Sync to iCloud with CKSyncEngine

actor MySyncManager : CKSyncEngineDelegate {
    
    init(container: CKContainer, localPersistence: MyLocalPersistence) {
        let configuration = CKSyncEngine.Configuration(
            database: container.privateCloudDatabase,
            stateSerialization: localPersistence.lastKnownSyncEngineState,
            delegate: self
        )
        self.syncEngine = CKSyncEngine(configuration)
    }
    
    func handleEvent(_ event: CKSyncEngine.Event, syncEngine: CKSyncEngine) async {
        switch event {
        case .stateUpdate(let stateUpdate):
            self.localPersistence.lastKnownSyncEngineState = stateUpdate.stateSerialization
        }
    }
}

func userDidEditData(recordID: CKRecord.ID) {
    // Tell the sync engine we need to send this data to the server.
    self.syncEngine.state.add(pendingRecordZoneChanges: [ .save(recordID) ])
}

func nextRecordZoneChangeBatch(
    _ context: CKSyncEngine.SendChangesContext, 
    syncEngine: CKSyncEngine
) async -> CKSyncEngine.RecordZoneChangeBatch? {

    let changes = syncEngine.state.pendingRecordZoneChanges.filter { 
        context.options.zoneIDs.contains($0.recordID.zoneID) 
    }

    return await CKSyncEngine.RecordZoneChangeBatch(pendingChanges: changes) { recordID in
        self.recordToSave(for: recordID)
    }
}

func handleEvent(_ event: CKSyncEngine.Event, syncEngine: CKSyncEngine) async {
    switch event {
        
    case .fetchedRecordZoneChanges(let recordZoneChanges):
        for modifications in recordZoneChanges.modifications {
            // Persist the fetched modification locally
        }

        for deletions in recordZoneChanges.deletions {
            // Remove the deleted data locally
        }

    case .fetchedDatabaseChanges(let databaseChanges):      
        for modifications in databaseChanges.modifications {
            // Persist the fetched modification locally
        }
      
        for deletions in databaseChanges.deletions { 
            // Remove the deleted data locally
        }

    // Perform any setup/cleanup necessary
    case .willFetchChanges, .didFetchChanges:
        break
      
    case .sentRecordZoneChanges(let sentChanges):

        for failedSave in sentChanges.failedRecordSaves {
            let recordID = failedSave.record.recordID

            switch failedSave.error.code {

            case .serverRecordChanged:
                if let serverRecord = failedSave.error.serverRecord {
                    // Merge server record into local data
                    syncEngine.state.add(pendingRecordZoneChanges: [ .save(recordID) ])
                }
            
            case .zoneNotFound: 
                // Tried to save a record, but the zone doesn't exist yet.
                syncEngine.state.add(pendingDatabaseChanges: [ .save(recordID.zoneID) ])
                syncEngine.state.add(pendingRecordZoneChanges: [ .save(recordID) ])
             
            // CKSyncEngine will automatically handle these errors
            case .networkFailure, .networkUnavailable, .serviceUnavailable, .requestRateLimited:
                break
              
            // An unknown error occurred
            default:
                break
            }
        }
      
    case .accountChange(let event):
        switch event.changeType {

        // Prepare for new user
        case .signIn:
            break
          
        // Delete local data
        case .signOut:
            break
          
        // Delete local data and prepare for new user
        case .switchAccounts: 
            break
        }
    }
}

let databases = [ container.privateCloudDatabase, container.sharedCloudDatabase ]

let syncEngines = databases.map {
    var configuration = CKSyncEngine.Configuration(
        database: $0,
        stateSerialization: lastKnownSyncEngineState($0.databaseScope),
        delegate: self
    )
    return CKSyncEngine(configuration)
}

func testSyncConflict() async throws {
    
    // Create two local databases to simulate two devices.
    let deviceA = MySyncManager()
    let deviceB = MySyncManager()
    
    // Save a value from the first device to the server.
    deviceA.value = "A"
    try await deviceA.syncEngine.sendChanges()
    
    // Try to save the value from the second device before it fetches changes.
    // The record save should fail with a conflict that includes the current server record.
    // In this example, we expect the value from the server to win.
    deviceB.value = "B"
    XCTAssertThrows(try await deviceB.syncEngine.sendChanges())
    XCTAssertEqual(deviceB.value, "A")
}