使用 NSPersistentCloudKitContainer 时预填充核心数据存储的最佳方法是什么？

Question

我遇到以下情况，我从 JSON 文件中解析对象并将它们存储到我的核心数据存储中。现在我正在使用 NSPersistentCloudKitContainer，当我在另一台设备上运行应用程序时，它还会解析 JSON 文件并将对象添加到核心数据。这会导致重复对象。

现在想知道有没有：

如果我可以检查一个实体是否已经远程存在，这是一种简单的方法吗？
还有其他方法可以避免对象在 CloudKit 中被保存两次吗？
从远程获取数据完成时收到通知？

Answer 1

也许现在回答为时已晚，但我最近正在处理同样的问题。经过几周的研究，我想把我学到的东西留在这里，希望能帮助遇到同样问题的人。

An easy way if I can check that an entity already exists remotely?

Any other way to avoid objects being saved twice in CloudKit?

是的，我们可以检查 iCloud 上是否已经存在该实体，但这不是决定是否解析 JSON 文件并将其保存到 CoreData persistentStore 的最佳方法。该应用程序可能未连接到 Apple ID / iCloud，或者存在一些网络问题，导致无法可靠地检查该实体是否存在于远程。

目前的解决方案是我们自己对数据进行去重，方法是为每个从 JSON 文件添加的数据 object 添加一个 UUID 字段，并删除具有相同 UUID 的 object . 大多数时候我还会添加一个 lastUpdate 字段，这样我们就可以保留最新的数据 object.

Getting notified when fetching data from remote has finished?

我们可以添加 NSPersistentStoreRemoteChange 的观察者，并在远程存储发生变化时收到通知。

Apple 提供了一个使用 CoreData 和 CloudKit 的演示项目，并很好地解释了重复数据删除。

将本地商店同步到云端 https://developer.apple.com/documentation/coredata/synchronizing_a_local_store_to_the_cloud

WWDC2019 session 202：将 CoreData 与 CloudKit 结合使用 https://developer.apple.com/videos/play/wwdc2019/202

整个想法是侦听远程存储中的更改，跟踪更改历史记录，并在有任何新数据进入时对我们的数据进行重复数据删除。（当然我们需要一些字段来确定数据是否重复）。持久存储提供历史跟踪功能，我们可以在它们合并到本地存储时获取这些事务，以及运行我们的重复数据删除过程。假设我们将在应用启动时解析 JSON 并导入标签：

// Use a custom queue to ensure only one process of history handling at the same time
private lazy var historyQueue: OperationQueue = {
    let queue = OperationQueue()
    queue.maxConcurrentOperationCount = 1
    return queue
}()

lazy var persistentContainer: NSPersistentContainer = {
    let container = NSPersistentCloudKitContainer(name: "CoreDataCloudKitDemo")

    ...
    // set the persistentStoreDescription to track history and generate notificaiton (NSPersistentHistoryTrackingKey, NSPersistentStoreRemoteChangeNotificationPostOptionKey)
    // load the persistentStores
    // set the mergePolicy of the viewContext
    ...

    // Observe Core Data remote change notifications.
    NotificationCenter.default.addObserver(
        self, selector: #selector(type(of: self).storeRemoteChange(_:)),
        name: .NSPersistentStoreRemoteChange, object: container.persistentStoreCoordinator)

    return container
}()

@objc func storeRemoteChange(_ notification: Notification) {
    // Process persistent history to merge changes from other coordinators.
    historyQueue.addOperation {
        self.processPersistentHistory()
    }
}

// To fetch change since last update, deduplicate if any new insert data, and save the updated token
private func processPersistentHistory() {
    // run in a background context and not blocking the view context.
    // when background context is saved, it will merge to the view context based on the merge policy
    let taskContext = persistentContainer.newBackgroundContext()
    taskContext.performAndWait {
        // Fetch history received from outside the app since the last token
        let historyFetchRequest = NSPersistentHistoryTransaction.fetchRequest!
        let request = NSPersistentHistoryChangeRequest.fetchHistory(after: lastHistoryToken)
        request.fetchRequest = historyFetchRequest

        let result = (try? taskContext.execute(request)) as? NSPersistentHistoryResult
        guard let transactions = result?.result as? [NSPersistentHistoryTransaction],
              !transactions.isEmpty
            else { return }

        // Tags from remote store
        var newTagObjectIDs = [NSManagedObjectID]()
        let tagEntityName = Tag.entity().name

        // Append those .insert change in the trasactions that we want to deduplicate
        for transaction in transactions where transaction.changes != nil {
            for change in transaction.changes!
                where change.changedObjectID.entity.name == tagEntityName && change.changeType == .insert {
                    newTagObjectIDs.append(change.changedObjectID)
            }
        }

        if !newTagObjectIDs.isEmpty {
            deduplicateAndWait(tagObjectIDs: newTagObjectIDs)
        }
        
        // Update the history token using the last transaction.
        lastHistoryToken = transactions.last!.token
    }
}

我们在这里保存添加标签的 ObjectID，以便我们可以在任何其他 object 上下文中对它们进行重复数据删除，

private func deduplicateAndWait(tagObjectIDs: [NSManagedObjectID]) {
    let taskContext = persistentContainer.backgroundContext()
    
    // Use performAndWait because each step relies on the sequence. Since historyQueue runs in the background, waiting won’t block the main queue.
    taskContext.performAndWait {
        tagObjectIDs.forEach { tagObjectID in
            self.deduplicate(tagObjectID: tagObjectID, performingContext: taskContext)
        }
        // Save the background context to trigger a notification and merge the result into the viewContext.
        taskContext.save(with: .deduplicate)
    }
}

private func deduplicate(tagObjectID: NSManagedObjectID, performingContext: NSManagedObjectContext) {
    // Get tag by the objectID
    guard let tag = performingContext.object(with: tagObjectID) as? Tag,
        let tagUUID = tag.uuid else {
        fatalError("###\(#function): Failed to retrieve a valid tag with ID: \(tagObjectID)")
    }

    // Fetch all tags with the same uuid
    let fetchRequest: NSFetchRequest<Tag> = Tag.fetchRequest()
    // Sort by lastUpdate, keep the latest Tag
    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "lastUpdate", ascending: false)]

    fetchRequest.predicate = NSPredicate(format: "uuid == %@", tagUUID)
    
    // Return if there are no duplicates.
    guard var duplicatedTags = try? performingContext.fetch(fetchRequest), duplicatedTags.count > 1 else {
        return
    }
    // Pick the first tag as the winner.
    guard let winner = duplicatedTags.first else {
        fatalError("###\(#function): Failed to retrieve the first duplicated tag")
    }
    duplicatedTags.removeFirst()
    remove(duplicatedTags: duplicatedTags, winner: winner, performingContext: performingContext)
}

最困难的部分（在我看来）是处理那些被删除的重复 object 的关系，假设我们的标签 object 与 one-to-many 有关系一个类别object（每个标签可能有多个类别）

private func remove(duplicatedTags: [Tag], winner: Tag, performingContext: NSManagedObjectContext) {
    duplicatedTags.forEach { tag in
        // delete the tag AFTER we handle the relationship
        // and be careful that the delete rule will also activate
        defer { performingContext.delete(tag) }
        
        if let categorys = tag.categorys as? Set<Category> {
            for category in categorys {
                // re-map those category to the winner Tag, or it will become nil when the duplicated Tag got delete
                category.ofTag = winner
            }
        }
    }
}

一个有趣的事情是，如果Category object也是从远程存储中添加的，那么当我们处理关系时它们可能还不存在，但那是另外一回事了。

使用 NSPersistentCloudKitContainer 时预填充核心数据存储的最佳方法是什么？

What's the best approach to prefill Core Data store when using NSPersistentCloudKitContainer?

swift

core-data

ios13

cloudkit