AssociatedObject关联对象原理实现

介绍

关联对象(AssociatedObject)是Objective-C 2.0运行时的一个特性,允许开发者对已经存在的类在扩展中添加自定义的属性。在实际生产过程中,比较常用的方式是给分类(Category)添加成员变量。

例子

#import <objc/runtime.h>

@interface NSObject (AssociatedObject)

@property (nonatomic, strong) id property;

@end

@implementation NSObject (AssociatedObject)
@dynamic property;

- (id)property {
    return objc_getAssociatedObject(self, _cmd);
}

- (void)setProperty:(NSString *)property {
    objc_setAssociatedObject(self, @selector(property), property, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

通过实现代码可以稍微分析下,objc_getAssociatedObject 拿着不变的指针地址(示例传入selector作为参数,实际是void*),从实例中获取需要的对象。objc_setAssociatedObject 根据传入的参数协议,保存指定的对象。

参数协议

typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
    OBJC_ASSOCIATION_ASSIGN = 0,           /**< Specifies a weak reference to the associated object. */
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object. The association is not made atomically. */
    OBJC_ASSOCIATION_COPY_NONATOMIC = 3,   /**< Specifies that the associated object is copied. The association is not made atomically. */
    OBJC_ASSOCIATION_RETAIN = 01401,       /**< Specifies a strong reference to the associated object. The association is made atomically. */
    OBJC_ASSOCIATION_COPY = 01403          /**< Specifies that the associated object is copied. The association is made atomically. */
};

其实这五个协议就是我们平时定义属性时使用的,需要注意的是,虽然苹果在注释中说 OBJC_ASSOCIATION_ASSIGN 相当于一个 weak reference,但其实等于 assign/unsafe_unretained

对于与weak的区别不在本文讨论范围内,浅显的区别在于变量释放后,weak 会把引用置空,unsafe_unretained会保留内存地址,一旦获取可能会野指针闪退。

总结

我们知道,如果类要添加变量,只有在objc_allocateClassPairobjc_registerClassPair之间addIvar。等类注册后,变量结构就不允许再被改变,这是为了防止两个相同类的实例拥有不同变量导致运行困惑。

那么在runtime时给实例添加变量,又不改变类内部变量结构,关联对象就是一个比较好的做法。


关联对象的实现

外部方法

//Sets an associated value for a given object using a given key and association policy.
void objc_setAssociatedObject(id object, const void * key, id value, objc_AssociationPolicy policy);

//Returns the value associated with a given object for a given key.
id objc_getAssociatedObject(id object, const void * key);

//Removes all associations for a given object.
void objc_removeAssociatedObjects(id object);

相比刚刚例子中的用法,多了一个objc_removeAssociatedObjects,那么可不可以用这个方法来删除不用的关联对象呢?

苹果的文档中解释说这个方法主要用来还原对象到类初始的状态,会移除所有的关联,包括其他模块添加的,因此应该用 objc_setAssociatedObject(..,nil,..) 的方式去卸载。


Setter实现

objc_setAssociatedObject实际调用的是_object_set_associative_reference

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);
    id new_value = value ? acquireValue(value, policy) : nil;
    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.associations());
        disguised_ptr_t disguised_object = DISGUISE(object);
        if (new_value) {
            // 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 {
            // 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);
}

内存管理

static id acquireValue(id value, uintptr_t policy) {
    switch (policy & 0xFF) {
    case OBJC_ASSOCIATION_SETTER_RETAIN:
        return objc_retain(value);
    case OBJC_ASSOCIATION_SETTER_COPY:
        return ((id(*)(id, SEL))objc_msgSend)(value, SEL_copy);
    }
    return value;
}

static void releaseValue(id value, uintptr_t policy) {
    if (policy & OBJC_ASSOCIATION_SETTER_RETAIN) {
        return objc_release(value);
    }
}

ObjcAssociation old_association(0, nil);
id new_value = value ? acquireValue(value, policy) : nil;
{
    old_association = ...
}
if (old_association.hasValue()) ReleaseValue()(old_association);

我们摘出与对象内存相关的代码仔细分析下,首先把新传入的对象,根据协议进行retain/copy,在赋值的过程中获取旧值,在方法结束前release


赋值

AssociationsManager manager;
AssociationsHashMap &associations(manager.associations());
disguised_ptr_t disguised_object = DISGUISE(object);
if (new_value) {
    //需要赋值
    AssociationsHashMap::iterator i = associations.find(disguised_object);
    if (i != associations.end()) {
        //找到了这个对象的关联表
        ObjectAssociationMap *refs = i->second;
        ObjectAssociationMap::iterator j = refs->find(key);
        if (j != refs->end()) {
            //找到了这个key的关联对象
            old_association = j->second;
            j->second = ObjcAssociation(policy, new_value);
        } else {
            //没找到,新增一个关联
            (*refs)[key] = ObjcAssociation(policy, new_value);
        }
    } else {
        //没找到,创建一个新的关联表
        ObjectAssociationMap *refs = new ObjectAssociationMap;
        associations[disguised_object] = refs;
        (*refs)[key] = ObjcAssociation(policy, new_value);
        object->setHasAssociatedObjects();
    }
}

先了解一下AssociationsManagerAssociationsHashMap

class AssociationsManager {
    static AssociationsHashMap *_map;
public:
    AssociationsHashMap &associations() {
        if (_map == NULL)
            _map = new AssociationsHashMap();
        return *_map;
    }
};

class AssociationsHashMap : public unordered_map<disguised_ptr_t, ObjectAssociationMap *, DisguisedPointerHash, DisguisedPointerEqual, AssociationsHashMapAllocator>;

class ObjectAssociationMap : public std::map<void *, ObjcAssociation, ObjectPointerLess, ObjectAssociationMapAllocator>;

AssociationsManager通过一个以指针地址为主键,值为关联表的哈希表,来管理应用内所有的关联对象。

首先以对象的指针地址去寻找关联表,再通过指定的键值查找关联关系,从而获取关联对象。

删除

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);
    }
}

和修改方法类似,找到关联关系后,执行哈希表的erase方法删除。


Getter实现

objc_getAssociatedObject实际调用的是_object_get_associative_reference

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;
}

查找哈希表的方法和Setter一样,区别在于如果策略中需要retain和autorelease的话,都需要处理。那么是怎么约定这些策略呢?

enum { 
    OBJC_ASSOCIATION_SETTER_ASSIGN      = 0,
    OBJC_ASSOCIATION_SETTER_RETAIN      = 1,
    OBJC_ASSOCIATION_SETTER_COPY        = 3,            // NOTE:  both bits are set, so we can simply test 1 bit in releaseValue below.
    OBJC_ASSOCIATION_GETTER_READ        = (0 << 8), 
    OBJC_ASSOCIATION_GETTER_RETAIN      = (1 << 8), 
    OBJC_ASSOCIATION_GETTER_AUTORELEASE = (2 << 8)
}; 

typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
    OBJC_ASSOCIATION_ASSIGN = 0,
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,
    OBJC_ASSOCIATION_COPY_NONATOMIC = 3,
    OBJC_ASSOCIATION_RETAIN = 01401, 
    OBJC_ASSOCIATION_COPY = 01403
};

OBJC_ASSOCIATION_RETAIN = 01401,其中01401开头是0,所以是八进制数字,翻译为二进制就是0000 0011 0000 0001,取位判断就是OBJC_ASSOCIATION_SETTER_RETAIN OBJC_ASSOCIATION_GETTER_RETAIN OBJC_ASSOCIATION_GETTER_AUTORELEASE

在保存的时候,需要retain,在获取的时候,需要先retain增加引用计数,再执行autorelease等待释放,从而实现原子性。

Remove实现

objc_removeAssociatedObjects会判断对象是否存在关联,然后再执行_object_set_associative_reference

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);
        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());
}

实现方式也可以看出为什么在介绍里不推荐使用,因为会遍历所有的关联对象,并且全部释放,可能会造成别的模块功能缺陷。

判断关联对象

比较有意思的是判断对象是否有关联对象的实现。

inline bool objc_object::hasAssociatedObjects()
{
    if (isTaggedPointer()) return true;
    if (isa.nonpointer) return isa.has_assoc;
    return true;
}
inline void objc_object::setHasAssociatedObjects()
{
    if (isTaggedPointer()) return;

 retry:
    isa_t oldisa = LoadExclusive(&isa.bits);
    isa_t newisa = oldisa;
    if (!newisa.nonpointer  ||  newisa.has_assoc) {
        ClearExclusive(&isa.bits);
        return;
    }
    newisa.has_assoc = true;
    if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry;
}

默认返回的结果都是true,只有在64位系统下,才保存一个标记位。这么处理我推测是为了加快释放周期速度,在析构对象时,会根据这个方法判断是否需要释放关联对象。试想如果每次都查询哈希表,执行效率必定会降低,不如都先通过,之后再做处理。

关于nonpointer不在本文介绍范围内,简单描述为在64位系统下,指针地址保存不仅仅为内存地址,还存有其他标记信息,包括本文涉及的has_assoc

taggedPointer是一种优化策略,把简单的数字或字符串信息直接保存在指针地址中,从而不申请额外内存加快运行效率。

总结

关联对象的实现不复杂,保存的方式为一个全局的哈希表,存取都通过查询表找到关联来执行。哈希表的特点就是牺牲空间换取时间,所以执行速度也可以保证。


问答

关联对象有什么应用?

关联对象可以在运行时给指定对象绑定一个有生命周期的变量。

1.由于不改变原类的实现,所以可以给原生类或者是打包的库进行扩展,一般配合Category实现完整的功能。

2.ObjC类定义的变量,由于runtime的特性,都会暴露到外部,使用关联对象可以隐藏关键变量,保证安全。

3.可以用于KVO,使用关联对象作为观察者,可以避免观察自身导致循环。

系统如何管理关联对象?

系统通过管理一个全局哈希表,通过对象指针地址和传递的固定参数地址来获取关联对象。根据setter传入的参数协议,来管理对象的生命周期。

其被释放的时候需要手动将其指针置空么?

当对象被释放时,如果设置的协议是OBJC_ASSOCIATION_ASSIGN,那么他的关联对象不会减少引用计数,其他的协议都会减少从而释放关联对象。

unsafe_unretain一般认为外部有对象控制,所以对象不用处理,因此不管什么协议,对象释放时都无需手动讲关联对象置空。

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