我们在runtime.h 文件中，有这么一部分代码

/* Associative References */

/**
 * 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. */
};

/** 
 * 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 _Nonnull object, const void * _Nonnull key,
                         id _Nullable value, objc_AssociationPolicy policy)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);

/** 
 * 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 _Nullable
objc_getAssociatedObject(id _Nonnull object, const void * _Nonnull key)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);

/** 
 * 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 _Nonnull object)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);

这部分就是我们经常用的关联引用部分，给category 增加一个假的成员变量。

基本用法

#import "ViewController.h"
#import <objc/runtime.h>
@interface ViewController ()

@end

@implementation ViewController

static NSString *_strFlag;

-(void)addValue{
    objc_setAssociatedObject(self, &_strFlag, @"你好", OBJC_ASSOCIATION_RETAIN_NONATOMIC);

}
-(void)getvaule{
  NSString * str =   objc_getAssociatedObject(self, &_strFlag);
    NSLog(@"getValue %@",str);
}
-(void)changeValue{
    objc_setAssociatedObject(self, &_strFlag, @"挺好的", OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [self getvaule];

}
-(void)deleteValue{
    objc_setAssociatedObject(self, &_strFlag, nil, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    [self getvaule];
}



- (void)viewDidLoad {
    [super viewDidLoad];
    [self addValue];
    [self getvaule   ];
    [self changeValue];
    [self deleteValue];
    // Do any additional setup after loading the view, typically from a nib.
}
@end

结果

2018-04-18 09:14:04.888137+0800 关联引用[30473:10915294] getValue 你好
2018-04-18 09:14:04.888323+0800 关联引用[30473:10915294] getValue 挺好的
2018-04-18 09:14:04.888412+0800 关联引用[30473:10915294] getValue (null)

使用场景

• 1.为现有的类添加私有变量
• 2.为现有的类添加公有属性
• 3.为KVO创建一个关联的观察者。

API

OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
__OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);
OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key)
__OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);
OBJC_EXPORT void objc_removeAssociatedObjects(id object)
__OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

在这API 中有个objc_AssociationPolicy 宏定义，我感觉这个图挺好的。就拿过来了。

参数的意义：

• 1.id object 设置关联对象的实例对象
• 2.const void *key 区分不同的关联对象的 key。其实这个key是个地址，只要是不变的地址就可以。（地址不变的有静态变量,函数指针等）
  举例
  使用 &AssociatedObjectKey 作为key值

static char AssociatedObjectKey = "AssociatedKey";

使用AssociatedKey 作为key值

static const void *AssociatedKey = "AssociatedKey";

使用@selector

@selector(associatedKey)

还有很多写法。这里就不做介绍了。

• 3.id value 关联的对象
• 4.objc_AssociationPolicy policy 关联对象的存储策略。对应关系如上图。

源码分析

objc_setAssociatedObject(id object, const void *key, id value,

                     objc_AssociationPolicy policy) 

objc_setAssociatedObject(id object, const void *key, id value, 
                         objc_AssociationPolicy policy) 
{
    objc_setAssociatedObject_non_gc(object, key, value, policy);
}

调用下面函数

void objc_setAssociatedObject_non_gc(id object, const void *key, id value, objc_AssociationPolicy policy) {
    _object_set_associative_reference(object, (void *)key, value, policy);
}

接着调用下面函数

void _object_set_associative_reference(id object, void *key, id value, uintptr_t policy) {
    // retain the new value (if any) outside the lock.
    ObjcAssociation old_association(0, nil);
    id new_value = value ? acquireValue(value, policy) : nil;
    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.associations());
        disguised_ptr_t disguised_object = DISGUISE(object);
        if (new_value) {
            // break any existing association.
            AssociationsHashMap::iterator i = associations.find(disguised_object);
            if (i != associations.end()) {
                // secondary table exists
                ObjectAssociationMap *refs = i->second;
                ObjectAssociationMap::iterator j = refs->find(key);
                if (j != refs->end()) {
                    old_association = j->second;
                    j->second = ObjcAssociation(policy, new_value);
                } else {
                    (*refs)[key] = ObjcAssociation(policy, new_value);
                }
            } else {
                // create the new association (first time).
                ObjectAssociationMap *refs = new ObjectAssociationMap;
                associations[disguised_object] = refs;
                (*refs)[key] = ObjcAssociation(policy, new_value);
                object->setHasAssociatedObjects();
            }
        } else {
            // setting the association to nil breaks the association.
            AssociationsHashMap::iterator i = associations.find(disguised_object);
            if (i !=  associations.end()) {
                ObjectAssociationMap *refs = i->second;
                ObjectAssociationMap::iterator j = refs->find(key);
                if (j != refs->end()) {
                    old_association = j->second;
                    refs->erase(j);
                }
            }
        }
    }
    // release the old value (outside of the lock).
    if (old_association.hasValue()) ReleaseValue()(old_association);
}

这个函数才是真正的实现

知识补充
ObjcAssociation old_association(0, nil); 相当于
ObjcAssociation * old_association；
ObjcAssociation._policy=0;
ObjcAssociation._value=nil;

AssociationsHashMap &associations(manager.associations());相当于
AssociationsHashMap & mm = manager.associations();
AssociationsHashMap &associations = mm;

流程图

关联引用.png

1.从流程图中，我们很容易能总结出来，关联引用的所有数据都是保存在一个全局map
2.全局map 的key 是object 转换成的数值，value 是一个newMap
3.newMap 的key 就是我们传入的key 。值是ObjcAssociation 对象。
4.ObjcAssociation 对象保存policy 和value 值。
结构图如下

结构图

5 从图中能看出来，我们要是传入的值是nil， 就相当于把值清空。
6 这里注意acquireValue(value, policy)。会根据外界传入的策略进行消息转发retain 或者copy

从这里有句话。

  object->setHasAssociatedObjects();

inline void
objc_object::setHasAssociatedObjects()
{
    if (isTaggedPointer()) return;
 retry:
    isa_t oldisa = LoadExclusive(&isa.bits);
    isa_t newisa = oldisa;
    if (!newisa.indexed) return;
    if (newisa.has_assoc) return;
    newisa.has_assoc = true;
    if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry;
}

从这里我们能看出来，objc-class 类中的 has_assoc 标志位是关联引用标志位，有关联引用，就标志位true。

id objc_getAssociatedObject(id object, const void *key)

id 
objc_getAssociatedObject(id object, const void *key) 
{
    return objc_getAssociatedObject_non_gc(object, key);
}

id objc_getAssociatedObject_non_gc(id object, const void *key) {
    return _object_get_associative_reference(object, (void *)key);
}

id _object_get_associative_reference(id object, void *key) {
    id value = nil;
    uintptr_t policy = OBJC_ASSOCIATION_ASSIGN;
    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.associations());
        disguised_ptr_t disguised_object = DISGUISE(object);
        AssociationsHashMap::iterator i = associations.find(disguised_object);
        if (i != associations.end()) {
            ObjectAssociationMap *refs = i->second;
            ObjectAssociationMap::iterator j = refs->find(key);
            if (j != refs->end()) {
                ObjcAssociation &entry = j->second;
                value = entry.value();
                policy = entry.policy();
                if (policy & OBJC_ASSOCIATION_GETTER_RETAIN) ((id(*)(id, SEL))objc_msgSend)(value, SEL_retain);
            }
        }
    }
    if (value && (policy & OBJC_ASSOCIATION_GETTER_AUTORELEASE)) {
        ((id(*)(id, SEL))objc_msgSend)(value, SEL_autorelease);
    }
    return value;
}

1.我们知道了关联引用的存储结构后，这里就很简单的看懂了。先获取全局的map
2.根据object 转换的数字，从map中招NewMap.没有找到就返回
3.从NewMap中招key 对应的ObjcAssociation 对象，没有找到就返回
4.找到后检查缓存策略是不是OBJC_ASSOCIATION_GETTER_RETAIN，是就发送消息retain 该对象

5 在获取到值的时候并且policy 是OBJC_ASSOCIATION_GETTER_AUTORELEASE 的时候，发送消息SEL_autorelease

void objc_removeAssociatedObjects(id object)

void objc_removeAssociatedObjects(id object) 
{
#if SUPPORT_GC
    if (UseGC) {
        auto_zone_erase_associative_refs(gc_zone, object);
    } else 
#endif
    {
        if (object && object->hasAssociatedObjects()) {
            _object_remove_assocations(object);
        }
    }
}

void _object_remove_assocations(id object) {
    vector< ObjcAssociation,ObjcAllocator<ObjcAssociation> > elements;
    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.associations());
        if (associations.size() == 0) return;
        disguised_ptr_t disguised_object = DISGUISE(object);
        AssociationsHashMap::iterator i = associations.find(disguised_object);
        if (i != associations.end()) {
            // copy all of the associations that need to be removed.
            ObjectAssociationMap *refs = i->second;
            for (ObjectAssociationMap::iterator j = refs->begin(), end = refs->end(); j != end; ++j) {
                elements.push_back(j->second);
            }
            // remove the secondary table.
            delete refs;
            associations.erase(i);
        }
    }
    // the calls to releaseValue() happen outside of the lock.
    for_each(elements.begin(), elements.end(), ReleaseValue());
}

1.这里我们能看出来。我们首先先检查对象有没有关联引用。没有就返回了。
2.这里我们先声明一个变量 vector< ObjcAssociation,ObjcAllocator<ObjcAssociation> > elements;
用来存放即将被删除的对象。
3.加入对象有关联引用。获取全局map，要是全局map没有值，那么也返回了。（不知道为什么这里没有清除标志位）
4.找到全局map中object 对应的NewMap.。
5.检查newMap中的数据，将数据保存到一个elements模板。
6.删除NewMap。
7.从elements模板中获取ObjcAssociation 对象，根据策略将对象释放。

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