介绍

在OC中要在对象中存放信息，我们需要从对象所属的类中继承一个子类，然后改用这个子类的对象。在不想使用子类的情况下，我们可以使用『关联对象』(Associated Object)来处理问题。
给对象关联其他对象，这些对象通过"键"来区分。在存储对象值的时候，可以设置"存储策略"。

typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
    OBJC_ASSOCIATION_ASSIGN = 0,           /**< Specifies a weak reference to the associated object. */
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object. 
                                            *   The association is not made atomically. */
    OBJC_ASSOCIATION_COPY_NONATOMIC = 3,   /**< Specifies that the associated object is copied. 
                                            *   The association is not made atomically. */
    OBJC_ASSOCIATION_RETAIN = 01401,       /**< Specifies a strong reference to the associated object.
                                            *   The association is made atomically. */
    OBJC_ASSOCIATION_COPY = 01403          /**< Specifies that the associated object is copied.
                                            *   The association is made atomically. */
};

这一点和设置属性相同，我们可以类比@propety来理解。
OBJC_ASSOCIATION_ASSIGN ------------------------- assign
OBJC_ASSOCIATION_RETAIN_NONATOMIC ------------ nonatomic, retain
OBJC_ASSOCIATION_COPY_NONATOMIC -------------- nonatomic,copy
OBJC_ASSOCIATION_RETAIN -------------------------- retain
OBJC_ASSOCIATION_COPY ---------------------------- copy

使用

我们可以使用这个特性给系统的类设置属性
给UIImageView设置imageURL

#import "objc/runtime.h"
#import "UIImageView+WebCache.h"

static const void *imageURLKey = &imageURLKey;

@implementation UIImageView (WebCache)
- (NSString *)imageURL {
    return objc_getAssociatedObject(self, imageURLKey);
}
- (void)setImageURL:(NSString *)imageURL {
    objc_setAssociatedObject(self, imageURLKey, imageURL, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
@end

在使用UIAlertView时，创建视图与处理按钮动作分开，很不方便阅读，我们可以这样处理。
1.设置"键"值。

static void *AlertViewKey = "AlertViewKey";

2.使用

- (void)askQuestion {
    UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"问题" message:@"你想要做…… ?" delegate:self cancelButtonTitle:@"取消" otherButtonTitles:@"确定", nil];
    void (^block)(NSInteger) = ^(NSInteger buttonIndex) {
        if (buttonIndex == 0) {
            [self doCancel];
        } else if (buttonIndex == 1) {
            [self doContinue];
        }
    };
    objc_setAssociatedObject(alertView, AlertViewKey, block, OBJC_ASSOCIATION_COPY);
    [alertView show];
}
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex {
    void (^block)(NSInteger) = objc_getAssociatedObject(alertView, AlertViewKey);
    block(buttonIndex);
}