Swift Actor Persistence

affaan-m/ECC/pi/core/skills/swift-actor-persistence

by affaan-mef648e01899ba3e8dc6371642deaaf64b4477775No license275K starsListed Oct 9, 2026Updated Oct 9, 2026Repository updated 4 days ago

Thread-safe data persistence in Swift using actors — in-memory cache with file-backed storage, eliminating data races by design. Use when persisting data in Swift and a data race or thread-safety problem needs designing out.

Instructions onlySoftware Development

AI-generated overview

Guides building thread-safe Swift data persistence with actors, combining an in-memory cache and file-backed JSON storage.

What it does: This skill provides patterns and code examples for a Swift actor-based repository that keeps data in an in-memory dictionary and persists it to a JSON file. It covers a generic LocalRepository type, usage from async call sites, and integration with an @Observable view model. It also lists design decisions, best practices, and anti-patterns for actor-based persistence.
When to use it: Use it when building a Swift 5.5+ persistence layer, when shared mutable state needs thread-safe access, or when replacing locks and DispatchQueue synchronization with actors. It also fits offline-first apps with local storage.
Requirements: Swift 5.5 or later with the actor model and Codable/Identifiable types; no scripts, packages, credentials, or network access are required.

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.

swift

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) })    }}

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:

swift

let repository = LocalRepository<Question>()
// Read — fast O(1) lookup from in-memory cachelet question = await repository.find(by: "q-001")let allQuestions = await repository.loadAll()
// Write — updates cache and persists to file atomicallytry await repository.save(newQuestion)try await repository.delete("q-001")

let repository = LocalRepository<Question>()
// Read — fast O(1) lookup from in-memory cachelet question = await repository.find(by: "q-001")let allQuestions = await repository.loadAll()
// Write — updates cache and persists to file atomicallytry await repository.save(newQuestion)try await repository.delete("q-001")

Combining with @Observable ViewModel

swift

@Observablefinal 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()    }}

@Observablefinal 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

Source and attribution

Source:affaan-m/ECCinpi/core/skills/swift-actor-persistenceat commitef648e0

License: No license

Content belongs to its original authors. SourceWeft indexes it from a public repository.

Report or request removal

Swift Actor Persistence

Swift Actors for Thread-Safe Persistence

When to Activate

Core Pattern

Actor-Based Repository

Usage

Combining with @Observable ViewModel

Key Design Decisions

Best Practices

Anti-Patterns to Avoid

When to Use

Source and attribution

More from affaan-m/ECC

Unified Notifications Ops

Verification Loop

Ui To Vue

Unified Memory

Visa Doc Translate

Team Builder

More in Software Development

Playground

M5 Onboard

Cardputer Buddy

Web Design Guidelines

Vercel React Native Skills

Vercel React Best Practices