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version: "1.0.1" name: swift-actor-persistence description: Thread-safe data persistence in Swift using actors — in-memory cache with file-backed storage, eliminating data races by design. origin: ECC

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) })
    }
}

Usage

All calls are automatically async due to actor isolation:

swift

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

swift

@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

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