weak引用表原理探究

一、weak引用实现原理探究

　　首先对《Xcode 10 下如何调试objc4-723》建立的objc源码调试工程表示感谢！

　　地址：https://www.jianshu.com/p/9e0fc8295c4b

　　大多数文章阐述了基本过程:

1.初始化一个weak对象时，runtime会调用一个objc_initWeak函数，初始化一个新的weak指针指向该对象的地址

2.在objc_initWeak函数中会继续调用objc_storeWeak函数，在这个过程是用来更新weak指针的指向，同时创建对应的弱引用表

3.在对象释放时，会调用clearDeallocating函数，这个函数会根据对象地址获取所有weak指针数组，然后遍历这个数组置为nil。最后把该条对象的记录从weak表中删除。

id objc_initWeak(id *location, id newObj) {
    // 查看对象实例是否有效
    // 无效对象直接导致指针释放
    if (!newObj) {
        *location = nil;
        return nil;
    }
    // 这里传递了三个 bool 数值
    // 使用 template 进行常量参数传递是为了优化性能
    return storeWeak
        (location, (objc_object*)newObj);
}

template 
static id 
storeWeak(id *location, objc_object *newObj)
{
    assert(HaveOld  ||  HaveNew);
    if (!HaveNew) assert(newObj == nil);

    Class previouslyInitializedClass = nil;
    id oldObj;
    SideTable *oldTable;
    SideTable *newTable;

    // Acquire locks for old and new values.
    // Order by lock address to prevent lock ordering problems. 
    // Retry if the old value changes underneath us.
 retry:
    if (HaveOld) {
        oldObj = *location;
        oldTable = &SideTables()[oldObj];
    } else {
        oldTable = nil;
    }
    if (HaveNew) {
        newTable = &SideTables()[newObj];
    } else {
        newTable = nil;
    }

    SideTable::lockTwo(oldTable, newTable);

    if (HaveOld  &&  *location != oldObj) {
        SideTable::unlockTwo(oldTable, newTable);
        goto retry;
    }

    // Prevent a deadlock between the weak reference machinery
    // and the +initialize machinery by ensuring that no 
    // weakly-referenced object has an un-+initialized isa.
    if (HaveNew  &&  newObj) {
        Class cls = newObj->getIsa();
        if (cls != previouslyInitializedClass  &&  
            !((objc_class *)cls)->isInitialized()) 
        {
            SideTable::unlockTwo(oldTable, newTable);
            _class_initialize(_class_getNonMetaClass(cls, (id)newObj));

            // If this class is finished with +initialize then we're good.
            // If this class is still running +initialize on this thread 
            // (i.e. +initialize called storeWeak on an instance of itself)
            // then we may proceed but it will appear initializing and 
            // not yet initialized to the check above.
            // Instead set previouslyInitializedClass to recognize it on retry.
            previouslyInitializedClass = cls;

            goto retry;
        }
    }

    // Clean up old value, if any.
    if (HaveOld) {
        weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
    }

    // Assign new value, if any.
    if (HaveNew) {
        newObj = (objc_object *)weak_register_no_lock(&newTable->weak_table, 
                                                      (id)newObj, location, 
                                                      CrashIfDeallocating);
        // weak_register_no_lock returns nil if weak store should be rejected

        // Set is-weakly-referenced bit in refcount table.
        if (newObj  &&  !newObj->isTaggedPointer()) {
            newObj->setWeaklyReferenced_nolock();
        }

        // Do not set *location anywhere else. That would introduce a race.
        *location = (id)newObj;
    }
    else {
        // No new value. The storage is not changed.
    }
    
    SideTable::unlockTwo(oldTable, newTable);

    return (id)newObj;
}

　　其中涉及到一个数据结构

struct SideTable {
    spinlock_t slock; // 因为操作对象的引用计数频率很快，因此系统在这里设置了一把自旋锁，保证是原子操作
    RefcountMap refcnts; // 引用计数器哈希表,根据对象地址查找对象的引用计数
    weak_table_t weak_table; // 维护weak指针的结构体
}

　　通过下面的代码取得

　　也就是全局的sidetables本身是一个hash表，总共大小为64；每一个value对应的是 sidetable，sidetable中保存引用计数表和weak引用表

 　　找到一个sidetable表之后，要根据weak所指对象的地址hash值，找到对应存储weak指针的value结构体

　　接下来的操作就是修改weak引用表了