弱引用管理相关

屏幕快照 2018-11-17 下午11.00.18.png

objc_initweak() 函数具体实现是怎样的？

添加weak 变量

屏幕快照 2018-11-17 下午11.02.11.png

id
objc_initWeak(id *location, id newObj)
{
    if (!newObj) {
        *location = nil;
        return nil;
    }

    return storeWeak<false/*old*/, true/*new*/, true/*crash*/>
        (location, (objc_object*)newObj);
}

template <bool HaveOld, bool HaveNew, bool CrashIfDeallocating>
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<HaveOld, HaveNew>(oldTable, newTable);

    if (HaveOld  &&  *location != oldObj) {
        SideTable::unlockTwo<HaveOld, HaveNew>(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<HaveOld, HaveNew>(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<HaveOld, HaveNew>(oldTable, newTable);

    return (id)newObj;
}

 

添加weak 变量流程：
一个被声明 __weak 变量的对象指针 经过 objc_initweak()经过一系列函数调用栈最终在 weak_register_no_lock()函数进行弱引用变量添加，添加位置是通过哈希算法来进行位置查找，如果经过查找对应位置有当前对象所对应的弱引用数组，就把新的弱引用变量添加到数组当中 ，没有新创建弱引用数组第一位 其他添加为nil

清除weak 变量流程：
当一个对象被dealloc 执行 weka_clear_no_lock(),再函数内部根据当前对象指针查找弱引用表,把当前对象相对应的弱引用都取出是个数组，遍历数组分别置为nil