Foil — правильная обертка для UserDefaults

final class AppSettings {
    static let shared = AppSettings()

    @FoilDefaultStorage(key: "flagEnabled")
    var flagEnabled = true

    @FoilDefaultStorage(key: "totalCount")
    var totalCount = 0

    @FoilDefaultStorageOptional(key: "timestamp")
    var timestamp: Date?
}

// Usage

func userDidToggleSetting(_ sender: UISwitch) {
    AppSettings.shared.flagEnabled = sender.isOn
}