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