runtime关联属性示例

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(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intfloat,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
 
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