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 ?? "")