objc_setAssociatedObject实现与weak等效的关联类型

设置关联对象可选的关联类型有5种：

/* 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. */
};

分别对应属性的内存管理语义的assing、nonatomic,retain、nonatomic,copy、retain、copy，
缺少了weak对应的类型,可以用一个中间对象实现与weak等效的关联类型：

用OBJC_ASSOCIATION_RETAIN_NONATOMIC的方式关联一个中间对象，这个对象相当于一个容器，里面弱引用了我们的目标对象
当我们需要拿到目标对象时，先取出中间对象，因为中间对象是被强引用的，所以一定存在，再取出其中的被弱引用的目标对象，如果目标对象已经delloc，指针自动置空，并不会造成野指针的崩溃。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

参考DZNEmptyDataSet：DZNWeakObjectContainer