We have an iOS application that is using a third party c library, which uses UNIX domain sockets to manage and save content to the device. We have found that this is possible by using Application Groups. We are now interested in using iCloud to sync the data between multiple devices. However, I've been unable to find any documentation on how to do this or if it's possible just using App groups. I've built a simple prototype, but I have had no success syncing App Group files through iCloud. Is this possible? If so, please post a link to documentation or an example of this being done.
iCloud and App Groups are very different features. I’m not sure why you’ve conflated them.
I’m also not sure what UNIX domain sockets have to do with saving files.
As to how you can sync content between devices, there’s a bunch of options here:
iCloud Documents, if a document-based approach makes sense
Core Data CloudKit integration , if you’re doing the whole database thing
CloudKit, if you want low-level control
Numerous third-party libraries
WWDC 2019 Session 202 Using Core Data With CloudKit
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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 I’m talking about the new stuff here, per WWDC 2019 Session 202 Using Core Data With CloudKit.