多线程核心数据有时返回零属性

我不熟悉核心数据。我有一个应用程序使用核心数据作为本地存储。写入和读取核心数据是由后台线程完成的。虽然这通常有效,但在极少数情况下,提取的数据是错误的,即提取的实体的属性是 nil .
为了检查这种情况,我编写了一个启动2个异步线程的单元测试:一个从核心数据中连续提取,另一个通过先删除所有数据,然后存储新数据来连续覆盖这些数据。
这个测试很快就会引发错误,但我不知道为什么。当然,我猜这是一个多线程的问题,但我不明白为什么,因为提取和删除+写入是在单个线程的单独管理上下文中完成的。 persistentContainer .
很抱歉,下面的代码很长,虽然被缩短了,但我认为没有它,就无法识别问题。
任何帮助都非常欢迎!

以下是我获取数据的函数:

func fetchShoppingItems(completion: @escaping (Set<ShoppingItem>?, Error?) -> Void) {
    persistentContainer.performBackgroundTask { (managedContext) in 
        let fetchRequest: NSFetchRequest<CDShoppingItem> = CDShoppingItem.fetchRequest()
        do {
            let cdShoppingItems: [CDShoppingItem] = try managedContext.fetch(fetchRequest)
            for nextCdShoppingItem in cdShoppingItems {
                nextCdShoppingItem.managedObjectContext!.performAndWait {
                    let nextname = nextCdShoppingItem.name! // Here, sometimes name is nil
                } // performAndWait
            } // for all cdShoppingItems
            completion(nil, nil)
            return
        } catch let error as NSError {
            // error handling
            completion(nil, error)
            return
        } // fetch error
    } // performBackgroundTask
} // fetchShoppingItems

我已经评论了这句话,它有时会使测试失败,因为 name 是 零 .

以下是我存储数据的功能:

func overwriteCD(shoppingItems: Set<ShoppingItem>,completion: @escaping () -> Void) {
    persistentContainer.performBackgroundTask { (managedContext) in 
        self.deleteAllCDRecords(managedContext: managedContext, in: "CDShoppingItem")
        let cdShoppingItemEntity = NSEntityDescription.entity(forEntityName: "CDShoppingItem",in: managedContext)!
        for nextShoppingItem in shoppingItems {
            let nextCdShoppingItem = CDShoppingItem(entity: cdShoppingItemEntity,insertInto: managedContext)
            nextCdShoppingItem.name = nextShoppingItem.name
        } // for all shopping items
        self.saveManagedContext(managedContext: managedContext)
        completion()
    } // performBackgroundTask
} // overwriteCD  

func deleteAllCDRecords(managedContext: NSManagedObjectContext, in entity: String) {
    let deleteFetch = NSFetchRequest<NSFetchRequestResult>(entityName: entity)
    let deleteRequest = NSBatchDeleteRequest(fetchRequest: deleteFetch)
    deleteRequest.resultType = .resultTypeObjectIDs
    do {
        let result = try managedContext.execute(deleteRequest) as? NSBatchDeleteResult
        let objectIDArray = result?.result as? [NSManagedObjectID]
        let changes = [NSDeletedObjectsKey: objectIDArray]
        NSManagedObjectContext.mergeChanges(fromRemoteContextSave: changes as [AnyHashable: Any], into: [managedContext])
    } catch let error as NSError {
        // error handling
    }
} // deleteAllCDRecords

func saveManagedContext(managedContext: NSManagedObjectContext) {
    if !managedContext.hasChanges { return }
    do {
        try managedContext.save()
    } catch let error as NSError {
        // error handling
    }
} // saveManagedContext