简单应用

关联对象常用的API如下：

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

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。

@interface Person (Extension)

@property (copy, nonatomic) NSString *name;

@end

@implementation Person (Extension)

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

- (NSString *)name {
    return objc_getAssociatedObject(self, _cmd);
}

关联对象的原理

实现关联对象技术的核心对象有

• AssociationsManager

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

• AssociationsHashMap

typedef DenseMap<DisguisedPtr<objc_object>, ObjectAssociationMap> AssociationsHashMap;

• ObjectAssociationMap

typedef DenseMap<const void *, ObjcAssociation> ObjectAssociationMap;

• ObjcAssociation

class ObjcAssociation {
    uintptr_t _policy;
    id _value;
}

关联对象的原理：

image.png

Runtime源码分析

id
_object_get_associative_reference(id object, const void *key)
{
    ObjcAssociation association{};

    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.get());
        AssociationsHashMap::iterator i = associations.find((objc_object *)object);
        if (i != associations.end()) {
            ObjectAssociationMap &refs = i->second;
            ObjectAssociationMap::iterator j = refs.find(key);
            if (j != refs.end()) {
                association = j->second;
                association.retainReturnedValue();
            }
        }
    }

    return association.autoreleaseReturnedValue();
}


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));

    DisguisedPtr<objc_object> disguised{(objc_object *)object};
    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;
            auto result = refs.try_emplace(key, std::move(association));
            if (!result.second) {
                association.swap(result.first->second);
            }
        } else {
            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);
                    if (refs.size() == 0) {
                        associations.erase(refs_it);

                    }
                }
            }
        }
    }

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

关联对象实现weak

associated object 本身并不支持添加具备 weak 特性的 property，但我们可以通过一个小技巧来完成，下面是一个简单的实现代码

- (void)setKey:(NSObject*)object {
    id __weak weakObject = object;
    id (^block)(void) = ^{ return weakObject; };
    objc_setAssociatedObject(self, @selector(key), block, OBJC_ASSOCIATION_COPY);
}

- (NSObject*)key {
    id (^block)(void) = objc_getAssociatedObject(self, _cmd);
    id curContext = block ? block() : nil;
    return curContext;
}