关联对象objc_setAssociatedObject和objc_getAssociatedObject

一、关于objc_setAssociatedObject和objc_getAssociatedObject的使用，首先需要包含头文件#import <objc/runtime.h>

1、objc_setAssociatedObject

1	`objc_setAssociatedObject(<#id` `_Nonnull object#>, <#const` `void` `* _Nonnull key#>, <#id` `_Nullable value#>, <#objc_AssociationPolicy policy#>)`

　　如上是objc_setAssociatedObject的各个参数第一个是要关联的对象，第二个是给个自定义的key，第三个是给value，第四个是对应的拷贝方式具体的参数如下所示

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. */
};

从上面的objc_setAssociatedObject参数可以看出相当于在关联的对象中有隐含的一个dictionary用于存储额外的数值，并且通过最后一个参数指定了value的拷贝方式

注意：如果该id、key、value已经存在，想将该value移除，则可以将value设空，比如：调用objc_setAssociatedObject(id, key, nil,OBJC_ASSOCIATION_ASSIGN); 将对应的value移除

2、objc_getAssociatedObject

objc_getAssociatedObject(<#id  _Nonnull object#>, <#const void * _Nonnull key#>)

如上是根据objc_setAssociatedObject设置时候的关联对象和key取出对应的值然后对值进行自己想要的操作

3、objc_removeAssociatedObjects

void
objc_removeAssociatedObjects(id _Nonnull object)

如上是要移除指定对象的所有关联key、value

如下是objc_setAssociatedObject和objc_getAssociatedObject这两个函数的可能用法

- (void) askuserAQuestion {
    void (^ block)() = ^() {
        NSLog(@"block alret");
    };
    objc_setAssociatedObject(self, "test_key", block, OBJC_ASSOCIATION_COPY_NONATOMIC);
    [self alertView];
}

- (void) alertView {
    void (^ block)() = objc_getAssociatedObject(self, "test_key");
    block();
}