Runtime初探(二)

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关联对象

在分类中可以定义property，但是编译器并不会@synthesize实例变量。一般有需求添加实例变量属性时会采用objc_setAssociatedObject和objc_getAssociatedObject方法绑定方法绑定，不过这种方法生成的与一个普通的实例变量完全是两码事。

objc_setAssociatedObject

分析源码后发现苹果为分类的属性建造了一个大的hashmap，这个大的hashmap的里面又根据对象划分了多个子hashmap，划分的依照object来划分。每个子hashmap中存放了各个分类添加的属性。

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

发现内部调用了_object_set_associative_reference

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void _object_set_associative_reference(id object, void *key, id value, uintptr_t policy) {
    if (!object && !value) return; //如果没有 需要被关联的对象和值则返回

    ObjcAssociation old_association(0, nil); // 旧值
    id new_value = value ? acquireValue(value, policy) : nil; // 新值
    {
        AssociationsManager manager;  // 全局管理关联的单例
        AssociationsHashMap &associations(manager.associations());  // 通过单例获取 hashmap
        disguised_ptr_t disguised_object = DISGUISE(object);  // 根据对象生成唯一 key
        if (new_value) { // 如果有新值 表示用新值 替换旧值
            // 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 { // 没有新值 表示需要移除已经存在的键值对
            // 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);
}

_object_get_associative_reference

取值比较简单，就是根据key和object对象进行遍历

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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;
}