How to sync records between Core Data and CloudKit efficiently

Question

I'm currently learning how to use CloudKit Framework and lack of documentation or examples showing how to sync Core Data and CloudKit.

I have watched all WWDC videos (2014, 2015, 2016) Dedicated to CloudKit, but none of them telling us how to implement syncing with Core Data. I can't find any fresh examples, tutorial or books, showing how to implement this syncing.

I know that it is effective to use Operations API by CloudKit (not Convenience API) and to Subscribe to changes as it said in the new WWDC 2016 videos, dedicated to CloudKit, but mapping with CoreData is a real problem.

For example, let's say I would like to create an app similar to Notes app. while offline, user can create his notes and work with them saving them to his core data database. When the device going online the app checks what changed on the server and saves newly created records to server (CloudKit).

When the app starts, it also fetches for changes from the CloudKit and if there are changes , it updates local cache (Core Data) with the new changes.

I would appreciate to have a common pattern of syncing. Where to place syncing with Core Data methods and how they should look like?

Would appreciate any information or help about this.

I'm using Swift 3, Xcode 8 , iOS 10.

Answer 1

Core Data already provides the user with the ability to sync to iCloud. There's no need to use CloudKit.

Design For Core Data In iCloud

But yes, Core Data with iCloud has been deprecated. Even so, it has not been discontinued, and there are no immediate plans at apple to discontinue it, they just want to discourage its use. But it also has problems with rationalising updates from multiple devices.

In any case, I have been looking into the question of how to do this with cloud kit myself. Two answers; the first is to use the following;

Seam in GitHub

The second is to do it manually;

Designing for CloudKit

The key here is that Cloud Kit needs the record metadata to be able to handle record updates reliably, so you have to save that metadata in your Core Data database. The CKRecord class includes a method encodeSystemFields(with:) which will encode those fields into a Data record that can be stored in your database, and then your can use the appropriate decoder when you need to restore the CKRecord.

Anyway, I am about to start doing this myself. I'll update this with more information when I have it.

Answer 2

Apple has recently published a guide that seems to answer this question. Check out Apple's Maintaining a Local Cache of CloudKit Records to see how to store CloudKit data on device.

While this guide doesn't provide sample code to write to the device, it does answer the rest of the question. This tells you how to fetch changes from CloudKit and create data which can be stored on the device.

Answer 3

As of iOS 13, there are new API's that simplify this synchronization for developers. I would recommend you to watch the WWDC19 session about the new synchronization between CoreData and CloudKit. Please note that these new API's only work for iOS 13+.

Video: https://developer.apple.com/videos/play/wwdc2019/202/

In short, you need to start using NSPersistentCloudKitContainer instead of NSPersistentContainer. This will let the syncing work automatically using automatic conflict resolution with a last-writer-wins merge strategy. If you want to build a good working app, you'll also need to do some modifications to improve the syncing for your app.

Official documentation can be found at:

• Setting Up Core Data with CloudKit
• Syncing a Core Data Store with CloudKit

---

Data modeling for collaboration (conflict-free replicated data type)

At the end of the session they also demonstrated an example of better sync merging than the default 'last-writer-wins merge strategy'. The usage of Causal Trees allow multiple users to edit the same string (and to some extend other types of data) without losing any data. I would really recommend everyone to read this article from Archagon that describes how this works and how to implement it (also with CloudKit syncing, but not the new automatic one). As demonstrated in the session, you can also implement this with the new automatic syncing between CoreData and CloudKit.