cloudkit-sync

CloudKit Sync (SwiftData & Core Data)

Sync a user's data across their devices with iCloud, for free, using their private database. This is the on-ramp to multi-device; for the local store and modeling, see ios-standards (SwiftData/@Model).

CloudKit's private database is per-user, on the user's iCloud account. You don't run a server. The trade-off: the schema must obey CloudKit's rules, and sync is eventual (seconds to minutes), not instant.

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SwiftData + CloudKit

let config = ModelConfiguration(
    "Main",
    schema: Schema([Trip.self, Stop.self]),
    cloudKitDatabase: .automatic        // or .private("iCloud.com.you.app")
)
let container = try ModelContainer(for: Trip.self, Stop.self, configurations: config)

Enable in the project: add the iCloud capability → CloudKit, pick/create a container, and add the Background Modes → Remote notifications capability (CloudKit uses silent pushes to trigger sync).

The schema rules CloudKit forces on you

These are the cause of ~90% of "it won't build / won't sync" issues:

• Every non-optional property needs a default value, or must be optional. CloudKit can't represent required-with-no-default.
• No @Attribute(.unique) — unique constraints aren't supported with CloudKit. Enforce uniqueness in app logic instead.
• Relationships must be optional and you generally need an inverse.
• No .deny delete rules that CloudKit can't model.

Violating these throws at ModelContainer init or silently disables sync. If a previously-local model won't sync, audit it against this list first.

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Core Data path

Use NSPersistentCloudKitContainer instead of NSPersistentContainer. Set the store's cloudKitContainerOptions and NSPersistentStoreRemoteChangeNotificationPostOptionKey to observe remote changes. Same schema constraints apply. Initialize the CloudKit schema during development with initializeCloudKitSchema(options:) (debug builds only — never ship that call).