系统底层源码分析(9)——Category(分类)关联属性

之前了解了分类的编译与运行和一些知识，其中提到分类不能直接加属性，今天就来看看分类怎么添加属性的，直接看例子：

#import <objc/runtime.h>

@implementation Person (Test)

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

- (NSString*)name{
    return objc_getAssociatedObject(self, @"NameKey");
}

@end

• 设置

1. 从objc4-750源码探究，先来看设置：

//通过 key : value 的形式给对象 object 设置关联属性
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
    _object_set_associative_reference(object, (void *)key, value, policy);
}

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);//每个关联属性的键值队----值和策略
    
    id new_value = value ? acquireValue(value, policy) : nil;//进行内存管理
    {
        AssociationsManager manager;//管理所有关联的属性的类，全局的
        AssociationsHashMap &associations(manager.associations());//初始化 HashMap
        
        disguised_ptr_t disguised_object = DISGUISE(object);//当前对象的地址按位取反后作为key
        if (new_value) {
            // break any existing association.
            AssociationsHashMap::iterator i = associations.find(disguised_object);//<fisrt : disguised_object , second : ObjectAssociationMap> 类似于元组，存着所有关联属性的键值对
            if (i != associations.end()) {
                // secondary table exists
                ObjectAssociationMap *refs = i->second;//一个对象的所有关联属性键值对
                ObjectAssociationMap::iterator j = refs->find(key);//<fisrt : 标识（自定义的） , second : ObjcAssociation>
                if (j != refs->end()) {
                    old_association = j->second;//新值覆盖
                    j->second = ObjcAssociation(policy, new_value);//ObjcAssociation存放关联属性
                } 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();//设置当前的isa指针
            }
        } else { ... }
    }
    // release the old value (outside of the lock).
    if (old_association.hasValue()) ReleaseValue()(old_association);
}

AssociationsManager里存着AssciationsHashMap，
AssciationsHashMap里存着<fisrt : disguised_ptr_t , second : ObjectAssociationMap>，
ObjectAssociationMap里存着<fisrt : 标识（自定义的） , second : ObjcAssociation>，
ObjcAssociation里存着策略policy和值value。
所以最终关联属性就是存放在ObjcAssociation里。

• 获取

2. 然后获取自然也是从ObjcAssociation类里获取：

//通过 key 获取关联的属性 object
id objc_getAssociatedObject(id object, const void *key) {
    return _object_get_associative_reference(object, (void *)key);
}

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);//取出
        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;
}

• 销毁

3. 当对象销毁时，关联属性也随之销毁：

- (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);//有关联对象时走这
    }
}

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);//移除关联属性
        obj->clearDeallocating();
    }

    return obj;
}

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);//key
        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());//移除
}

4. 当然也可以手动移除：

//移除对象所关联的属性
void objc_removeAssociatedObjects(id object) 
{
    if (object && object->hasAssociatedObjects()) {
        _object_remove_assocations(object);
    }
}

分类关联属性其实就是利用它的AssociationsManager管理保存在ObjcAssociation里的关联属性。