Swift Actor Persistence

作者 affaan-mef648e01899b無授權條款275K 個星標收錄於 2026年10月8日更新於 2026年10月8日儲存庫3 天前更新

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

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AI 產生的概覽

使用 Swift actor 實現執行緒安全的本地持久化，結合記憶體快取與檔案儲存。

功能: 提供使用 actor 建構執行緒安全持久化層的 Swift 程式碼模式，將記憶體字典快取與 JSON 檔案儲存結合。內容包含泛型 actor 儲存庫及其 save、delete、find、loadAll 操作、@Observable 檢視模型範例、設計決策表、最佳實務與反模式。產出為設計指引與範例程式碼，而非可直接執行的指令碼。
適用情境: 適用於在 Swift 5.9+ 的 iOS 或 macOS 專案中建構本地資料持久化層、需要對共享可變狀態進行執行緒安全存取、以 actor 取代手動鎖或 DispatchQueue 同步，以及建構離線優先的本地儲存應用程式。不適用於非 Swift 專案或伺服器端資料庫工作。
執行需求: 不附帶指令碼或工具，僅為說明性內容。鎖定 Swift 5.9+ 專案（iOS 17+、macOS 14+），需熟悉 Swift 並行、Codable 與檔案儲存 API。

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

來源與署名

來源：affaan-m/ecc位於.kiro/skills/swift-actor-persistence提交ef648e0

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檢舉或申請下架

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

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