app-storage.swift

/*
 The @AppStorage property wrapper does not fully work unless installed
 directly in a view. In particular, it does not work when used in an 
 observable object.

 Paste the following into a fresh project, run it in the simulator,
 and notice that tapping the "Toggle model.isOn" button causes the view
 to update propertly, but tapping the "Toggle UserDefaults directly" 
 button does not. This means that writing directly to user defaults 
 does cause @AppStorage to update, _unless_ @AppStorage is used in the
 view.
 */

import SwiftUI

@main
struct MainApp: App {
  var body: some Scene {
    WindowGroup {
      FeatureView()
    }
  }
}

class FeatureModel: ObservableObject {
  @AppStorage("isOn") var isOn = false
}

struct FeatureView: View {
  @ObservedObject var model = FeatureModel()
  var body: some View {
    Form {
      Section {
        Text("model.isOn: \(model.isOn.description)")
      }
      Section {
        Button("Toggle model.isOn") { model.isOn.toggle() }
        Button("Toggle UserDefaults directly") {
          print("model.isOn before", model.isOn)
          UserDefaults.standard.setValue(
            !UserDefaults.standard.bool(forKey: "isOn"),
            forKey: "isOn"
          )
          print("model.isOn after", model.isOn)
        }
      }
    }
  }
}