Testable @UserDefaults property wrapper

UserDefaultsPropertyWrapper.swift

import Foundation

public protocol UserDefaultsInterface {
    func set(_ value: Any?, forKey defaultName: String)
    func object(forKey defaultName: String) -> Any?
}

extension UserDefaults: UserDefaultsInterface {}

class UserDefaultsMock: UserDefaultsInterface {
    var valueSet: Any?
    var valueSetKey: String?
    var objectKey: String?
    var objectToBeReturned: Any?

    func set(_ value: Any?, forKey defaultName: String) {
        valueSet = value
        valueSetKey = defaultName
    }

    func object(forKey defaultName: String) -> Any? {
        objectKey = defaultName
        return objectToBeReturned
    }
}

enum UserDefaultsKeys: String {
    case userSeenPopUp = "user_seen_popup"
}

@propertyWrapper
struct Preferences<T> {
    var defaults: UserDefaultsInterface = UserDefaults.standard
    let key: UserDefaultsKeys
    let defaultValue: T

    var wrappedValue: T {
        get {
            defaults.object(forKey: key.rawValue) as? T ?? defaultValue
        } set {
            defaults.set(newValue, forKey: key.rawValue)
        }
    }

    init(wrappedValue: T, key: UserDefaultsKeys) {
        defaultValue = wrappedValue
        self.key = key
    }
}

class SomeClass {
    @Preferences(key: .userSeenPopUp)
    var hasUserSeenPopUp = false

    init(defaults: UserDefaultsInterface = UserDefaults.standard) {
        _hasUserSeenPopUp.defaults = defaults
    }

    func showPopUpIfNecessary() {
        guard !hasUserSeenPopUp else { return }
        // show pop up
        hasUserSeenPopUp.toggle()
    }
}

let mock = UserDefaultsMock()
let test = SomeClass(defaults: mock) // Inject mocked instance into the property wrapper
assert(!test.hasUserSeenPopUp)
test.showPopUpIfNecessary()

assert(mock.valueSet as? Bool == true)
assert(mock.valueSetKey == UserDefaultsKeys.userSeenPopUp.rawValue)