默认情况下，因为分类底层结构的限制，不能添加成员变量到分类中。但可以通过关联对象来间接实现

关联对象提供了以下API

添加关联对象

void objc_setAssociatedObject(id object, const void * key,
                                id value, objc_AssociationPolicy policy)

获得关联对象

id objc_getAssociatedObject(id object, const void * key)

移除所有的关联对象

void objc_removeAssociatedObjects(id object)

key的常见用法

1. static void *MyKey = &MyKey;
   objc_setAssociatedObject(obj, MyKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC)
   objc_getAssociatedObject(obj, MyKey)

2. static char MyKey;
   objc_setAssociatedObject(obj, &MyKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC)
   objc_getAssociatedObject(obj, &MyKey)

3. 使用属性名作为key
   objc_setAssociatedObject(obj, @"property", value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
   objc_getAssociatedObject(obj, @"property");

4. 使用get方法的@selecor作为key
   objc_setAssociatedObject(obj, @selector(getter), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC)
   objc_getAssociatedObject(obj, @selector(getter))

示例：

- (void)setName:(NSString *)name
{
    objc_setAssociatedObject(self, @selector(name), name, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

- (NSString *)name
{
    // 隐式参数
    // _cmd == @selector(name)
    return objc_getAssociatedObject(self, _cmd);
}

objc_AssociationPolicy

image.png

关联对象的原理

实现关联对象技术的核心对象有
AssociationsManager
AssociationsHashMap
ObjectAssociationMap
ObjcAssociation

objc4源码解读：objc-references.mm

image.png

image.png

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);
            if (i != associations.end()) {
                // secondary table exists
                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 {
                // create the new association (first time).
                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.
            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);
}


class AssociationsManager {
    // associative references: object pointer -> PtrPtrHashMap.
    static AssociationsHashMap *_map;
public:
    AssociationsManager()   { AssociationsManagerLock.lock(); }
    ~AssociationsManager()  { AssociationsManagerLock.unlock(); }
    
    AssociationsHashMap &associations() {
        if (_map == NULL)
            _map = new AssociationsHashMap();
        return *_map;
    }
};

注意

1. 关联对象不会强引用对象，而是保存了对象的值给了一个新的对象
2. 关联对象没有弱引用的效果，使用时需要注意