# IOS Persistence

> Implement local persistence with SwiftData, Core Data, and secure storage. Use when setting up SwiftData models, Core Data stacks, or local persistence in iOS.

- Skill: `hoangnguyen0403/ios-persistence` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds add hoangnguyen0403/ios-persistence`
- Raw SKILL.md: https://api.skillmd.com/api/skills/hoangnguyen0403/ios-persistence/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: HoangNguyen0403 (https://skillmd.com/u/hoangnguyen0403)
- Updated: 2026-08-19
- Page: https://skillmd.com/skills/hoangnguyen0403/ios-persistence

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# iOS Persistence

## **Priority: P0 (CRITICAL)**

## Implementation Workflow

1. **Choose storage tier** — SwiftData for iOS 17+, Core Data for legacy, Apple secure credential storage 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 heavy I/O on `viewContext`.
5. **Secure sensitive data** — Use secure storage for tokens and PII; never store in `UserDefaults`.

See [SwiftData and Core Data implementation examples](references/implementation.md)

## Anti-Patterns

- **No Heavy I/O on `viewContext`**: Use private background contexts
- **No String-based Predicates**: Use KeyPaths or generated helpers
- **No Missing Merge Strategy**: Set the merge policy (`mergePolicy`) explicitly (e.g., `mergeByPropertyObjectTrump`)

## References

- [SwiftData & Core Data Implementation](references/implementation.md)