很多开源库都使用了关联，我们可以不用修改类的定义而为其对象增加存储空间。这在我们无法访问到类的源码的时候或者是考虑到二进制兼容性的时候是非常有用。
关联是基于关键字的，因此，我们可以为任何对象增加任意多的关联，每个都使用不同的关键字即可。关联是可以保证被关联的对象在关联对象的整个生命周期都是可用的（在垃圾自动回收环境下也不会导致资源不可回收）。
在类的定义之外为类增加额外的存储空间
- objc_setAssociatedObject
/**
* 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
*/
OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);
objc_setAssociatedObject把一个对象与另外一个对象进行关联。该函数需要四个参数：源对象，关键字，关联的对象和一个关联策略
关联策略
/* Associative References */
/**
* Policies related to associative references.
* These are options to objc_setAssociatedObject()
*/
typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
//指定一个弱引用相关联的对象
OBJC_ASSOCIATION_ASSIGN = 0, /**< Specifies a weak reference to the associated object. */
//强引用关联对象，非原子操作
OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object.
* The association is not made atomically. */
// 复制关联对象，非原子操作
OBJC_ASSOCIATION_COPY_NONATOMIC = 3, /**< Specifies that the associated object is copied.
* The association is not made atomically. */
// 强引用关联对象，原子操作
OBJC_ASSOCIATION_RETAIN = 01401, /**< Specifies a strong reference to the associated object.
* The association is made atomically. */
// 复制关联对象，非原子操作
OBJC_ASSOCIATION_COPY = 01403 /**< Specifies that the associated object is copied.
* The association is made atomically. */
};
OBJC_ASSOCIATION_ASSIGN 等价于 @property(assign)。
OBJC_ASSOCIATION_RETAIN_NONATOMIC等价于 @property(strong, nonatomic)。
OBJC_ASSOCIATION_COPY_NONATOMIC等价于@property(copy, nonatomic)。
OBJC_ASSOCIATION_RETAIN等价于@property(strong,atomic)。
OBJC_ASSOCIATION_COPY等价于@property(copy, atomic)。
/**
//获取关联的对象
* 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
*/
OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);
//断开关联
//或者直接使用objc_setAssociatedObject函数，传入nil值即可
/**
* 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
*/
OBJC_EXPORT void objc_removeAssociatedObjects(id object)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);
* 例子
//给一个可变数组添加一个关联
static char overviewKey;
NSMutableArray * marrDemo =[[NSMutableArray alloc] initWithObjects:@"1", @"2", @"3", nil];
NSString * overview = [[NSString alloc] initWithFormat:@"%@",@"First three numbers"];
objc_setAssociatedObject(marrDemo, &overviewKey, overview, OBJC_ASSOCIATION_RETAIN);
//获取相关联的对象
NSString * associatedObject = (NSString *)objc_getAssociatedObject(marrDemo, &overviewKey);
//断开关联
//断开关联是使用objc_setAssociatedObject函数，传入nil值即可
objc_setAssociatedObject(marrDemo, &overviewKey, nil, OBJC_ASSOCIATION_RETAIN);
分类，是不能添加属性的，调用会崩溃的。
查了下文档发现，OC的分类允许给分类添加属性，但不会自动生成getter、setter方法。有没有解决方案呢？有，通过运行时建立关联引用。
#import <UIKit/UIKit.h>
@interface UIViewController (ICEDemo)
@property (nonatomic, copy) NSString *str;
@end
#import "UIViewController+ICEDemo.h"
#import <objc/runtime.h>
static void *strKey = &strKey;
@implementation UIViewController (ICEDemo)
- (void)setStr:(NSString *)str {
objc_setAssociatedObject(self, & strKey, str, OBJC_ASSOCIATION_COPY);
}
-(NSString *)str
{
return objc_getAssociatedObject(self, &strKey);
}
@end
