关联对象

分类中不能添加成员变量

默认情况下，因为分类底层结构的限制，分类中不能添加成员变量。如果强行添加会报错“Instance variables may not be placed in categories实例变量不能放在类别中实例变量不能放在类别中”如下图

image.png
如果通过@property (assign, nonatomic) int a;不会添加成员变量，只会生成(没有实现设值和取值)的setter和getter方法.

通过关联对象变相的给分类“添加成员变量”

设置关联对象

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

获取关联对象

objc_getAssociatedObject(id object, const void * key)

移除类所有的关联对象

objc_removeAssociatedObjects(id object)

key的常见用法,保证key的唯一性

取唯一的指针地址值
static void *MyKey = &MyKey;
objc_setAssociatedObject(obj, MyKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC)
objc_getAssociatedObject(obj, MyKey)
static NSString *MyKey1;
objc_setAssociatedObject(obj, &MyKey1, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC)
objc_getAssociatedObject(obj, &MyKey1)
使用属性名作为key
objc_setAssociatedObject(obj, @"property", value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
objc_getAssociatedObject(obj, @"property");
使用get方法的方法编号@selecor作为key
objc_setAssociatedObject(obj, @selector(getter), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC)
objc_getAssociatedObject(obj, @selector(getter))

objc_AssociationPolicy

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

objc4源码解读`objc_setAssociatedObject(id object, const void * key, id value, objc_AssociationPolicy policy)

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) {
    // 初始化第三个表。
    ObjcAssociation old_association(0, nil);
    id new_value = value ? acquireValue(value, policy) : nil;
    {
        AssociationsManager manager;
        第一个表AssociationsHashMap
        AssociationsHashMap &associations(manager.associations());
        disguised_ptr_t disguised_object = DISGUISE(object);//object作为key
        if (new_value) {
  
            //通过disguised_object取value值ObjectAssociationMap
            AssociationsHashMap::iterator i = associations.find(disguised_object);
            if (i != associations.end()) {//i不是最后一个
                // 第二个表ObjectAssociationMap类表
                ObjectAssociationMap *refs = i->second;
                //ObjectAssociationMap对象表中通过key取出value值ObjcAssociation
                ObjectAssociationMap::iterator j = refs->find(key);
                if (j != refs->end()) {//j不是最后一个
                     //第三个表，ObjcAssociation类表
                    old_association = j->second;
                    //第三个表设置新值
                    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 {
            // 设置关联为nil会移除关联值。
            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);//擦除
                }
            }
        }
    }
    // release the old value (outside of the lock).
    if (old_association.hasValue()) ReleaseValue()(old_association);
}

实现关联对象技术的核心对象有

AssociationsManager
AssociationsHashMap//类表1
ObjectAssociationMap//类表2
ObjcAssociatio//类表3

class AssociationsManager {
    static AssociationsHashMap *_map
};
类表1
class AssociationsHashMap : public unordered_map<disguised_ptr_t(key), ObjectAssociationMap *(value)>
类表2
class ObjectAssociationMap : public std::map<void *(key), ObjcAssociation(value)>
类表3
class ObjcAssociation {
        uintptr_t _policy;//key
        id _value;//value
};

objc4源码解读：objc-references.mm

原理图解： image.png