Swift Actor Persistence

用于线程安全持久化的 Swift Actor

使用 Swift actor 构建线程安全数据持久化层的模式。结合内存缓存与文件支持的存储，利用 actor 模型在编译时消除数据竞争。

何时激活

• 在 Swift 5.5+ 中构建数据持久化层
• 需要对共享可变状态进行线程安全访问
• 希望消除手动同步（锁、DispatchQueue）
• 构建具有本地存储的离线优先应用

核心模式

基于 Actor 的存储库

Actor 模型保证了序列化访问 —— 没有数据竞争，由编译器强制执行。

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) })
    }
}