OC中给分类添加成员变量

在OC中，由于分类底层结构的限制，并不能添加成员变量到分类中，但是可以通过关联对象实现。

涉及到的相关api

//添加关联对象
// objece 需要添加关联对象的对象
// key     用来去关联值得key
// value  关联的值
// policy  政策(retain、assin)
void objc_setAssociatedObject(id object, const void * key,
                                id value, objc_AssociationPolicy policy)

// 获取关联对象
// object 需要获取关联对象的对象
// key     用来取关联对象的key
id objc_getAssociatedObject(id object, const void * key)

// 溢出对象的所有关联对象
void objc_removeAssociatedObjects(id object)

实现为分类添加"属性的代码如下":

#import <Foundation/Foundation.h>
#import "LBPerson.h"
#import "LBPerson+Associate.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        LBPerson *person = [[LBPerson alloc] init];
        person.name = @"muzi";
        NSLog(@"%@",person.name);
    }
    return 0;
}

#import "LBPerson.h"

NS_ASSUME_NONNULL_BEGIN

@interface LBPerson (Associate)
@property (nonatomic, copy) NSString *name;
@end

#import "LBPerson+Associate.h"
#import <objc/runtime.h>

@implementation LBPerson (Associate)
- (void)setName:(NSString *)name {
//    OBJC_ASSOCIATION_ASSIGN = 0,            assign
//    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,  strong, nonatomic
//    OBJC_ASSOCIATION_COPY_NONATOMIC = 3,    copy, nonatomic
//    OBJC_ASSOCIATION_RETAIN = 01401,        strong, atomic
//    OBJC_ASSOCIATION_COPY = 01403           copy, atomic
    objc_setAssociatedObject(self, @"name", name, OBJC_ASSOCIATION_COPY);
}

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

设置关联对象的源码如下:

typedef DenseMap<const void *, ObjcAssociation> ObjectAssociationMap;
typedef DenseMap<DisguisedPtr<objc_object>, ObjectAssociationMap> AssociationsHashMap;

// class AssociationsManager manages a lock / hash table singleton pair.
// Allocating an instance acquires the lock

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

public:
    AssociationsManager()   { AssociationsManagerLock.lock(); }
    ~AssociationsManager()  { AssociationsManagerLock.unlock(); }

    AssociationsHashMap &get() {
        return _mapStorage.get();
    }

    static void init() {
        _mapStorage.init();
    }
};

AssociationsManager::Storage AssociationsManager::_mapStorage;

} // namespace objc

using namespace objc;

void
_objc_associations_init()
{
    AssociationsManager::init();
}

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

    {
        AssociationsManager manager;                      // 获取AssociationsManaget的对象
        AssociationsHashMap &associations(manager.get()); // return _mapStorage.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();
}

// Unlike setting/getting an associated reference,
// this function is performance sensitive because of
// raw isa objects (such as OS Objects) that can't track
// whether they have associated objects.
void
_object_remove_assocations(id object)
{
    ObjectAssociationMap refs{};

    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.get());
        AssociationsHashMap::iterator i = associations.find((objc_object *)object);
        if (i != associations.end()) {
            refs.swap(i->second);
            associations.erase(i);
        }
    }

    // release everything (outside of the lock).
    for (auto &i: refs) {
        i.second.releaseHeldValue();
    }
}

本来还想注释一波源码，奈何C++学的太菜，大致总结如下:

与关联对象有关的四个

AssociationsManager
AssociationsHashMap
ObjectAssociationMap
ObjcAssociation

大致调用，先取出mannager中的AssociationsHashMap，然后再通过DisguisedPtr作key从AssociationsHashMap取出ObjectAssociationMap，再用key取ObjcAssociation

，ObjcAssociation中放的就是策略和报错的关联值，顺序如下图所示: