原文出自：标哥的技术博客

前言

关联API介绍

/**
 * Sets an associated value for a given object using a given key and association policy.
 *
 * @param object The source object for the association.
 * @param key The key for the association.
 * @param value The value to associate with the key key for object. Pass nil to clear an existing association.
 * @param policy The policy for the association. For possible values, see “Associative Object Behaviors.”
 *
 * @see objc_setAssociatedObject
 * @see objc_removeAssociatedObjects
 */
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)

/**
 * Returns the value associated with a given object for a given key.
 *
 * @param object The source object for the association.
 * @param key The key for the association.
 *
 * @return The value associated with the key \e key for \e object.
 *
 * @see objc_setAssociatedObject
 */
id objc_getAssociatedObject(id object, const void *key)

/**
 * Removes all associations for a given object.
 *
 * @param object An object that maintains associated objects.
 *
 * @note The main purpose of this function is to make it easy to return an object
 *  to a "pristine state”. You should not use this function for general removal of
 *  associations from objects, since it also removes associations that other clients
 *  may have added to the object. Typically you should use \c objc_setAssociatedObject
 *  with a nil value to clear an association.
 *
 * @see objc_setAssociatedObject
 * @see objc_getAssociatedObject
 */
void objc_removeAssociatedObjects(id object)

实际上，我们几乎不会使用到objc_removeAssociatedObjects函数，这个函数的功能是移除指定的对象上所有的关联。

设置关联值

对于设置关联，我们需要使用下面的API关联起来：

void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)

参数说明：

• object：与谁关联，通常是传self
• key：唯一键，在获取值时通过该键获取，通常是使用static const void *来声明
• value：关联所设置的值
• policy：内存管理策略，比如使用copy

获取关联值

如果我们要获取所关联的值，需要通过key来获取，调用如下函数：

id objc_getAssociatedObject(id object, const void *key)

参数说明：

• object：与谁关联，通常是传self，在设置关联时所指定的与哪个对象关联的那个对象
• key：唯一键，在设置关联时所指定的键

关联策略

我们先看看设置关联时所指定的policy，它是一个枚举类型，看官方说明：

/**
 * 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. */
};

我们说明一下各个值的作用：

• OBJC_ASSOCIATION_ASSIGN：表示弱引用关联，通常是基本数据类型，如int、float
• OBJC_ASSOCIATION_RETAIN_NONATOMIC：表示强（strong）引用关联对象
• OBJC_ASSOCIATION_COPY_NONATOMIC：表示关联对象copy
• OBJC_ASSOCIATION_RETAIN：表示强（strong）引用关联对象，但不是线程安全的
• OBJC_ASSOCIATION_COPY：表示关联对象copy，但不是线程安全的

扩展属性

扩展头文件声明：

#import <UIKit/UIKit.h>

typedef void (^HYBTouchUpBlock)(id sender);

@interface UIControl (HYBBlock)

@property (nonatomic, copy) HYBTouchUpBlock hyb_touchUpBlock;

@end

扩展实现文件：

#import "UIControl+HYBBlock.h"
#import <objc/runtime.h>

static const void *sHYBUIControlTouchUpEventBlockKey = "sHYBUIControlTouchUpEventBlockKey";

@implementation UIControl (HYBBlock)

- (void)setHyb_touchUpBlock:(HYBTouchUpBlock)hyb_touchUpBlock {
  objc_setAssociatedObject(self,
                           sHYBUIControlTouchUpEventBlockKey,
                           hyb_touchUpBlock,
                           OBJC_ASSOCIATION_COPY);

  [self removeTarget:self
              action:@selector(hybOnTouchUp:)
    forControlEvents:UIControlEventTouchUpInside];

  if (hyb_touchUpBlock) {
    [self addTarget:self
             action:@selector(hybOnTouchUp:)
   forControlEvents:UIControlEventTouchUpInside];
  }
}

- (HYBTouchUpBlock)hyb_touchUpBlock {
  return objc_getAssociatedObject(self, sHYBUIControlTouchUpEventBlockKey);
}

- (void)hybOnTouchUp:(UIButton *)sender {
  HYBTouchUpBlock touchUp = self.hyb_touchUpBlock;

  if (touchUp) {
    touchUp(sender);
  }
}

@end

使用起来很简单吧！！！

写在最后

且看且珍惜！！！

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