An app using CRDT and CloudKit.
- There is a single list in this app. Users can edit it across devices.
- Users can add an item to or delete one from the list.
- New items can be inserted at the beginning of the app or after any given item.
- Items will show their creation time.
- Items would be in different color for different devices, but items from the local device is always in pink.
- It should support remote push notification.
- Two singleton objects are used to coordinate CloudKit and the user interface:
StorageLayerandListModel. StorageLayerholds a weave in aCurrentValueSubject.- Both
ListModelandStorageLayercould write to the weave. The former for locally created edits, and the latter for downloaded remote edits. - When the weave changes,
ListModelrecomputes its current list and update the SwiftUI accordingly. StorageLayerkeeps track of the last local edit uploaded. When the weave changes, it uploads new edits made by this device to CloudKit.StorageLayeralso keeps a list of pending edits not yet weaved into the weave. It tries to weave those edits after each successful synchronization.- Local edits can also be in the pending edit list. But those edits are already contained in the weave, so they are skipped entirely.
- Apps running on a simulator cannot receive push notifications.
- We did not handle any CloudKit error. Rather, we
fatalErrorwhenever one happens. This helps debugging but do not use this in production code! - We once encountered an error from
NSURLSession. Turns out the Wi-Fi had some issue. Curiously, the error was not anCKError.
- The CRDT blog post, highly recommended.
- The chronofold paper written by Victor Grischenko. Not used, but I like it so much.