Agent Skills: iOS Persistence

iOS Persistence

Priority: P0

Implementation Workflow

1. Choose storage tier — SwiftData for iOS 17+, Core Data for legacy, Keychain for secrets, UserDefaults for flags only.
2. Define models — Use @Model macro (SwiftData) or .xcdatamodeld (Core Data).
3. Configure container — Use @MainActor for ModelContainer (SwiftData) or NSPersistentContainer (Core Data).
4. Perform background writes — Use newBackgroundContext() (Core Data) to avoid UI lag; never do heavy I/O on viewContext.
5. Secure sensitive data — Use Keychain for tokens and PII; never store in UserDefaults.

See SwiftData and Core Data implementation examples

Anti-Patterns

• ❌ Heavy I/O on viewContext — use private background contexts
• ❌ String-based predicates — use KeyPaths or generated helpers
• ❌ Missing merge strategy — set mergePolicy explicitly (e.g., mergeByPropertyObjectTrump)

References

• SwiftData & Core Data Implementation