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LocalData/Proposal.md
Matt Bruce 126b9cf318 Docs: update docs for Proposal, README 
Summary:
- Docs: update docs for Proposal, README

Stats:
- 2 files changed, 18 insertions(+), 6 deletions(-)
2026-01-18 13:43:08 -06:00

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LocalData Package Proposal

Goal

Create a single, typed, discoverable namespace for persisted app data with consistent security guarantees and clear ownership. This makes it obvious what is stored, where it is stored, how it is secured, how it is serialized, who owns it, and which platforms it belongs to or should sync to.

Package Placement

  • localPackages/LocalData/
    • Sources/LocalData/
    • Tests/LocalDataTests/

Dependencies

  • Foundation
  • Security (Keychain)
  • CryptoKit (encryption)
  • WatchConnectivity (sync helpers)

Architecture

Core Components

  • StorageKey protocol - Defines storage configuration for each data type
  • StorageRouter actor - Main entry point coordinating all storage operations
  • StorageProviding protocol - Abstraction for storage operations
  • StorageKeyCatalog protocol - Catalog of keys for auditing/validation

Isolated Helper Classes (Actors)

Each helper is a dedicated actor providing thread-safe access to a specific storage domain:

  • KeychainHelper - All keychain operations (set, get, delete, exists, deleteAll)
  • EncryptionHelper - AES-256-GCM or ChaCha20-Poly1305 with PBKDF2/HKDF
  • FileStorageHelper - File system operations (read, write, delete, list, size), including App Group containers
  • UserDefaultsHelper - UserDefaults operations with suite and App Group support
  • SyncHelper - WatchConnectivity sync operations

Models

  • StorageDomain - userDefaults, appGroupUserDefaults, keychain, fileSystem, encryptedFileSystem, appGroupFileSystem
  • SecurityPolicy - none, keychain (with accessibility/accessControl), encrypted (AES-256 or ChaCha20-Poly1305)
  • Serializer - JSON, property list, raw Data, or custom encode/decode with named serializer metadata
  • KeyMaterialSource - Identifier for external key material providers
  • PlatformAvailability - all, phoneOnly, watchOnly, phoneWithWatchSync
  • SyncPolicy - never, manual, automaticSmall
  • KeychainAccessibility - All 7 iOS options (whenUnlocked, afterFirstUnlock, etc.)
  • KeychainAccessControl - All 6 options (userPresence, biometryAny, devicePasscode, etc.)
  • FileDirectory - documents, caches, custom URL
  • StorageError - Comprehensive error types
  • StorageKeyDescriptor - Audit snapshot of a key’s storage metadata
  • AnyStorageKey - Type-erased storage key for catalogs
  • AnyCodable - Type-erased Codable for mixed-type payloads

Usage Pattern

Apps extend StorageKeys with their own key types and use StorageRouter.shared. This follows the Notification.Name pattern for discoverable keys.

Audit & Validation

Apps can register a StorageKeyCatalog to generate audit reports and enforce key registration. Registration validates:

  • Duplicate key names
  • Missing descriptions
  • Unregistered keys at runtime (debug assertions)

Sync Behavior

StorageRouter can call WCSession.updateApplicationContext for manual or automaticSmall sync policies when availability allows it. Session activation and receiving data are owned by the app.

Platforms

  • iOS 17+
  • watchOS 10+

Future Ideas (Not Implemented)

  • Migration helpers for legacy storage
  • Key rotation strategies for encrypted data
  • Watch-optimized data representations

Any future changes should keep LocalData documentation in sync with code changes.