这玩意儿已经在前面的文章里多次提到，但一直没深入，这一篇就来研究研究。
runtime提供的和associated object有关的接口有3个：
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy);
id objc_getAssociatedObject(id object, const void *key);
void objc_removeAssociatedObjects(id object) ;
选第一个作为切入点，详细分析一下，其他两个方法稍微说一说。
objc_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) {
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) {
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;
j->second = ObjcAssociation(policy, new_value);
} else {
(*refs)[key] = ObjcAssociation(policy, new_value);
}
} else {
ObjectAssociationMap *refs = new ObjectAssociationMap;
associations[disguised_object] = refs;
(*refs)[key] = ObjcAssociation(policy, new_value);
object->setHasAssociatedObjects();
}
} else {
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);
}
}
}
}
if (old_association.hasValue()) ReleaseValue()(old_association);
}
想要理解associated object的存取过程，就必须要对这个方法中提到的几个类有足够的了解，按照层次依次是：
- AssociationsManager
// class AssociationsManager manages a lock / hash table singleton pair.
// Allocating an instance acquires the lock, and calling its assocations() method
// lazily allocates it.
class AssociationsManager {
static spinlock_t _lock;
static AssociationsHashMap *_map; // associative references: object pointer -> PtrPtrHashMap.
public:
AssociationsManager() { _lock.lock(); }
~AssociationsManager() { _lock.unlock(); }
AssociationsHashMap &associations() {
if (_map == NULL)
_map = new AssociationsHashMap();
return *_map;
}
};
注释写了，AssociationsManager管理了一个自旋锁到哈希表的单例映射。通过associations()方法可以取到管理的AssociationsHashMap单例。
- AssociationsHashMap
class AssociationsHashMap : public unordered_map
{
public:
void *operator new(size_t n) { return ::malloc(n); }
void operator delete(void *ptr) { ::free(ptr); }
};
这是一个无序哈希表，存储的是对象地址(set方法的第一个参数取反)到ObjectAssociationMap的映射。
- ObjectAssociationMap
class ObjectAssociationMap : public std::map
{
public:
void *operator new(size_t n) { return ::malloc(n); }
void operator delete(void *ptr) { ::free(ptr); }
};
此表存储了key(set方法的第二个参数)到被关联对象ObjcAssociation的映射。
- ObjcAssociation
class ObjcAssociation {
uintptr_t _policy;
id _value;
public:
ObjcAssociation(uintptr_t policy, id value) : _policy(policy), _value(value) {}
ObjcAssociation() : _policy(0), _value(nil) {}
uintptr_t policy() const { return _policy; }
id value() const { return _value; }
bool hasValue() { return _value != nil; }
};
存储关联对象的信息，_value对应第三个参数，_policy对应第四个参数。
至此set方法的四个参数都用上了，再回过去把set方法分解一哈就很简单了：
ObjcAssociation old_association(0, nil);
先声明一个对象用来存放可能存在的旧的value，即如果你对一个对象调用了两次set方法，那么在第二次set的时候，需要把第一个set进去的value释放掉。这个对象就是存储前一次的value用于释放。
id new_value = value ? acquireValue(value, policy) : nil;
static id acquireValue(id value, uintptr_t policy) {
switch (policy & 0xFF) {
case OBJC_ASSOCIATION_SETTER_RETAIN:
return ((id(*)(id, SEL))objc_msgSend)(value, SEL_retain);
case OBJC_ASSOCIATION_SETTER_COPY:
return ((id(*)(id, SEL))objc_msgSend)(value, SEL_copy);
}
return value;
}
typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
OBJC_ASSOCIATION_ASSIGN = 0,
OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,
OBJC_ASSOCIATION_COPY_NONATOMIC = 3,
OBJC_ASSOCIATION_RETAIN = 01401,
OBJC_ASSOCIATION_COPY = 01403
};
根据调用set方法传的policy调用对应的方法。比如retain或者copy。
AssociationsManager manager;
AssociationsHashMap &associations(manager.associations());
disguised_ptr_t disguised_object = DISGUISE(object);
inline disguised_ptr_t DISGUISE(id value) { return ~ uintptr_t(value); }
typedef uintptr_t disguised_ptr_t;
先获取AssociationsManager，在构造函数中会进行加锁。接着通过associations()方法获取AssociationsHashMap单例，如果前两步看起来有些不习惯，重写一下就清楚了：
AssociationsManager manager = AssociationsManager();
AssociationsHashMap &associations = manager.associations();
最后一行就是把object的地址取反，后面会用作AssociationsHashMap的键。
if (new_value) {
AssociationsHashMap::iterator i = associations.find(disguised_object);
(1) if (i != associations.end()) {
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
(2) if (j != refs->end()) {
old_association = j->second;
j->second = ObjcAssociation(policy, new_value);
} else {
(*refs)[key] = ObjcAssociation(policy, new_value);
}
} else {
ObjectAssociationMap *refs = new ObjectAssociationMap;
associations[disguised_object] = refs;
(*refs)[key] = ObjcAssociation(policy, new_value);
object->setHasAssociatedObjects();
}
}
先判断是否传入了新的value，如果传入新的value，就进入了设置关联对象的流程。
首先通过上一步处理好的对象地址进行查找：
(1) == true
根据AssociationsHashMap的定义，直接获取ObjectAssociationMap。(2) == true
代表曾经设置过关联对象，把原先的值存到old_association中，留作后面释放，再把新值通过ObjcAssociation(policy, new_value)构造函数存放进去。(2) == false
通过key找不到关联对象，直接构造一个新的ObjcAssociation对象作为key的value。(1) == false
即通过对象地址就查找不到，代表从未设置过关联对象，那么该创建的创建，该关联的关联。最后通过：
object->setHasAssociatedObjects()
将isa的has_assoc字段设为true。
随着源代码的阅读，最开始那篇Runtime源码 —— 对象、类和isa中对isa有些不熟悉的字段也就一个一个见到了。有种融会贯通的感觉，哈哈。
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);
}
}
}
如果调用set方法没有传入新的值，那么就把关联对象从ObjectAssociationMap中删除掉。就好像把对象设为nil要release一样。
// release the old value (outside of the lock).
if (old_association.hasValue()) ReleaseValue()(old_association);
struct ReleaseValue {
void operator() (ObjcAssociation &association) {
releaseValue(association.value(), association.policy());
}
};
static void releaseValue(id value, uintptr_t policy) {
if (policy & OBJC_ASSOCIATION_SETTER_RETAIN) {
((id(*)(id, SEL))objc_msgSend)(value, SEL_release);
}
}
最后一步，把原先的值释放掉。当然只有policy是retain的才需要释放，assign和copy的的就不需要了。
这就是set方法的全部过程。因为嵌套了两层map，看起来有点绕。
objc_getAssociatedObject
理解了set的过程，再通过get方法的参数：
id _object_get_associative_reference(id object, void *key)
猜测一下get的过程应该是这样的：
- 先获取AssociationsManager单例，进而获取AssociationsHashMap
- 通过object获取ObjectAssociationMap
- 通过key获取ObjcAssociation
- 取出ObjcAssociation中的value并返回
看起来合情合理，再看看源码：
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;
}
跟预测的流程几乎一样，只是从ObjcAssociation中获取value的时候，需要根据policy进行相应的retain，autorelease。
objc_removeAssociatedObjects
这个方法用于删除对象的全部关联对象，参数就一个，就是想要清理的对象：
void objc_removeAssociatedObjects(id object)
{
if (object && object->hasAssociatedObjects()) {
_object_remove_assocations(object);
}
}
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());
}
先判断是否存在关联对象，如果存在，才需要进行清除。
清除的过程分为几个部分：
- 存储ObjectAssociationMap中存在的所有的ObjcAssociation
- 释放ObjectAssociationMap内存并从AssociationsHashMap中删除
- 释放所有的ObjcAssociation，最后那个ReleaseValue()方法就是对有需要的对象调用release
全部完成之后AssociationsHashMap中就不存在此对象的任何关联对象了。
总结
- 被关联的对象与关联对象在数据结构上并没有什么关系，关联对象是由AssociationsManager统一管理
- 通常情况下不需要主动调用objc_removeAssociatedObjects(...)方法，移除关联对象会在对象release -> dealloc的时候自动调用，如果想要移除某个关联对象，调用objc_setAssociatedObject(...)方法，并设置value为nil。
- 在category中通过关联对象实现的get/set接口只是让属性看起来像是属性，category本身还是无法添加实例变量。