在日常开发的过程中经常会用到在category中添加属性的需求，在添加属性后是默认不会自动生成setter和getter方法的，需要我们自己手动添加，这就用到了runtime的两个方法，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
这个分类的关联对象到底是这么实现的，我们在objc-runtime.mm文件中或许会找到答案
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);
// 通过acquireValue方法对传入的value进行retain或者copy，得到新的new_value
id new_value = value ? acquireValue(value, policy) : nil;
{
// 创建manager，manager里面有一个AssociationsHashMap类型的哈希表
AssociationsManager manager;
AssociationsHashMap &associations(manager.associations());
// 将object简单处理成~uintptr_t(value)类型
disguised_ptr_t disguised_object = DISGUISE(object);
if (new_value) {
// break any existing association.
// 通过disguised_object找到对应的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;
// 通过key找到对应的ObjectAssociationMap
ObjectAssociationMap::iterator j = refs->find(key);
if (j != refs->end()) {
// 如果对应的ObjectAssociationMap存在，取出旧值释放，更新新值
// j->first 绑定关联对象传入的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键值对的形式存入表中
(*refs)[key] = ObjcAssociation(policy, new_value);
}
} else {
// create the new association (first time).
// 如果disguised_object对应的哈希表不存在
ObjectAssociationMap *refs = new ObjectAssociationMap;
// 将disguised_object作为 key ，refs（ObjectAssociationMap）作为value
// 存入AssociationsHashMap表中
associations[disguised_object] = refs;
// 将传入的key 为key，ObjcAssociation为value存入表中
(*refs)[key] = ObjcAssociation(policy, new_value);
// 设置当前对象绑定关联对象
object->setHasAssociatedObjects();
}
} else {
// setting the association to nil breaks the association.
// 如果设置的value 为nil，找到对应的key删除
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).
// 释放 old_association
if (old_association.hasValue()) ReleaseValue()(old_association);
}
用下面的一张图来说明AssociationsManager，AssociationsHashMap,ObjectAssociationMap,ObjectAssociations它们之间的关系：
set_associative.png
至此，通过上图可以看出它们之间的关系：
关联对象并没有添加在原来的类或者分类上，而是放在了一张哈希表中，将传入的object转化为disguised_object作为key，ObjectAssociationMap作为value放在了AssociationsHashMap表中，而ObjectAssociationMap表中存放的是以传入的key作为key，ObjcAssociation作为value的数据关系，ObjcAssociation中保存的是传入的value和policy。
那我们接下来看看它是这么取值的吧
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;
}
取值看起来就比较简单了，通过disguised_object和key找到对应ObjcAssociation，返回里面保存的value。这也就是为什么在取值的时候只需要传入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());
}
上述源码可以看出_object_remove_assocations函数将object对象所有关联对象全部删除。
