Associated Objects

Associated Objects—或者 Associative References，正如他们起初所被认识的那样—是Objective-C 2.0 runtime的特征，在OS X Snow Leopard（IOS 4）中被引进。具体指的就是

@interface NSObject (AssociatedObject)
@property (nonatomic, strong) id associatedObject;
@end

NSObject+AssociatedObject.m

@implementation NSObject (AssociatedObject)
@dynamic associatedObject;

- (void)setAssociatedObject:(id)object {
objc_setAssociatedObject(self, @selector(associatedObject), object, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (id)associatedObject {
return objc_getAssociatedObject(self, @selector(associatedObject));
}