在日常開發(fā)的過程中經(jīng)常會用到在category中添加屬性的需求，在添加屬性后是默認(rèn)不會自動生成setter和getter方法的，需要我們自己手動添加，這就用到了runtime的兩個方法，objc_setAssociatedObject和objc_getAssociatedObject。

@interface Person (category1)
@property (nonatomic, copy) NSString * address;
@end
static const NSString * associate_address = @"address";
@implementation Person (category1)
- (void)setAddress:(NSString *)address{
    objc_setAssociatedObject(self, 
                             &associate_address,
                             address, 
                             OBJC_ASSOCIATION_COPY_NONATOMIC);
}

- (NSString *)address{
    return objc_getAssociatedObject(self, &associate_address);
}

@end

這個分類的關(guān)聯(lián)對象到底是這么實(shí)現(xiàn)的，我們在objc-runtime.mm文件中或許會找到答案

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

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


void objc_removeAssociatedObjects(id object) 
{
    if (object && object->hasAssociatedObjects()) {
        _object_remove_assocations(object);
    }
}

objc_setAssociatedObject

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);
   // 通過acquireValue方法對傳入的value進(jìn)行retain或者copy，得到新的new_value
    id new_value = value ? acquireValue(value, policy) : nil;
    {
        // 創(chuàng)建manager，manager里面有一個AssociationsHashMap類型的哈希表
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.associations());
        // 將object簡單處理成~uintptr_t(value)類型
        disguised_ptr_t disguised_object = DISGUISE(object);
        if (new_value) {
            // break any existing association.
            // 通過disguised_object找到對應(yīng)的AssociationsHashMap
            AssociationsHashMap::iterator i = associations.find(disguised_object);
            if (i != associations.end()) {
                // secondary table exists
                // i->first disguised_object ,i->second ObjectAssociationMap
                ObjectAssociationMap *refs = i->second;
                //   通過key找到對應(yīng)的ObjectAssociationMap
                ObjectAssociationMap::iterator j = refs->find(key);
                if (j != refs->end()) {
                    // 如果對應(yīng)的ObjectAssociationMap存在，取出舊值釋放，更新新值
                    // j->first 綁定關(guān)聯(lián)對象傳入的key
                    // j->second ObjcAssociation(policy, new_value)
                    old_association = j->second;
                    j->second = ObjcAssociation(policy, new_value);
                } else {
                    // 如果不存在，以key-ObjcAssociation鍵值對的形式存入表中
                    (*refs)[key] = ObjcAssociation(policy, new_value);
                }
            } else {
                // create the new association (first time).
                // 如果disguised_object對應(yīng)的哈希表不存在
                ObjectAssociationMap *refs = new ObjectAssociationMap;
                // 將disguised_object作為 key ，refs（ObjectAssociationMap）作為value
                // 存入AssociationsHashMap表中
                associations[disguised_object] = refs;
                // 將傳入的key 為key，ObjcAssociation為value存入表中
                (*refs)[key] = ObjcAssociation(policy, new_value);
                // 設(shè)置當(dāng)前對象綁定關(guān)聯(lián)對象
                object->setHasAssociatedObjects();
            }
        } else {
            // setting the association to nil breaks the association.
            // 如果設(shè)置的value 為nil，找到對應(yīng)的key刪除
            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).
   // 釋放 old_association
    if (old_association.hasValue()) ReleaseValue()(old_association);
}

用下面的一張圖來說明AssociationsManager，AssociationsHashMap,ObjectAssociationMap,ObjectAssociations它們之間的關(guān)系：

set_associative.png

至此，通過上圖可以看出它們之間的關(guān)系：
關(guān)聯(lián)對象并沒有添加在原來的類或者分類上，而是放在了一張哈希表中，將傳入的object轉(zhuǎn)化為disguised_object作為key，ObjectAssociationMap作為value放在了AssociationsHashMap表中，而ObjectAssociationMap表中存放的是以傳入的key作為key，ObjcAssociation作為value的數(shù)據(jù)關(guān)系，ObjcAssociation中保存的是傳入的value和policy。

那我們接下來看看它是這么取值的吧

objc_getAssociatedObject

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) {
                    objc_retain(value);
                }
            }
        }
    }
    if (value && (policy & OBJC_ASSOCIATION_GETTER_AUTORELEASE)) {
        objc_autorelease(value);
    }
    return value;
}

取值看起來就比較簡單了，通過disguised_object和key找到對應(yīng)ObjcAssociation，返回里面保存的value。這也就是為什么在取值的時候只需要傳入object和key，就可以取到值了。

objc_removeAssociatedObjects

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());
}

上述源碼可以看出_object_remove_assocations函數(shù)將object對象所有關(guān)聯(lián)對象全部刪除。