CloudKit Reference

Purpose: Comprehensive CloudKit reference for database-based iCloud storage and sync
Availability: iOS 10.0+ (basic), iOS 17.0+ (CKSyncEngine), iOS 17.0+ (SwiftData integration)
Context: Modern CloudKit sync via CKSyncEngine (WWDC 2023) or SwiftData integration

When to Use This Skill

Use this skill when:

• Implementing structured data sync to iCloud
• Choosing between SwiftData+CloudKit, CKSyncEngine, or raw CloudKit APIs
• Setting up public/private/shared databases
• Implementing conflict resolution
• Debugging CloudKit sync issues
• Monitoring CloudKit performance

NOT for: Simple file sync (use axiom-icloud-drive-ref instead)

Overview

CloudKit is for STRUCTURED DATA sync (records with relationships), not simple file sync.

Three modern approaches:

1. SwiftData + CloudKit (Easiest, iOS 17+)
2. CKSyncEngine (Custom persistence, iOS 17+, WWDC 2023)
3. Raw CloudKit APIs (Maximum control, more complexity)

---

Approach 1: SwiftData + CloudKit (Recommended)

When to use: iOS 17+ apps with SwiftData models

Limitations:

• Private database only (no public/shared)
• Automatic sync (less control)
• SwiftData constraints apply

// ✅ CORRECT: SwiftData with CloudKit sync
import SwiftData

@Model
class Task {
    var title: String
    var isCompleted: Bool
    var dueDate: Date

    init(title: String, isCompleted: Bool = false, dueDate: Date) {
        self.title = title
        self.isCompleted = isCompleted
        self.dueDate = dueDate
    }
}

// Configure CloudKit container
let container = try ModelContainer(
    for: Task.self,
    configurations: ModelConfiguration(
        cloudKitDatabase: .private("iCloud.com.example.app")
    )
)

// That's it! Sync happens automatically

Entitlements required:

• iCloud capability
• CloudKit container

Use axiom-swiftdata skill for SwiftData details

---

Approach 2: CKSyncEngine (Modern, WWDC 2023)

When to use: Custom persistence (SQLite, GRDB, JSON) with cloud sync

Advantages over raw CloudKit:

• Manages fetch/upload cycles automatically
• Handles conflicts
• Manages account changes
• Recommended over manual CKDatabase operations

// ✅ CORRECT: CKSyncEngine setup
import CloudKit

class SyncManager {
    let syncEngine: CKSyncEngine

    init() throws {
        let config = CKSyncEngine.Configuration(
            database: CKContainer.default().privateCloudDatabase,
            stateSerialization: loadSyncState(),
            delegate: self
        )

        syncEngine = try CKSyncEngine(config)
    }

    // Implement delegate methods
}

extension SyncManager: CKSyncEngineDelegate {
    // Handle events
    func handleEvent(_ event: CKSyncEngine.Event, syncEngine: CKSyncEngine) async {
        switch event {
        case .stateUpdate(let stateUpdate):
            saveSyncState(stateUpdate.stateSerialization)

        case .accountChange(let change):
            handleAccountChange(change)

        case .fetchedDatabaseChanges(let changes):
            applyDatabaseChanges(changes)

        case .fetchedRecordZoneChanges(let changes):
            applyRecordChanges(changes)

        case .sentRecordZoneChanges(let changes):
            handleSentChanges(changes)

        case .willFetchChanges, .didFetchChanges,
             .willSendChanges, .didSendChanges:
            // Optional lifecycle events
            break

        @unknown default:
            break
        }
    }

    // Next batch of changes to send
    func nextRecordZoneChangeBatch(
        _ context: CKSyncEngine.SendChangesContext,
        syncEngine: CKSyncEngine
    ) async -> CKSyncEngine.RecordZoneChangeBatch? {
        // Return pending local changes
        let pendingChanges = getPendingLocalChanges()
        return CKSyncEngine.RecordZoneChangeBatch(
            pendingSaves: pendingChanges,
            recordIDsToDelete: []
        )
    }
}

Key concepts:

• State serialization: Persist sync state between app launches
• Events: Delegate receives events for changes
• Batches: You provide pending changes, engine uploads them
• Automatic conflict resolution: Engine handles basic conflicts

---

Approach 3: Raw CloudKit APIs (Legacy)

When to use: Only if CKSyncEngine doesn't fit (rare)

Core types:

• CKContainer — Entry point
• CKDatabase — Public/private/shared scope
• CKRecord — Individual data record
• CKRecordZone — Logical grouping
• CKAsset — Binary file storage

Basic Operations

// ✅ Container and database
let container = CKContainer.default()
let privateDatabase = container.privateCloudDatabase
let publicDatabase = container.publicCloudDatabase

// ✅ Create record
let record = CKRecord(recordType: "Task")
record["title"] = "Buy groceries"
record["isCompleted"] = false
record["dueDate"] = Date()

// ✅ Save record
try await privateDatabase.save(record)

// ✅ Fetch record
let recordID = CKRecord.ID(recordName: "task-123")
let fetchedRecord = try await privateDatabase.record(for: recordID)

// ✅ Query records
let predicate = NSPredicate(format: "isCompleted == NO")
let query = CKQuery(recordType: "Task", predicate: predicate)
let (matchResults, _) = try await privateDatabase.records(matching: query)

for result in matchResults {
    if case .success(let record) = result.1 {
        print("Task: \(record["title"] as? String ?? "")")
    }
}

// ✅ Delete record
try await privateDatabase.deleteRecord(withID: recordID)

Conflict Resolution

// ✅ Handle conflicts with savePolicy
let operation = CKModifyRecordsOperation(
    recordsToSave: [record],
    recordIDsToDelete: nil
)

// Save only if server version unchanged
operation.savePolicy = .ifServerRecordUnchanged

// OR: Always overwrite server
operation.savePolicy = .changedKeys  // Only changed fields

operation.modifyRecordsResultBlock = { result in
    switch result {
    case .success:
        print("Saved")
    case .failure(let error as CKError):
        if error.code == .serverRecordChanged {
            // Conflict - merge manually
            let serverRecord = error.serverRecord
            let clientRecord = error.clientRecord
            let merged = mergeRecords(server: serverRecord, client: clientRecord)
            // Retry with merged record
        }
    }
}

privateDatabase.add(operation)

---

Database Scopes

Scope	Accessibility	SwiftData Support	Use Case
Private	User only	✅ Yes	Personal user data
Public	All users	❌ No	Shared/public content
Shared	Invited users	❌ No	Collaboration

Private Database

// ✅ Private database (most common)
let privateDB = CKContainer.default().privateCloudDatabase

// User must be signed into iCloud
// Data syncs across user's devices
// Not visible to other users

Public Database

// ✅ Public database (for shared content)
let publicDB = CKContainer.default().publicCloudDatabase

// Accessible to all app users
// Even unauthenticated users can read
// Writes require authentication
// Use for: Leaderboards, public content, discovery

Shared Database

// ✅ Shared database (collaboration)
let sharedDB = CKContainer.default().sharedCloudDatabase

// For CKShare-based collaboration
// Users invited to specific record zones
// Use for: Shared documents, team data

---

CloudKit Assets (Files)

// ✅ Store files as CKAsset
let imageURL = saveImageToTempFile(image)  // Must be file URL
let asset = CKAsset(fileURL: imageURL)

let record = CKRecord(recordType: "Photo")
record["image"] = asset
record["caption"] = "Sunset"

try await privateDatabase.save(record)

// ✅ Retrieve asset
let fetchedRecord = try await privateDatabase.record(for: recordID)
if let asset = fetchedRecord["image"] as? CKAsset,
   let fileURL = asset.fileURL {
    let imageData = try Data(contentsOf: fileURL)
    let image = UIImage(data: imageData)
}

Important: CKAsset requires a file URL, not Data. Write data to temp file first.

---

CloudKit Console (Monitoring - WWDC 2024)

Developer Notifications

Set up alerts for:

• Schema changes
• Quota exceeded
• High error rates
• Custom thresholds

Telemetry

Monitor:

• Request count
• Error rate
• Latency (p50, p95, p99)
• Bandwidth usage

Logs

View:

• Individual requests
• Error details
• Performance bottlenecks

Access: https://icloud.developer.apple.com/dashboard

---

Common Patterns

Pattern 1: Initial Sync

// ✅ Fetch all records on first launch
func performInitialSync() async throws {
    let predicate = NSPredicate(value: true)  // All records
    let query = CKQuery(recordType: "Task", predicate: predicate)

    let (results, _) = try await privateDatabase.records(matching: query)

    for result in results {
        if case .success(let record) = result.1 {
            saveToLocalDatabase(record)
        }
    }
}

Pattern 2: Incremental Sync

// ✅ Use CKServerChangeToken for incremental fetches
func fetchChanges(since token: CKServerChangeToken?) async throws {
    let zoneID = CKRecordZone.ID(zoneName: "Tasks")

    let config = CKFetchRecordZoneChangesOperation.ZoneConfiguration(
        previousServerChangeToken: token
    )

    let operation = CKFetchRecordZoneChangesOperation(
        recordZoneIDs: [zoneID],
        configurationsByRecordZoneID: [zoneID: config]
    )

    operation.recordWasChangedBlock = { recordID, result in
        if case .success(let record) = result {
            updateLocalDatabase(with: record)
        }
    }

    operation.recordWithIDWasDeletedBlock = { recordID, _ in
        deleteFromLocalDatabase(recordID)
    }

    operation.recordZoneFetchResultBlock = { zoneID, result in
        if case .success(let (token, _, _)) = result {
            saveChangeToken(token)  // For next fetch
        }
    }

    try await privateDatabase.add(operation)
}

---

Entitlements

Required entitlements in Xcode:

<!-- iCloud capability -->
<key>com.apple.developer.icloud-services</key>
<array>
    <string>CloudKit</string>
</array>

<!-- CloudKit container -->
<key>com.apple.developer.icloud-container-identifiers</key>
<array>
    <string>iCloud.com.example.app</string>
</array>

Setup:

1. Xcode → Target → Signing & Capabilities
2. "+ Capability" → iCloud
3. Check "CloudKit"
4. Select or create container

---

Quick Reference

Task	Modern API (iOS 17+)	Legacy API
Structured data sync	SwiftData + CloudKit	CKSyncEngine or CKDatabase
Custom persistence sync	CKSyncEngine	CKDatabase
Conflict resolution	Automatic (SwiftData/CKSyncEngine)	Manual (savePolicy)
Account changes	Handled automatically	Manual detection
Monitoring	CloudKit Console telemetry	Manual logging

---

Related Skills

• axiom-swiftdata — SwiftData implementation details
• axiom-storage — Choose CloudKit vs iCloud Drive
• axiom-icloud-drive-ref — File-based iCloud sync
• axiom-cloud-sync-diag — Debug CloudKit sync issues

---

Last Updated: 2025-12-12
Skill Type: Reference
Minimum iOS: 10.0 (basic), 17.0 (CKSyncEngine, SwiftData integration)
WWDC Sessions: 2023-10188 (CKSyncEngine), 2024-10122 (CloudKit Console)