1. 三个方法
objc_setAssociatedObject
objc_getAssociatedObject
objc_removeAssociatedObjects
2原理
2.1objc_setAssociatedObject
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
_object_set_associative_reference(object, (void *)key, value, policy);
}
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);
//同一个对象只会创建一个ObjectAssociationMap对象。？
if (i != associations.end()) {
// secondary table exists
//当前对象已经创建关联对象时
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);//对象置nil后再关联对象
}
} else {
// create the new association (first time).
//当前对象没有创建关联对象时
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.
// 当前对象value设置为nil时，释放ObjectAssociationMap对象
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);
}
static id acquireValue(id value, uintptr_t policy) {
switch (policy & 0xFF) {
case OBJC_ASSOCIATION_SETTER_RETAIN:
return objc_retain(value);
case OBJC_ASSOCIATION_SETTER_COPY:
return ((id(*)(id, SEL))objc_msgSend)(value, SEL_copy);
}
return value;
}
- 全局维护AssociationsManager,管理类中存放的是AssociationsHashMap。
- acquireValue如果是OBJC_ASSOCIATION_SETTER_RETAIN或OBJC_ASSOCIATION_SETTER_COPY，对value进行retain和copy操作。与引用计数管理相同。
- 存储时，通过DISGUISE(object)作为key查找，associations没有存储时会走else
ObjectAssociationMap *refs = new ObjectAssociationMap;
associations[disguised_object] = refs;
(*refs)[key] = ObjcAssociation(policy, new_value);
object->setHasAssociatedObjects();
inline void
objc_object::setHasAssociatedObjects()
{
...
newisa.has_assoc = true;//关联对象标志位
if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry;
}
ObjcAssociation是构造存储对象。
即一个对象会生成一个ObjectAssociationMap，并通过disguised_object存储在associations中，new_value存储在ref中，setHasAssociatedObjects设置对象有关联对象的标志位。
refs存储当前对象的所有关联对象
- associations已经分配时，通过key进行存储
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
if (j != refs->end()) {
old_association = j->second;
j->second = ObjcAssociation(policy, new_value);//key已经存在时，找到对应位置更新旧的
} else {
(*refs)[key] = ObjcAssociation(policy, new_value);//key没有存储时，直接存储
}
如果key存储过值，找到对应的value的位置，替换旧的。
如果key没有，ObjcAssociation构造的对象。
关联对象存储分三种情况:
第一种:对象第一次使用关联对象初始化，ObjectAssociationMap，使用策略和value构造存储对象
第二种:对象的关联对象已经初始化，在ObjectAssociationMap查找，没找到按key直接存储。
第三种:对象的关联对象已经初始化，在ObjectAssociationMap查找，找到按key替换原有的存储。
2.2 objc_getAssociatedObject
id objc_getAssociatedObject(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) {
objc_retain(value);
}
}
}
}
if (value && (policy & OBJC_ASSOCIATION_GETTER_AUTORELEASE)) {
objc_autorelease(value);
}
return value;
}
这一段在associations查找，如果策略是OBJC_ASSOCIATION_GETTER_RETAIN，会有个计数+1objc_retain(value);如果是OBJC_ASSOCIATION_GETTER_AUTORELEASE，通过objc_autorelease延迟释放。
2.3objc_removeAssociatedObjects
void objc_removeAssociatedObjects(id object)
{
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);//删除
}
}
// the calls to releaseValue() happen outside of the lock.
for_each(elements.begin(), elements.end(), ReleaseValue());
}
这个方法在dealloc中通过hasAssociatedObjects标志位释放，
找到关联对象elements，遍历进行释放。
总结:
1.不用修改原有对象就能为其添加属性，不消耗对象分配空间。
2.关联对象释放不用担心，系统会在对象销毁时在dealloc中根据标志位进行释放。
3.关联对象与分类的关系？关联对象很好的解决了分类不能添加属性的问题。
4.关联对象的本质:由AssociationsManager管理并在AssociationsHashMap存储。
所有对象的关联内容都在同一个全局容器中。
