- (void)viewDidLoad
{
    [super viewDidLoad];
    NSString * overview = @"overview";
    self.model= [[DataModel alloc] init];
    [self.model setValue:@"searph" forKey:@"stockName"];
    [self.model setValue:@"10.0" forKey:@"price"];
    [self.model addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:NULL];
    <span style="color:#ff0000;">objc_setAssociatedObject</span>(self.model, &overviewKey, overview, OBJC_ASSOCIATION_RETAIN);
}
 

-(IBAction)btnpressed:(id)sender
{
    [self.model setValue:@"20.0" forKey:@"price"];
}

<pre name="code" class="objc">-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if([keyPath isEqualToString:@"price"]){
        NSLog(@"changeDic = %@",change);
        NSString * associatedObject = (NSString *)<span style="color:#ff0000;">objc_getAssociatedObject</span>(self.model, &overviewKey);
        NSLog(@"associatedObject >>>>>>> %@",associatedObject);
    }
}
 

2014-09-22 23:01:25.470 TestAssociatedObject[61080:60b] changeDic = {
    kind = 1;
    new = 20;
    old = 10;
}
2014-09-22 23:01:25.471 TestAssociatedObject[61080:60b] associatedObject >>>>>>> overview

建立關聯：

建立關聯要使用到Objective-C的執行時函式：objc_setAssociatedObject來把一個物件與另外一個物件進行關聯。

<span style="color:#ff0000;">objc_setAssociatedObject</span>(self.model, &overviewKey, overview, OBJC_ASSOCIATION_RETAIN);

關聯策略表明了相關的物件是通過賦值，保留引用還是複製的方式進行關聯的；

獲取相關聯的物件：

NSString * associatedObject = (NSString *)<span style="color:#ff0000;">objc_getAssociatedObject</span>(self.model, &overviewKey);

斷開關聯：

 斷開關聯是使用objc_setAssociatedObject函式，傳入nil值即可

<span style="color: rgb(255, 0, 0);">objc_setAssociatedObject</span>(self.model, &overviewKey, nil, OBJC_ASSOCIATION_RETAIN);