关联对象

  • 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

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

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

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

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))

objc_AssociationPolicy，关联策略

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objc_AssociationPolicy                    对应的修饰符
OBJC_ASSOCIATION_ASSIGN                        assign
OBJC_ASSOCIATION_RETAIN_NONATOMIC       strong, nonatomic
OBJC_ASSOCIATION_COPY_NONATOMIC         copy, nonatomic
OBJC_ASSOCIATION_RETAIN                    strong, atomic
OBJC_ASSOCIATION_COPY                    copy, atomic

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：objc-references.mm

AssociationsManager

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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;
    }
};

AssociationsHashMap

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class AssociationsHashMap : public unordered_map<disguised_ptr_t, ObjectAssociationMap *, DisguisedPointerHash, DisguisedPointerEqual, AssociationsHashMapAllocator> {
public:
    void *operator new(size_t n) { return ::malloc(n); }
    void operator delete(void *ptr) { ::free(ptr); }
};

ObjectAssociationMap

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class ObjectAssociationMap : public std::map<void *, ObjcAssociation, ObjectPointerLess, ObjectAssociationMapAllocator> {
public:
    void *operator new(size_t n) { return ::malloc(n); }
    void operator delete(void *ptr) { ::free(ptr); }
};

ObjcAssociation

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class ObjcAssociation {
    
    uintptr_t _policy;
    id _value;
    
public:
    ObjcAssociation(uintptr_t policy, id value) : _policy(policy), _value(value) {}
    ObjcAssociation() : _policy(0), _value(nil) {}

    uintptr_t policy() const { return _policy; }
    id value() const { return _value; }
    
    bool hasValue() { return _value != nil; }
};

notes:

  • 关联对象并不是存储在被关联对象本身内存中
  • 关联对象存储在全局的统一的一个AssociationsManager中
  • 设置关联对象为nil，就相当于是移除关联对象
  • 即为以下的关系：
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    AssociationsManager
    AssociationsHashMap *_map;

    AssociationsHashMap
    disguised_ptr_t    ObjectAssociationMap
    disguised_ptr_t    ObjectAssociationMap
    ...    ...

    AssociationsMap
    void *    ObjectAssociation
    void *    ObjectAssociation
    ...    ...

    ObjectAssociation
    uintptr_t _policy;
    id _value;

参考