内存管理策略:
typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
OBJC_ASSOCIATION_ASSIGN = 0, /**< Specifies a weak reference to the associated object. */
OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object.
* The association is not made atomically. */
OBJC_ASSOCIATION_COPY_NONATOMIC = 3, /**< Specifies that the associated object is copied.
* The association is not made atomically. */
OBJC_ASSOCIATION_RETAIN = 01401, /**< Specifies a strong reference to the associated object.
* The association is made atomically. */
OBJC_ASSOCIATION_COPY = 01403 /**< Specifies that the associated object is copied.
* The association is made atomically. */
};
对于四个参数理解完了之后让我们看看它真正的实现函数_object_set_associative_reference
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);//得到对象地址
if (new_value) {
// break any existing association.
AssociationsHashMap::iterator i = associations.find(disguised_object);//首先通过对象的地址获取对象的hashmap
if (i != associations.end()) {//判断是否已经存在，已经存在
// secondary table exists
ObjectAssociationMap *refs = i->second;//取值，对应的map
ObjectAssociationMap::iterator j = refs->find(key);//通过key查找
if (j != refs->end()) {//如果已经存在
old_association = j->second;//取到原来老的值,以便后边对其释放
j->second = ObjcAssociation(policy, new_value);//存储新的值
} else {//不存在
(*refs)[key] = ObjcAssociation(policy, new_value);
}
} else {//如果不存在,创建一个
// create the new association (first time).
ObjectAssociationMap *refs = new ObjectAssociationMap;
associations[disguised_object] = refs;
(*refs)[key] = ObjcAssociation(policy, new_value);
object->setHasAssociatedObjects();
}
} else {//不存在则创建一个
// 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);
}
}
}
}
// release the old value (outside of the lock).
if (old_association.hasValue()) ReleaseValue()(old_association);
}
