float price;
}
@end
@implementation StockData
@end

2.定义此model为Controller的属性，实例化它，监听它的属性，并显示在当前的View里边。

- (void)viewDidLoad
{
    [super viewDidLoad];

    stockForKVO = [[StockData alloc] init];
    [stockForKVO setValue:@"searph" forKey:@"stockName"];
    [stockForKVO setValue:@"10.0" forKey:@"price"];    
    [stockForKVO addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:NULL];

    myLabel = [[UILabel alloc]initWithFrame:CGRectMake(100, 100, 100, 30 )];
    myLabel.textColor = [UIColor redColor];
    myLabel.text = [stockForKVO valueForKey:@"price"];
    [self.view addSubview:myLabel];
   
    UIButton * b = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    b.frame = CGRectMake(0, 0, 100, 30);
    [b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:b];

}

3.当点击button的时候，调用buttonAction方法，修改对象的属性。

-(void) buttonAction
{
    [stockForKVO setValue:@"20.0" forKey:@"price"];
}

4. 实现回调方法

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if([keyPath isEqualToString:@"price"])
    {
        myLabel.text =[NSString stringWithFormat:@"%@",[stockForKVO valueForKey:@"price"]] ;6     }
}

5.增加观察与取消观察是成对出现的，所以需要在最后的时候，移除观察者

- (void)dealloc
{
    [super dealloc];
    [stockForKVO removeObserver:self forKeyPath:@"price"];
    [stockForKVO release];
}

转载自：http://magicalboy.com/kvc_and_kvo/

           http://blog.csdn.net/yuquan0821/article/details/6646400