Coredata Sync

Problem

// NSPersistentStoreRemoteChange fires when disk changes
// Often triggers reload that races with in-flight save
NotificationCenter.observe(.NSPersistentStoreRemoteChange) {
    dataManager.loadData()  // Can overwrite correct data with stale!
}

Solution

var isMutating = false

func moveItems(...) {
    isMutating = true
    defer {
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.75) {
            self.isMutating = false
        }
    }
    // ... mutation code
}

func loadData() {
    guard !isMutating else { return }  // Skip during mutation
    // ... load code
}

CloudKit Sync Configuration

Pattern

// Use NSPersistentCloudKitContainer for automatic sync
let container = NSPersistentCloudKitContainer(name: "Model")

// Configure merge policy
context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy

// Monitor sync events
NotificationCenter.default.addObserver(
    forName: NSPersistentCloudKitContainer.eventChangedNotification,
    object: container,
    queue: .main
) { notification in
    guard let event = notification.userInfo?[
        NSPersistentCloudKitContainer.eventNotificationUserInfoKey
    ] as? NSPersistentCloudKitContainer.Event else { return }

    if event.endDate != nil {
        if let error = event.error {
            handleSyncError(error)
        }
    }
}

Context Management

Pattern

// Use appropriate context for operation type
func performBackgroundSync(_ data: SyncData) async throws {
    try await container.performBackgroundTask { context in
        context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy

        for item in data.items {
            let entity = ItemEntity.findOrCreate(id: item.id, in: context)
            entity.update(from: item)
        }

        try context.save()
    }
}

Antipattern

// Never pass managed objects between threads
let item = mainContext.fetch(...)  // Main thread
Task.detached {
    item.name = "Updated"  // BUG: Wrong thread!
}

Common Failures

"Merge conflict with no resolution"

• Cause: NSMergePolicy not set or inappropriate
• Fix: Configure mergeByPropertyObjectTrump or custom policy
• Prevention: Always set merge policy explicitly

"Sync stuck in pending state"

• Cause: CloudKit quota exceeded or network issues
• Fix: Check CloudKit Dashboard, verify container permissions
• Prevention: Monitor sync events, implement timeout

"Data appears on one device but not another"

• Cause: Different iCloud accounts or sync delay
• Fix: Verify accounts match, wait for sync, check logs
• Prevention: Add sync status indicators to UI

"Duplicate entities after sync"

• Cause: Missing unique constraint or improper upsert
• Fix: Add unique constraint, use findOrCreate pattern
• Prevention: Always use upsert logic for synced entities