swift-actor-persistence

SKILL.md

Swift Actors for Thread-Safe Persistence

Patterns for building thread-safe data persistence layers using Swift actors. Combines in-memory caching with file-backed storage, leveraging the actor model to eliminate data races at compile time.

When to Activate

Building a data persistence layer in Swift 5.5+

Need thread-safe access to shared mutable state

Want to eliminate manual synchronization (locks, DispatchQueues)

Building offline-first apps with local storage

Core Pattern

Actor-Based Repository

The actor model guarantees serialized access — no data races, enforced by the compiler.
public actor LocalRepository<T: Codable & Identifiable> where T.ID == String {
    private var cache: [String: T] = [:]
    private let fileURL: URL

    public init(directory: URL = .documentsDirectory, filename: String = "data.json") {
        self.fileURL = directory.appendingPathComponent(filename)
        // Synchronous load during init (actor isolation not yet active)
        self.cache = Self.loadSynchronously(from: fileURL)
    }

    // MARK: - Public API

    public func save(_ item: T) throws {
        cache[item.id] = item
        try persistToFile()
    }

    public func delete(_ id: String) throws {
        cache[id] = nil
        try persistToFile()
    }

    public func find(by id: String) -> T? {
        cache[id]
    }

    public func loadAll() -> [T] {
        Array(cache.values)
    }

    // MARK: - Private

    private func persistToFile() throws {
        let data = try JSONEncoder().encode(Array(cache.values))
        try data.write(to: fileURL, options: .atomic)
    }

    private static func loadSynchronously(from url: URL) -> [String: T] {
        guard let data = try? Data(contentsOf: url),
              let items = try? JSONDecoder().decode([T].self, from: data) else {
            return [:]
        }
        return Dictionary(uniqueKeysWithValues: items.map { ($0.id, $0) })
    }
}

Usage

All calls are automatically async due to actor isolation:

let repository = LocalRepository<Question>()

// Read — fast O(1) lookup from in-memory cache
let question = await repository.find(by: "q-001")
let allQuestions = await repository.loadAll()

// Write — updates cache and persists to file atomically
try await repository.save(newQuestion)
try await repository.delete("q-001")

Combining with @Observable ViewModel

@Observable
final class QuestionListViewModel {
    private(set) var questions: [Question] = []
    private let repository: LocalRepository<Question>

    init(repository: LocalRepository<Question> = LocalRepository()) {
        self.repository = repository
    }

    func load() async {
        questions = await repository.loadAll()
    }

    func add(_ question: Question) async throws {
        try await repository.save(question)
        questions = await repository.loadAll()
    }
}

Key Design Decisions

Decision
Rationale

Actor (not class + lock)
Compiler-enforced thread safety, no manual synchronization

In-memory cache + file persistence
Fast reads from cache, durable writes to disk

Synchronous init loading
Avoids async initialization complexity

Dictionary keyed by ID
O(1) lookups by identifier

Generic over Codable & Identifiable
Reusable across any model type

Atomic file writes (.atomic)
Prevents partial writes on crash

Best Practices

Use Sendable types for all data crossing actor boundaries

Keep the actor's public API minimal — only expose domain operations, not persistence details

Use .atomic writes to prevent data corruption if the app crashes mid-write

Load synchronously in init — async initializers add complexity with minimal benefit for local files

Combine with @Observable ViewModels for reactive UI updates

Anti-Patterns to Avoid

Using DispatchQueue or NSLock instead of actors for new Swift concurrency code

Exposing the internal cache dictionary to external callers

Making the file URL configurable without validation

Forgetting that all actor method calls are await — callers must handle async context

Using nonisolated to bypass actor isolation (defeats the purpose)

When to Use

Local data storage in iOS/macOS apps (user data, settings, cached content)

Offline-first architectures that sync to a server later

Any shared mutable state that multiple parts of the app access concurrently

Replacing legacy DispatchQueue-based thread safety with modern Swift concurrency