数据持久化

iOS 本地存储分层：UserDefaults 存偏好、Keychain 存敏感凭证、Core Data / SwiftData 存结构化数据。对照 /roadmaps/ios「Data Persistence」节点。

学习目标

• 选择合适的存储方案（轻量 vs 关系型）
• 使用 UserDefaults 与 Keychain 读写配置与 Token
• 了解 Core Data 实体建模与 CRUD 基础
• 对照 Spring JPA 与浏览器 localStorage

核心概念

UserDefaults — 轻量键值

// 写入
UserDefaults.standard.set(true, forKey: "hasSeenOnboarding")
UserDefaults.standard.set("dark", forKey: "theme")

// 读取
let seen = UserDefaults.standard.bool(forKey: "hasSeenOnboarding")
let theme = UserDefaults.standard.string(forKey: "theme") ?? "light"

适合主题、引导页标记等小数据。不要存 Token 或密码。

Keychain — 安全存储

import Security

enum KeychainHelper {
    static func save(_ data: Data, service: String, account: String) throws {
        let query: [CFString: Any] = [
            kSecClass: kSecClassGenericPassword,
            kSecAttrService: service,
            kSecAttrAccount: account,
            kSecValueData: data,
        ]
        SecItemDelete(query as CFDictionary)
        let status = SecItemAdd(query as CFDictionary, nil)
        guard status == errSecSuccess else { throw KeychainError.unhandled(status) }
    }

    static func load(service: String, account: String) throws -> Data? {
        let query: [CFString: Any] = [
            kSecClass: kSecClassGenericPassword,
            kSecAttrService: service,
            kSecAttrAccount: account,
            kSecReturnData: true,
        ]
        var result: AnyObject?
        let status = SecItemCopyMatching(query as CFDictionary, &result)
        guard status == errSecSuccess else { return nil }
        return result as? Data
    }
}

enum KeychainError: Error {
    case unhandled(OSStatus)
}

Keychain 加密存储，适合 JWT、API Key；系统级安全，卸载 App 行为可配置。

Core Data 基础

Core Data 是 Apple 的 ORM 框架：Entity → NSManagedObject，支持关系、迁移、iCloud 同步。

// 简化的 CRUD 思路（需 Xcode Data Model 定义 LessonEntity）
func saveLesson(_ lesson: Lesson, context: NSManagedObjectContext) throws {
    let entity = LessonEntity(context: context)
    entity.id = Int64(lesson.id)
    entity.title = lesson.title
    entity.completed = lesson.completed
    try context.save()
}

func fetchLessons(context: NSManagedObjectContext) throws -> [Lesson] {
    let request = LessonEntity.fetchRequest()
    request.sortDescriptors = [NSSortDescriptor(key: "id", ascending: true)]
    let results = try context.fetch(request)
    return results.map { Lesson(id: Int($0.id), title: $0.title ?? "", completed: $0.completed) }
}