https://www.fivestars.blog/swiftui/app-scene-storage.html

app-scene-storage.swift

// Original article here: https://www.fivestars.blog/swiftui/app-scene-storage.html

import Combine
import SwiftUI

// MARK: Appearance

enum Appearance: String, Codable, CaseIterable, Identifiable {
  case dark
  case light
  case system

  var id: String { rawValue }
}

struct Preferences: Codable {
  var appearance: Appearance

  static let defaultValue = Preferences(appearance: .system)
}

// MARK: ContentView

class ContentViewModel: ObservableObject {
  @Published("userPreferences") var preferences: Preferences = .defaultValue
}

struct ContentView: View {
  @StateObject var model = ContentViewModel()

  var body: some View {
    Picker("Appearance", selection: $model.preferences.appearance) {
      ForEach(Appearance.allCases, id: \.self) {
        Text(verbatim: $0.rawValue)
      }
    }.pickerStyle(SegmentedPickerStyle())
  }
}

// MARK: Published+UserDefaults

private var cancellableSet: Set<AnyCancellable> = []

extension Published where Value: Codable {
  init(wrappedValue defaultValue: Value, _ key: String, store: UserDefaults? = nil) {
    let _store: UserDefaults = store ?? .standard

    if
      let data = _store.data(forKey: key),
      let value = try? JSONDecoder().decode(Value.self, from: data) {
      self.init(initialValue: value)
    } else {
      self.init(initialValue: defaultValue)
    }

    projectedValue
      .sink { newValue in
        let data = try? JSONEncoder().encode(newValue)
        _store.set(data, forKey: key)
      }
      .store(in: &cancellableSet)
  }
}

// MARK: Published+UISceneSession

extension Published where Value: Codable {
  init(wrappedValue defaultValue: Value, _ key: String, session: UISceneSession) {
    if
      let data = session.userInfo?[key] as? Data,
      let value = try? JSONDecoder().decode(Value.self, from: data) {
      self.init(initialValue: value)
    } else {
      self.init(initialValue: defaultValue)
    }

    projectedValue
      .sink { newValue in
        let data = try? JSONEncoder().encode(newValue)
        session.userInfo?[key] = data
      }
      .store(in: &cancellableSet)
  }
}