UserDefaults

What is UserDefaults?

UserDefaults is a simple key-value store for small amounts of user preferences. It persists data across app launches but is NOT encrypted.

Writing Values

let defaults = UserDefaults.standard

defaults.set("Alice", forKey: "username")
defaults.set(42, forKey: "launchCount")
defaults.set(true, forKey: "hasCompletedOnboarding")
defaults.set(Date(), forKey: "lastLoginDate")

Reading Values

let username = defaults.string(forKey: "username")
let launchCount = defaults.integer(forKey: "launchCount")
let hasOnboarded = defaults.bool(forKey: "hasCompletedOnboarding")
let lastLogin = defaults.object(forKey: "lastLoginDate") as? Date

Removing Values

defaults.removeObject(forKey: "username")

Register Defaults

Set default values for first launch:

defaults.register(defaults: [
    "username": "Guest",
    "launchCount": 0,
    "hasCompletedOnboarding": false,
    "theme": "system"
])

This does not write to disk until a value is explicitly set.

What to Store

Store only small, non-sensitive data:

• User preferences (theme, language)
• Feature flags
• Launch count
• Last run version

Do NOT store:

• Sensitive data (use Keychain)
• Large data (use file system)
• Complex objects (use Codable + FileManager)

@AppStorage for SwiftUI

@AppStorage provides a clean property wrapper for UserDefaults in SwiftUI:

struct SettingsView: View {
    @AppStorage("theme") private var theme = "system"
    @AppStorage("notificationsEnabled") private var notifications = true
    @AppStorage("fontSize") private var fontSize = 16.0

    var body: some View {
        Form {
            Picker("Theme", selection: $theme) {
                Text("Light").tag("light")
                Text("Dark").tag("dark")
                Text("System").tag("system")
            }

            Toggle("Notifications", isOn: $notifications)

            Slider(value: $fontSize, in: 12...24, step: 1)
        }
    }
}

@AppStorage vs @State

• @State: In-memory, lost when view recreates
• @AppStorage: Persists in UserDefaults, shared across the app

@AppStorage with Custom Defaults

Specify a custom UserDefaults suite:

@AppStorage("color", store: UserDefaults(suiteName: "com.app.group"))
private var color = "blue"

This is useful for App Groups where multiple extensions share data.

App Groups

Share UserDefaults between app and extensions:

let sharedDefaults = UserDefaults(suiteName: "group.com.app.shared")
sharedDefaults?.set("value", forKey: "key")

Combining with Observable

@Observable
class UserPreferences {
    var theme: String {
        get { UserDefaults.standard.string(forKey: "theme") ?? "system" }
        set { UserDefaults.standard.set(newValue, forKey: "theme") }
    }

    var notificationsEnabled: Bool {
        get { UserDefaults.standard.bool(forKey: "notificationsEnabled") }
        set { UserDefaults.standard.set(newValue, forKey: "notificationsEnabled") }
    }
}

Keep It Small

UserDefaults is not a database. Store only preferences and settings, not application data.

Thread Safety

UserDefaults is thread-safe for individual reads and writes. However, batch operations should be performed on a single thread.

Synchronize

defaults.synchronize()

This is rarely needed in modern iOS as UserDefaults auto-saves, but can be useful before app termination.

Testing

Use a custom suite for testing to avoid polluting shared defaults:

let testDefaults = UserDefaults(suiteName: "TestSuite")!
testDefaults.removePersistentDomain(forName: "TestSuite")

Migration

When changing default values, handle migrations:

func migrateDefaults() {
    let version = defaults.integer(forKey: "dataVersion")
    if version < 2 {
        // Migrate from v1 to v2
        defaults.set(2, forKey: "dataVersion")
    }
}

Security

UserDefaults stores data in plaintext plist files. Never store sensitive information like passwords, tokens, or personal data. Use Keychain for that purpose.