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Swift User Defaults

A series of Swift friendly utilities for Foundation's UserDefaults class.

Features

  • 🔑 Constant Keys - Manage default keys using a specialized type to help prevent bugs and keep your project organized.
  • 🦺 Type Safety - Automatically cast to the right types and forget about Any?.
  • 🔍 Observations - Effortless observations in Swift.
  • 👩‍💻 Codable and RawRepresentable Support - Consistently encode and decode Codable and RawRepresentable types with no additional effort.
  • 🧪 Mocking in UI Tests - Inject default values from your UI test suite directly into your application.
  • 🎁 Property Wrappers - Bringing the power of SwiftUI's @AppStorage wrapper to Swift with @UserDefault.

🔑 Constant Keys

With UserDefaults today, you store values against a given 'key'. This key is a String and over time using string's can lead to easy to avoid bugs unless you are defining your own constants somewhere.

You likely have to do something like the following in a project today:

let userDefaults = UserDefaults.standard
var value = (userDefaults.object(forKey: "UserCount") as? Int) ?? 0
value += 1
userDefaults.set(value, forKey: "UserCoumt")

As you can see from the example above, reusing strings can lead to bugs through typos so a common way to guard against this is to define constants:

struct Constants {
    static let userCountDefaultsKey = "UserCount"
}

// ...

let userDefaults = UserDefaults.standard
var value = (userDefaults.object(forKey: Constants.userCountDefaultsKey) as? Int) ?? 0
value += 1
userDefaults.set(value, forKey: Constants.userCountDefaultsKey)

This is much better because you can be safe knowing that you're using the correct key, but we can do better.

Similar to Foundation's Notification.Name, SwiftUserDefaults provides a new UserDefaults.Key type that acts as a namespace for you to provide your own constants that can be conveniently used around your app without having to worry about typos or other issues that might occur during refactoring.

import Foundation
import SwiftUserDefaults

extension UserDefaults.Key {
    /// The number of users interacted with.
    static let userCount = Self("UserCount")

    /// The name of the user.
    static let userName = Self("UserName")

    /// The last visit.
    static let lastVisit = Self("LastVisit")
}

SwiftUserDefaults then provides a series of additional APIs built on top of this type. Continue reading to learn how to use them.

🦺 Type Safety

When using UserDefaults, you must only attempt to set booleans, data, dates, numbers or strings, as well as dictionaries or arrays consisting of those types otherwise you'll experience a runtime crash with no protections from the Compiler.

SwiftUserDefaults provides safer APIs that combined with UserDefaults.Key offer a much safer experience with UserDefaults:

let userDefaults = UserDefaults.standard
var value = userDefaults.x.object(Int.self, forKey: .userCount) ?? 0
value += 1
userDefaults.x.set(value, forKey: .userCount)

In the above example, the key argument uses UserDefaults.Key constants and the value is automatically cast to a known type all by accessing the safer API via the x extension.

Additionally, the compiler can help to catch mistakes when passing unsupported types into set(_:forKey:).

struct User {
    let id: UUID
}

func updateCurrentUser(_ user: User) {
    // ❌ Runtime Crash
    userDefaults.set(user.id, forKey: "UserId")
    // SIGABRT
    //
    // Attempt to insert non-property list object
    // DAE8F83E-5760-475D-B28D-D493F695E765 for key UserId

    // ✅ Compile Time Error
    userDefaults.x.set(user.id, forKey: .userId)
    // Instance method 'set(_:forKey:)' requires that 'UUID' conform to 'UserDefaultsStorable'
}

🔍 Observations

UserDefaults is key-value observing compliant however you can't use Swift's key-path based overlay since the stored defaults don't associate to actual properties. SwiftUserDefaults helps solve this problem by providing a wrapper around the Objective C based KVO methods:

import Foundation
import SwiftUserDefaults

class MyViewController: UIViewController {
    let store = UserDefaults.standard
    var observation: UserDefaults.Observation?

    // ...

    override func viewDidLoad() {
        super.viewDidLoad()

        // ...

        observation = store.x.observeObject(String.self, forKey: .userName) { change in
            self.nameLabel.text = change.value
        }
    }

    deinit {
        observation?.invalidate()
    }
}

The change property is the UserDefaults.Change enum which consists of two cases to represent both the .initial value and any subsequent .update's. If you don't care about this, you can access the underlying value via the value property.

👩‍💻 Codable and RawRepresentable Support

In addition to supporting the default value types for UserDefaults, convenience methods have also been provided to facilitate the use of Codable and RawRepresentable types (including enums).

For RawRepresentable types, you can use them exactly like String and Int values and SwiftUserDefaults will automatically read and write the rawValue to the underlying store:

enum Tab: String { // String and Int backed enum's are `RawRepresentable`.
    case home, search, create
}

let initialTab = userDefaults.x.object(Tab.self, forKey: .lastTab) ?? .home
showTab(initialTab)

// ...

func tabDidChange(_ tab: Tab) {
    userDefaults.x.set(tab, forKey: .lastTab)
}

For Codable types, you pass an additional CodingStrategy parameter (.json or .plist) to dictate the format of encoding to use when reading and writing the value:

struct Activity: Codable {
    let id: UUID
    let name: String
}

let restoredActivity = userDefaults.x.object(Activity.self, forKey: .currentActivity, strategy: .json)

func showActivity(_ activity: Activity) {
    userDefaults.x.set(activity, forKey: .currentActivity, strategy: .json)
}

⚠️ Warning: While these APIs can make it tempting to encode large models to UserDefaults, you should continue to remember that some platforms have strict limits for the size of the UserDefaults store.

For more information, see the official Apple Developer Documentation.

🧪 Mocking in UI Tests

SwiftUserDefaults provides a structured way to inject values into UserDefaults of your App target from the UI Testing target. This works by formatting a payload of launch arguments that UserDefaults will read into the NSArgumentDomain.

MyAppCommon Target

import SwiftUserDefaults

extension UserDefaults.Key {
    /// The current level of the user
    public static let currentLevel = Self("CurrentLevel")
    /// The name of the user using the app
    public static let userName = Self("UserName")
    /// The unique identifier assigned to this user
    public static let userGUID = Self("UserGUID")
}

MyAppUITests Target

import MyAppCommon
import SwiftUserDefaults
import XCTest

struct MyAppConfiguration: LaunchArgumentEncodable {
    @UserDefaultOverride(.currentLevel)
    var currentLevel: Int?
    
    @UserDefaultOverride(.userName)
    var userName: String?
    
    @UserDefaultOverride(.userGUID)
    var userGUID = "FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF"
}

final class MyAppTests: XCTestCase {
    func testMyApp() throws {
        var configuration = MyAppConfiguration()
        container.currentLevel = 8
        container.userName = "John Doe"

        let app = XCUIApplication()
        app.launchArguments = try configuration.encodeLaunchArguments()
        app.launch()

        // ...
    }
}

MyApp Target

import SwiftUserDefaults
import UIKit

class ViewController: UIViewController {
    // ...

    override func viewDidLoad() {
        super.viewDidLoad()

        let store = UserDefaults.standard
        store.x.object(Int.self, for: .currentLevel) // 8
        store.x.object(String.self, for: .userName) // "John Doe"
        store.x.object(String.self, for: .userGUID) // "FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF"
    }
}

🎁 Property Wrappers

SwiftUserDefaults brings UserDefaults.Key to SwiftUI's @AppStorage property wrapper, and in addition, it introduces an @UserDefault property wrapper with similar behavior that is suitable outside of SwiftUI.

The simplest way to use the property wrapper is as follows:

import SwiftUserDefaults

class MyStore {
    @UserDefault(.userName)
    var userName: String?

    @UserDefault(.currentLevel)
    var currentLevel: Int = 1

    @UserDefault(.difficulty)
    var difficulty: Difficulty = .medium
}

If you need to be able to inject dependencies into MyStore, you can also do so as follows:

import SwiftUserDefaults

class MyStore {
    @UserDefault var userName: String?
    @UserDefault var currentLevel: Int
    @UserDefault var difficulty: Difficulty

    init(userDefaults store: UserDefaults) {
        _userName = UserDefault(.userName, store: store)
        _currentLevel = UserDefault(.currentLevel, store: store, defaultValue: 1)
        _difficulty = UserDefault(.difficulty, store: store, defaultValue: .medium)
    }
}

Finally, through the projected value, @UserDefault allows you to reset and observe the stored value:

let store = MyStore(userDefaults: .standard)

// Removes the value from user defaults
store.$userName.reset()

// Observes the user default, respecting the default value
let observer = store.$currentLevel.addObserver { change in
    change.value // Int, 1
}

As with the UserDefault.X APIs, the property wrapper supports primitive, RawRepresentable and Codable types.

Installation

CocoaPods

CocoaPods is a dependency manager for Cocoa projects. For usage and installation instructions, visit their website. To integrate SwiftUserDefaults into your Xcode project using CocoaPods, specify it in your Podfile:

pod 'swift-user-defaults'

Swift Package Manager

Add the following to your Package.swift

dependencies: [
    .package(url: "https://github.com/cookpad/swift-user-defaults.git", .upToNextMajor(from: "0.1.0"))
]

Or use the https://github.com/cookpad/swift-user-defaults.git repository link in Xcode.