前言
在开发中经常需要给已有的类添加方法和属性，但是Objective-C是不允许给已有类通过分类添加属性的，因为类分类是不会自动生成成员变量的。但是，我们可以通过运行时机制就可以做到了。
本篇文章适合新手阅读，手把手教你如何在项目中使用关联属性！
API介绍
我们先看看Runtime提供的关联API，只有这三个API，使用也是非常简单的：
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/**
* 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)
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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。既然我们要添加关联属性，几乎不会存在需要手动取消关联的场合。
设置关联值（Setter）
对于设置关联，我们需要使用下面的API关联起来：
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void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
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参数说明：
object：与谁关联，通常是传selfkey：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（Getter）
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：
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id objc_getAssociatedObject(id object, const void *key)
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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：
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/**
* 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. */
};
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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，非线程安全 - OBJC_ASSOCIATION_RETAIN_NONATOMIC：表示强（strong）引用关联对象，非线程安全
- OBJC_ASSOCIATION_COPY_NONATOMIC：表示关联对象copy，非线程安全
- OBJC_ASSOCIATION_RETAIN：表示强（strong）引用关联对象，是线程安全的
- OBJC_ASSOCIATION_COPY：表示关联对象copy，是线程安全的
扩展属性
我们来写一个例子，扩展UIControl添加Block版本的TouchUpInside事件。
扩展头文件声明：
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#import <UIKit/UIKit.h>
typedef void (^HYBTouchUpBlock)(id sender);
@interface UIControl (HYBBlock)
@property (nonatomic, copy) HYBTouchUpBlock hyb_touchUpBlock;
@end
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扩展实现文件：
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#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
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