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) |
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) |
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) |
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. */ }; |
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 |
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 |