十四、关联对象

分类与类扩展

1. category∶类别，分类

• 专门用来给类添加新的方法
• 不能给类添加成员属性，添加了成员变量，也无法取到
• 注意∶其实可以通过runtime给分类添加属性
• 分类中用 @property 定义变量，只会生成变量的 getter，setter 方法的声明，不能生成方法实现和带下划线的成员变量。

2. extension∶类扩展

• 可以说成是特殊的分类，也称作匿名分类
• 可以给类添加成员属性，但是是私有变量
• 可以给类添加方法，也是私有方法

在分类中我们只能通过runtime给分类添加属性

- (void)setCate_name:(NSString *)cate_name{
    /**
     1: 对象
     2: 标识符
     3: value
     4: 策略
     */
    objc_setAssociatedObject(self, "cate_name", cate_name, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

- (NSString *)cate_name{
    return  objc_getAssociatedObject(self, "cate_name");
}

objc_setAssociatedObject到底做了什么呢，我们随着源码一探究竟

void
objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
{
    SetAssocHook.get()(object, key, value, policy);
}

static ChainedHookFunction<objc_hook_setAssociatedObject> SetAssocHook{_base_objc_setAssociatedObject};

static void
_base_objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
{
  _object_set_associative_reference(object, key, value, policy);
}

我们最好找到了方法实现的地方_object_set_associative_reference

void
_object_set_associative_reference(id object, const void *key, id value, uintptr_t policy)
{
    // This code used to work when nil was passed for object and key. Some code
    // probably relies on that to not crash. Check and handle it explicitly.
    // rdar://problem/44094390
    if (!object && !value) return;

    if (object->getIsa()->forbidsAssociatedObjects())
        _objc_fatal("objc_setAssociatedObject called on instance (%p) of class %s which does not allow associated objects", object, object_getClassName(object));
    // 包装了一下 对象
    DisguisedPtr<objc_object> disguised{(objc_object *)object};
    // 包装一下 policy - value
    ObjcAssociation association{policy, value};

    // retain the new value (if any) outside the lock.
    association.acquireValue();

    {
        AssociationsManager manager;
    
        AssociationsHashMap &associations(manager.get());

        if (value) {
            auto refs_result = associations.try_emplace(disguised, ObjectAssociationMap{});
            if (refs_result.second) {
                /* it's the first association we make */
                object->setHasAssociatedObjects();
            }

            /* establish or replace the association */
            auto &refs = refs_result.first->second; // 空的桶子
            auto result = refs.try_emplace(key, std::move(association));向空的桶子里插入
            if (!result.second) {
                association.swap(result.first->second);
            }
        } else {//value没有
            auto refs_it = associations.find(disguised);
            if (refs_it != associations.end()) {
                auto &refs = refs_it->second;
                auto it = refs.find(key);
                if (it != refs.end()) {
                    association.swap(it->second);
                    refs.erase(it);//消除
                    if (refs.size() == 0) {
                        associations.erase(refs_it);//消除

                    }
                }
            }
        }
    }

    // release the old value (outside of the lock).
    association.releaseHeldValue();
}

disguised就是对object进行了包装一下，association对policy 和 value进行了包装

    inline void acquireValue() {
        if (_value) {
            switch (_policy & 0xFF) {
            case OBJC_ASSOCIATION_SETTER_RETAIN:
                _value = objc_retain(_value);
                break;
            case OBJC_ASSOCIATION_SETTER_COPY:
                _value = ((id(*)(id, SEL))objc_msgSend)(_value, @selector(copy));
                break;
            }
        }
    }

在acquireValue对value 根据policy进行了处理

class AssociationsManager {
    using Storage = ExplicitInitDenseMap<DisguisedPtr<objc_object>, ObjectAssociationMap>;
    static Storage _mapStorage;

public:
    AssociationsManager()   { AssociationsManagerLock.lock(); }
    ~AssociationsManager()  { AssociationsManagerLock.unlock(); }

    AssociationsHashMap &get() {
        return _mapStorage.get();
    }

    static void init() {
        _mapStorage.init();
    }
};

manager只是加了个锁，而associations是从manager. get()获取的，_mapStorage是一个静态变量，所以AssociationsHashMap获取同一个表

  template <typename... Ts>
  std::pair<iterator, bool> try_emplace(const KeyT &Key, Ts &&... Args) {
    BucketT *TheBucket;
    if (LookupBucketFor(Key, TheBucket))  // 找桶子
      return std::make_pair(
               makeIterator(TheBucket, getBucketsEnd(), true),
               false); // Already in map.

    // Otherwise, insert the new element.
    TheBucket = InsertIntoBucket(TheBucket, Key, std::forward<Ts>(Args)...);
    return std::make_pair(
             makeIterator(TheBucket, getBucketsEnd(), true),
             true);
  }

定义个空的桶子，首先去寻找，找到，就返回false，没找到，插入个桶子返回true

        if (value) {
            auto refs_result = associations.try_emplace(disguised, ObjectAssociationMap{});//先查询总表
            if (refs_result.second) {
                /* it's the first association we make */
                object->setHasAssociatedObjects();
            }

            /* establish or replace the association */
            auto &refs = refs_result.first->second; // 空的桶子
            auto result = refs.try_emplace(key, std::move(association));向空的桶子里插入
            if (!result.second) {
                association.swap(result.first->second);
            }
        } 

bucket.png

请问关联对象是否需要移除

- (void)dealloc {
    _objc_rootDealloc(self);
}

void
_objc_rootDealloc(id obj)
{
    ASSERT(obj);

    obj->rootDealloc();
}

inline void
objc_object::rootDealloc()
{
    if (isTaggedPointer()) return;  // fixme necessary?

    if (fastpath(isa.nonpointer  &&  
                 !isa.weakly_referenced  &&  
                 !isa.has_assoc  &&  
                 !isa.has_cxx_dtor  &&  
                 !isa.has_sidetable_rc))
    {
        assert(!sidetable_present());
        free(this);
    } 
    else {
        object_dispose((id)this);
    }
}

id 
object_dispose(id obj)
{
    if (!obj) return nil;

    objc_destructInstance(obj);    
    free(obj);

    return nil;
}
void *objc_destructInstance(id obj) 
{
    if (obj) {
        // Read all of the flags at once for performance.
        bool cxx = obj->hasCxxDtor();
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        if (cxx) object_cxxDestruct(obj);
        if (assoc) _object_remove_assocations(obj);
        obj->clearDeallocating();
    }

    return obj;
}

void
_object_remove_assocations(id object)
{
    ObjectAssociationMap refs{};

    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.get());
        AssociationsHashMap::iterator i = associations.find((objc_object *)object);
        if (i != associations.end()) {
            refs.swap(i->second);
            associations.erase(i);
        }
    }

    // release everything (outside of the lock).
    for (auto &i: refs) {
        i.second.releaseHeldValue();
    }
}

析构的时候都会移除掉