A property. wrapper for store/restore variables backed by the given user defaults.

UserDefaultsStorage.swift

import Foundation

@propertyWrapper
public struct UserDefaultStorage<T: Codable> {
    private let key: String
    private let defaultValue: T

    private let userDefaults: UserDefaults

    public init(key: String, default: T, store: UserDefaults = .standard) {
        self.key = key
        self.defaultValue = `default`
        self.userDefaults = store
    }

    public var wrappedValue: T {
        get {
            guard let data = userDefaults.data(forKey: key) else { return defaultValue }
            do {
                let value = try JSONDecoder().decode(T.self, from: data)
                return value
            } catch {
                assertionFailure(error.localizedDescription)
                return defaultValue
            }
        }
        set {
            let data: Data?
            do {
                data = try JSONEncoder().encode(newValue)
            } catch {
                assertionFailure(error.localizedDescription)
                data = nil
            }
            userDefaults.set(data, forKey: key)
        }
    }
}