由于分类是不能直接添加成员变量的，因此需要使用关联对象技术为分类添加成员变量，由此引出几个问题：
- 问题1❓:什么是成员变量，成员变量和实例变量还有属性的区别是什么?
- 问题2❓:分类中是如何添加成员变量的，其实现原理是什么？
- 问题3❓:分类中添加的成员变量被保存在哪？(在宿主类上还是其他什么地方)
void _object_set_associative_reference(id object, void *key, id value, uintptr_t policy) {
//1.创建一个空的ObjcAssociation对象
ObjcAssociation old_association(0, nil);
// 2.对关联对象进行内存管理操作retain/copy
id new_value = value ? acquireValue(value, policy) : nil;
{
//3.创建AssociationsManager对象和其成员变量AssociationsHashMap对象
AssociationsManager manager;
AssociationsHashMap &associations(manager.associations());
//4.将被关联对象的指针地址反转
disguised_ptr_t disguised_object = DISGUISE(object);
// 5.判断传入的关联对象是否为空
if (new_value) {
//如果不为空
/*通过被关联对象的指针地址从AssociationsHashMap中获取对应的
ObjectAssociationMap
*/
AssociationsHashMap::iterator i = associations.find(disguised_object);
//如果该被关联对象已经存在了对应的ObjectAssociationMap
if (i != associations.end()) {
// 通过参数key获取到ObjectAssociationMap对应的ObjcAssociation
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
//如果存在对应的ObjcAssociation
if (j != refs->end()) {
//把开始创建的空的ObjcAssociation对象设置为找到的这个ObjcAssociation
old_association = j->second;
//把当前的关联对象封装为ObjcAssociation并赋值给ObjectAssociationMap中key对应的值
j->second = ObjcAssociation(policy, new_value);
} else {//如果没有找到存在对应的ObjcAssociation，那就根据参数key直接设置
(*refs)[key] = ObjcAssociation(policy, new_value);
}
} else { //如果该被关联对象不存在了对应的ObjectAssociationMap
// 那么就创建新的ObjectAssociationMap
ObjectAssociationMap *refs = new ObjectAssociationMap;
//以被关联对象的反转地址为key，新创建的ObjectAssociationMap为value，映射为ObjectAssociationHashMap
associations[disguised_object] = refs;
//把参数key作为ObjectAssociationMap的key，ObjcAssociation对象为value
(*refs)[key] = ObjcAssociation(policy, new_value);
//最后设置这个被关联对象为有关联对象
object->setHasAssociatedObjects();
}
} else {//如果传入的关联对象为nil，会将被关联对象的对应关联对象擦除
// 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);
}
}
}
}
// 最后将old_association擦除
if (old_association.hasValue()) ReleaseValue()(old_association);
}
id _object_get_associative_reference(id object, void *key) {
id value = nil;
uintptr_t policy = OBJC_ASSOCIATION_ASSIGN;
{
AssociationsManager manager;
// 初始化hashmap
AssociationsHashMap &associations(manager.associations());
// 当前对象的地址按位取反（key）
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;
}
inline void
objc_object::setHasAssociatedObjects()
{
if (isTaggedPointer()) return;
retry:
// 设置当前的isa指针的标识
isa_t oldisa = LoadExclusive(&isa.bits);
isa_t newisa = oldisa;
if (!newisa.nonpointer || newisa.has_assoc) {
ClearExclusive(&isa.bits);
return;
}
newisa.has_assoc = true;
if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry;
}
inline void
objc_object::rootDealloc()
{
if (isTaggedPointer()) return; // fixme necessary?
if (fastpath(isa.nonpointer &&
!isa.weakly_referenced &&
!isa.has_assoc &&
!isa.has_cxx_dtor &&
!isa.has_sidetable_rc))
{
assert(!sidetable_present());
free(this);
}
else {
object_dispose((id)this);
}
}
id
object_dispose(id obj)
{
if (!obj) return nil;
objc_destructInstance(obj);
free(obj);
return nil;
}
void *objc_destructInstance(id obj)
{
if (obj) {
// Read all of the flags at once for performance.
bool cxx = obj->hasCxxDtor();
bool assoc = obj->hasAssociatedObjects();
// This order is important.
if (cxx) object_cxxDestruct(obj);
if (assoc) _object_remove_assocations(obj);
obj->clearDeallocating();
}
return obj;
}
void _object_remove_assocations(id object) {
vector< ObjcAssociation,ObjcAllocator > 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());
}
关联对象并不是存储在被关联对象本身内存中，而是存储在全局的统一的一个AssociationsManager中，如果设置关联对象为nil，就相当于是移除关联对象。