UserDefaults

1. Cơ bản

// Write
UserDefaults.standard.set("John", forKey: "username")
UserDefaults.standard.set(25, forKey: "age")
UserDefaults.standard.set(true, forKey: "isPremium")

// Read
let name = UserDefaults.standard.string(forKey: "username")
let age = UserDefaults.standard.integer(forKey: "age")
let isPremium = UserDefaults.standard.bool(forKey: "isPremium")

2. @AppStorage

@AppStorage("username") var username = ""
@AppStorage("theme") var theme = "light"

// Trong View, tự động bind
TextField("Name", text: $username)

3. Custom Objects

struct Settings: Codable {
    var volume: Double = 0.5
    var language: String = "en"
}

// Save
let settings = Settings()
if let data = try? JSONEncoder().encode(settings) {
    UserDefaults.standard.set(data, forKey: "settings")
}

// Load
if let data = UserDefaults.standard.data(forKey: "settings"),
   let settings = try? JSONDecoder().decode(Settings.self, from: data) {
    // Use settings
}

4. Keys Enum

enum UserDefaultsKeys: String {
    case username
    case email
    case isLoggedIn
}

UserDefaults.standard.set("John", forKey: UserDefaultsKeys.username.rawValue)

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📝 Limits

• Không dùng cho data lớn
• Không encrypt (đừng lưu passwords)
• Synchronous (có thể block main thread)