設置關聯對象可選的關聯類型有5種：

/* Associative References */

/**
 * Policies related to associative references.
 * These are options to objc_setAssociatedObject()
 */
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. */
};

分別對應屬性的內存管理語義的assing、nonatomic,retain、nonatomic,copy、retain、copy，
缺少了weak對應的類型,可以用一個中間對象實現與weak等效的關聯類型：

• 用OBJC_ASSOCIATION_RETAIN_NONATOMIC的方式關聯一個中間對象，這個對象相當于一個容器，里面弱引用了我們的目標對象
• 當我們需要拿到目標對象時，先取出中間對象，因為中間對象是被強引用的，所以一定存在，再取出其中的被弱引用的目標對象，如果目標對象已經delloc，指針自動置空，并不會造成野指針的崩潰。OBJC_ASSOCIATION_ASSIGN是做不到的。

- (id)delegate {
    LLSWeakObjectContainer *container = objc_getAssociatedObject(self, xxx);
    return container.weakObject;
}

- (void)setDelegate:(id)delegate {
    objc_setAssociatedObject(self, xxx, [[LLSWeakObjectContainer alloc] initWithWeakObject:delegate], OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@interface LLSWeakObjectContainer : NSObject

@property (nonatomic, readonly, weak) id weakObject;

- (instancetype)initWithWeakObject:(id)object;

@end

@implementation LLSWeakObjectContainer

- (instancetype)initWithWeakObject:(id)object
{
    self = [super init];
    if (self) {
        _weakObject = object;
    }
    return self;
}

@end

參考DZNEmptyDataSet：DZNWeakObjectContainer