SwiftData + CKSyncEngine

1-Is it possible to use SwiftData for local data storage and CKSyncEngine to sync this local data storage with iCloud?

Yes. CKSyncEngine takes care the synchronization. Making a local cache for the CloudKit data is in a separate layer, which can be done with whatever persistence technology you like, including SwiftData.

2-If yes, is there any example code to implement this?

We don't have any SwiftData + CKSyncEngine sample code as of today.

3-Also, to make sure I don't run into problems later - is it okay to fire data upload (sendChanges) and download (fetchChanges) manually with CKSyncEngine and do it often? Are there any limits how often these functions can be called to not get "blocked" by the server?

It is OK to call fetchChanges(_:) and sendChanges(_:) in your app. Doing that frequently is much less so, because that can trigger CloudKit throttles, as discussed in TN3162: Understanding CloudKit throttles.

We don't have a clear documentation about how often is too often, but the technote covers how to identify and handle CloudKit throttles, which hopefully helps.

Best,
——
Ziqiao Chen
 Worldwide Developer Relations.

Thanks for the reply.

I've been trying to get CKSyncEngine working with SwiftData. Since you don't any sample code, here's my code and some questions about it.

CKSyncEngineDelegate has a function called "handleEvent". In that function I handle the "fetchedRecordZoneChanges" event amongst other events. Here's my function to handle that event (it's basically copy paste from the sample app):

func handleFetchedRecordZoneChanges(_ event: CKSyncEngine.Event.FetchedRecordZoneChanges) {
    
    guard let localContainer else { return }
    let newContext = ModelContext(localContainer)
    let localHabits = SwiftDataPersistence.fetch(FetchDescriptor<Habit>(), predicate: nil, sortBy: [], fetchLimit: nil, context: newContext)
    
    for modification in event.modifications {
        
        // The sync engine fetched a record, and we want to merge it into our local persistence.
        let serverHabit = modification.record
        let serverHabitID = serverHabit.recordID.recordName
        
        // If we already have this object locally, let's merge the data from the server.
        if let localHabit = localHabits.first(where: { habit in
            habit.identifier == serverHabitID
        }) {
            localHabit.mergeFromServerData(serverHabit)
            localHabit.setLastKnownRecordIfNewer(serverHabit)
        }
        
        // Otherwise, let's create a new local object.
        else {
            let newHabit = Habit(record: serverHabit)
            newContext.insert(newHabit)
        }
    }
    
    for deletion in event.deletions {
        // TO DO
    }
    
    // If we had any changes, let's save to disk.
    if !event.modifications.isEmpty || !event.deletions.isEmpty {
        SwiftDataPersistence.save(newContext)
        NotificationCenter.default.post(name: newContextDidSaveNotification, object: nil)
    }
}

Last line of code: I'm sending a notification to my SwiftUI view (that shows habits) to fetch edited/new habits.

Here's my view code:

struct HabitsListView: View {
    @State private var habits = [Habit]()
    
    @Environment(\.modelContext) private var context
    
    var body: some View {
        LazyVStack(alignment: .leading, spacing: 8) {
            ForEach(habits) { habit in
                HabitRow(habit)
            }
        }
        .onNewContextDidSave {
            fetchHabits()
        }
    }
    
    func fetchHabits() {
        self.habits = SwiftDataPersistence.fetch(FetchDescriptor<Habit>(), predicate: nil, sortBy: [SortDescriptor(\Habit.name)], fetchLimit: nil, context: context)
    }
}

This approach seems to work, but it's not ideal and I'm not sure it's a correct way to do it. It works with only 1 custom notification, but I ran into problems when I added more notifications - then the notifications stopped firing or the view didn't catch them.

But my main question at this moment is: How to get @Query to update my habits list, so I don't have to manually fetch habits at every change.

I would like to use a view like this:

struct HabitsListView: View {
    @Query private var habits: [Habit]
    
    var body: some View {
        LazyVStack(alignment: .leading, spacing: 8) {
            ForEach(habits) { habit in
                HabitRow(habit)
            }
        }
    }
}

Thanks! Martin

SwiftData provides .willSave and .didSave notifications, and so you probably don't need to define your own (newContextDidSaveNotification in your code).

Secondly, @Query is supposed to detect the change from other model context and trigger a SwiftUI update for the relevant view. There may still have bugs in that area though that prevent SwiftDat from achieving that, as discussed in this post.

If you do see on the latest version systems that @Query doesn't detect the change and trigger the SwiftUI update, I’d suggest that you file a feedback report for the SwiftData team to take a look – If you do so, please share your report ID here for folks to track.

Best,
——
Ziqiao Chen
 Worldwide Developer Relations.