Foil: A Swift Property Wrapper for UserDefaults

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Foil

A lightweight property wrapper for UserDefaults done right

About

Read the post: A better approach to writing a UserDefaults Property Wrapper

Why the name?

Foil, as in "let me quickly and easily wrap and store this leftover food in some foil so I can eat it later." 🌯 😉

Foil: noun North America A very thin, pliable, easily torn sheet of aluminum used for cooking, packaging, cosmetics, and insulation.

Usage

You can use @FoilDefaultStorage for non-optional values and @FoilDefaultStorageOptional for optional ones. You may wish to store all your user defaults in one place, however, that is not necessary. Any property on any type can use this wrapper.

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
}

There is also an included example app project.

Using `enum` keys

If you prefer using an enum for the keys, writing an extension specific to your app is easy. However, this is not required. In fact, unless you have a specific reason to reference the keys, this is completely unnecessary.

enum AppSettingsKey: String, CaseIterable {
    case flagEnabled
    case totalCount
    case timestamp
}

extension FoilDefaultStorage {
    init(wrappedValue: T, _ key: AppSettingsKey) {
        self.init(wrappedValue: wrappedValue, key: key.rawValue)
    }
}

extension FoilDefaultStorageOptional {
    init(_ key: AppSettingsKey) {
        self.init(key: key.rawValue)
    }
}

Observing changes

There are many ways to observe property changes. The most common are by using Key-Value Observing or a Combine Publisher. KVO observing requires the object with the property to inherit from NSObject and the property must be declared as @objc dynamic.

final class AppSettings: NSObject {
    static let shared = AppSettings()

    @FoilDefaultStorageOptional(key: "userId")
    @objc dynamic var userId: String?

    @FoilDefaultStorageOptional(key: "average")
    var average: Double?
}

Using KVO

let observer = AppSettings.shared.observe(\.userId, options: [.new]) { settings, change in
    print(change)
}

Using Combine

Note

The average does not need the @objc dynamic annotation, .receiveValue will fire immediately with the current value of average and on every change after.

AppSettings.shared.$average
    .sink {
        print($0)
    }
    .store(in: &cancellable)

Combine Alternative with KVO

Note

In this case, userId needs the @objc dynamic annotation and AppSettings needs to inherit from NSObject. Then receiveValue will fire only on changes to wrapped object's value. It will not publish the initial value as in the example above.

AppSettings.shared
    .publisher(for: \.userId, options: [.new])
    .sink {
        print($0)
    }
    .store(in: &cancellable)

Supported types

The following types are supported by default for use with @FoilDefaultStorage.

Note

While the UserDefaultsSerializable protocol defines a failable initializer, init?(storedValue:), it is possible to provide a custom implementation with a non-failable initializer, which still satisfies the protocol requirements.

For all of Swift's built-in types (Bool, Int, Double, String, etc.), the default implementation of UserDefaultsSerializable is non-failable.

Important

Adding support for custom types is possible by conforming to UserDefaultsSerializable. However, this is highly discouraged as all plist types are supported by default. UserDefaults is not intended for storing complex data structures and object graphs. You should probably be using a proper database (or serializing to disk via Codable) instead.

While Foil supports storing Codable types by default, you should use this sparingly and only for small objects with few properties.

Bool

Int

UInt

Float

Double

String

URL

Date

Data

Array

Set

Dictionary

RawRepresentable types

Codable types

Notes on Codable types

Warning

If you are storing custom Codable types and using the default implementation of UserDefaultsSerializable provided by Foil, then you must use the optional variant of the property wrapper, @FoilDefaultStorageOptional. This will allow you to make breaking changes to your Codable type (e.g., adding or removing a property). Alternatively, you can provide a custom implementation of Codable that supports migration, or provide a custom implementation of UserDefaultsSerializable that handles encoding/decoding failures. See the example below.

Codable Example:

// Note: uses the default implementation of UserDefaultsSerializable
struct User: Codable, UserDefaultsSerializable {
    let id: UUID
    let name: String
}

// Yes, do this
@FoilDefaultStorageOptional(key: "user")
var user: User?

// NO, do NOT this
// This will crash if you change User by adding/removing properties
@FoilDefaultStorage(key: "user")
var user = User()

Notes on RawRepresentable types

Using RawRepresentable types, especially as properties of a Codable type require special considerations. As mentioned above, Codable types must use @FoilDefaultStorageOptional out-of-the-box, unless you provide a custom implementation of UserDefaultsSerializable. The same is true for RawRepresentable types.

Warning

RawRepresentable types must use @FoilDefaultStorageOptional in case you modify the cases of your enum (or otherwise modify your RawRepresentable with a breaking change). Additionally, RawRepresentable types have a designated initializer that is failable, init?(rawValue:), and thus could return nil.

Or, if you are storing a Codable type that has RawRepresentable properties, by default those properties should be optional to accommodate the optionality described above.

If you wish to avoid these edge cases with RawRepresentable types, you can provide a non-failable initializer:

extension MyStringEnum: UserDefaultsSerializable {
    // Default init provided by Foil
    // public init?(storedValue: RawValue.StoredValue) { ... }

    // New, non-failable init using force-unwrap.
    // Only do this if you know you will not make breaking changes.
    public init(storedValue: String) { self.init(rawValue: storedValue)! }
}

Additional Resources

NSUserDefaults in Practice, the excellent guide by David Smith

UserDefaults documentation

Preferences and Settings Programming Guide

Property List Programming Guide

Supported Platforms

iOS 13.0+

tvOS 13.0+

watchOS 6.0+

macOS 11.0+

visionOS 1.0+

Requirements

Swift 6.0+

Xcode 16.0+

SwiftLint

Installation

Swift Package Manager

dependencies: [
    .package(url: "https://github.com/jessesquires/Foil.git", from: "6.0.0")
]

Alternatively, you can add the package directly via Xcode.

Documentation

You can read the documentation here. Generated with jazzy. Hosted by GitHub Pages.

Documentation is also available on the Swift Package Index.

Contributing

Interested in making contributions to this project? Please review the guides below.

Contributing Guidelines

Code of Conduct

Support and Help

Security Policy

Also consider sponsoring this project or buying my apps! ✌️

Credits

Created and maintained by Jesse Squires.

License

Released under the MIT License. See LICENSE for details.

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Completed work

Posted Aug 12, 2025

Developed Foil, a property wrapper for UserDefaults in Swift.

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Foil: A Swift Property Wrapper for UserDefaults

Foil

About

Usage

Using enum keys

Observing changes

Supported types

Additional Resources

Supported Platforms

Requirements

Installation

Swift Package Manager

Documentation

Contributing

Credits

License

Using `enum` keys