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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：

/*** 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)

设置关联

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。

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)

object：与谁关联，通常是传self参数说明：

1. 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
2. value：关联所设置的值
3. 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)

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参数说明：

1. 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. */};

OBJC_ASSOCIATION_ASSIGN：表示弱引用关联，通常是基本数据类型，如int、float，非线程安全我们说明一下各个值的作用：

1. OBJC_ASSOCIATION_RETAIN_NONATOMIC：表示强（strong）引用关联对象，非线程安全
2. OBJC_ASSOCIATION_COPY_NONATOMIC：表示关联对象copy，非线程安全
3. OBJC_ASSOCIATION_RETAIN：表示强（strong）引用关联对象，是线程安全的
4. 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：

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