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runtime关联属性

前言

在开发中经常需要给已有的类扩展添加方法和属性，但是 Objective-C 是不允许给已有类扩展属性的，因为类扩展是不会自动生成成员变量的。但是，苹果提供了 runtime ，我们可以通过 runtime 使用关联 API 就可以做到了。

关联API介绍

我们先看看关联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_AssociationPolicypolicy)   /** * 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_AssociationPolicypolicy)

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

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);      [selfremoveTarget:self               action:@selector(hybOnTouchUp:)     forControlEvents:UIControlEventTouchUpInside];      if (hyb_touchUpBlock) {     [selfaddTarget: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！！！