Swift Actor Persistence

by affaan-mef648e01899bNo license275K starsListed Oct 8, 2026Updated Oct 8, 2026Repository updated 3 days ago

Thread-safe data persistence in Swift using actors — in-memory cache with file-backed storage, eliminating data races by design.

Instructions onlySoftware Development

AI-generated overview

Swift actor patterns for thread-safe local persistence with an in-memory cache and file-backed storage.

What it does: Provides Swift code patterns for building a thread-safe persistence layer using actors, combining an in-memory dictionary cache with JSON file storage. It includes a generic actor repository with save, delete, find, and loadAll operations, an example @Observable view model, a table of design decisions, best practices, and anti-patterns. The deliverable is guidance and example code rather than a runnable script.
When to use it: Use when building a local data persistence layer in Swift 5.9+ for iOS or macOS, when shared mutable state needs thread-safe access, when replacing manual locks or DispatchQueue synchronization with actors, or when building offline-first apps with local storage. It is not intended for non-Swift projects or server-side database work.
Requirements: No scripts or tools are shipped; it is instructions only. It targets Swift 5.9+ projects (iOS 17+, macOS 14+) and assumes familiarity with Swift concurrency, Codable, and file storage APIs.

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 5.9+ (iOS 17+, macOS 14+)
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(items.map { ($0.id, $0) }, uniquingKeysWith: { _, latest in latest })    }}

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(items.map { ($0.id, $0) }, uniquingKeysWith: { _, latest in latest })    }}

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/eccin.kiro/skills/swift-actor-persistenceat commitef648e0

License: No license

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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

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