Using FetchRequest in swiftUI view model

Question

I'm trying to follow the MVVM pattern in swiftUI and I’m running into a problem with core data and fetch request. All of the videos I've seen and articles I have read on it, have a @FetchRequest in the view, that accesses and modifies the core data. How would I put this in the SettingsVCModel? I can't seem to figure it out and as such I have kept the fetch request inside the view struct(SettingsVC) and used it there. However this has so far been with buttons, where you are able to perform an action. Now I need to do it with a toggle, that only has a binding variable associated with it and no action like buttons have. I tried the didSet approach on the @Published bio inside SettingsVCModel, but they you don't have access to the fetch request. Any help would be greatly appreciated. The code I’m working with is below.

struct SettingsVC: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    @FetchRequest(fetchRequest: UserSettings.getUserSettings()) var userSettings : FetchedResults<UserSettings>
    @ObservedObject var model = SettingsVCModel()

    var body: some View {
        NavigationView {
            Form{
                Section(header: Text("Application")){
                    Toggle(isOn: $model.bio, label: {Text(model.determineBiometricType())})
                    Picker(selection: $model.unitSelection, label: Text("Units")) {
                        Text("Imperial").tag(0)
                        Text("Metric").tag(1)
                    }
                    SettingsButton(toggleButton: $model.openSettings, title: "System Authorizations")
                }
                Section(header: Text("Feedback")){
                    NavigationLink(destination: ContactVC()){
                        Text("Contact Me")
                    }
                    SettingsButton(toggleButton: $model.rateApp, title: "Please Rate Body Insights")
                    SettingsButton(toggleButton: $model.tellAFriend, title: "Tell a Friend")
                }
                Section(header: Text("General")){
                    NavigationLink(destination: AboutVC()){
                        Text("About")
                    }
                    SettingsButton(toggleButton: $model.openPrivacyPolicy, title: "Privacy Policy", openPrivacyPolicy: true)
                }
            }
            .onAppear{
                self.model.bio = self.userSettings.first!.useBiometricUnlock
                self.model.unitSelection = self.userSettings.first!.usesMetric ? 1 : 0
            }
            .navigationBarTitle("Settings")
            .sheet(isPresented: $model.tellAFriend, content: {
                ShareSheetView(activityItems: ["Hey, check out this cool app! https://apps.apple.com/uy/app/body-insights/id1397531585"])
            })

        }
    }
} 


final class SettingsVCModel : ObservableObject{
    @Published var unitSelection = 0
    @Published var tellAFriend = false
    @Published var openPrivacyPolicy = false
    @Published var bio = false
    @Published var openSettings = false {
        didSet{
            if openSettings{
                openAppSettingsApp()
                openSettings = false
            }
        }
    }
    @Published var rateApp = false {
        didSet{
            if rateApp{
                openRateApp()
                rateApp = false
            }
        }
    }

    func openRateApp() {
        let appID = "1397531585"
        let urlString = "https://itunes.apple.com/us/app/appName/id\(appID)?mt=8&action=write-review"
        let url = URL(string: urlString)!
        UIApplication.shared.open(url, options: [:], completionHandler: nil)
    }

    func openAppSettingsApp() {
        guard
            let settingsURL = URL(string: UIApplication.openSettingsURLString),
            UIApplication.shared.canOpenURL(settingsURL)
            else {
                return
        }

        UIApplication.shared.open(settingsURL)
        return
    }

    func determineBiometricType() -> String {
        let authContext = LAContext()
        let _ = authContext.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: nil)
        switch(authContext.biometryType) {
        case .none:
            return "Not Avaliable"
        case .touchID:
            return "TouchID"
        case .faceID:
            return "FaceID"
        @unknown default:
            return "Not Avaliable"
        }
    }
}

public class UserSettings : NSManagedObject, Identifiable {
    @NSManaged public var useBiometricUnlock : Bool
    @NSManaged public var usesMetric : Bool
    @NSManaged public var name : String
    @NSManaged public var birthday : Date
    @NSManaged public var age : Int

    static func getUserSettings() -> NSFetchRequest<UserSettings> {
        let request : NSFetchRequest<UserSettings> = UserSettings.fetchRequest() as! NSFetchRequest<UserSettings>
        request.sortDescriptors = []
        return request
    }

    static func save(){
        let appDelegate = UIApplication.shared.delegate as! AppDelegate
        let context = appDelegate.persistentContainer.viewContext

        do {
            try context.save()
        } catch{
            print(error)
        }
    }

    static func preloadData(){
        let preloadKey: String  = "preloadKey"
        let isPreloaded = UserDefaults.standard.bool(forKey: preloadKey)

        if !isPreloaded {
            let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
            let defaultSettings = UserSettings(context: context)
            let deviceName = UIDevice.current.name
            let firstName = deviceName.components(separatedBy: " ").first
            let isMetric = NSLocale.current.usesMetricSystem

            defaultSettings.name = firstName ?? ""
            defaultSettings.useBiometricUnlock = false
            defaultSettings.usesMetric = isMetric
            defaultSettings.age = 0
            defaultSettings.birthday = Date()

            UserDefaults.standard.set(true, forKey: preloadKey)
            UserSettings.save()
        }
    }
}

Answer 1

It appears you can only use SwiftUI's FetchRequest inside a view. If you check its definition, FetchRequest conforms to DynamicProperty, and if you read the documentation around both they imply it is designed to be used in a SwiftUI view.

FetchRequest:

/// Property wrapper to help Core Data clients drive views from the results of
/// a fetch request. The managed object context used by the fetch request and
/// its results is provided by @Environment(\.managedObjectContext).
@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
@propertyWrapper public struct FetchRequest<Result> : DynamicProperty where Result : NSFetchRequestResult {

DynamicProperty:

/// Represents a stored variable in a `View` type that is dynamically
/// updated from some external property of the view. These variables
/// will be given valid values immediately before `body()` is called.
@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
public protocol DynamicProperty {

    /// Called immediately before the view's body() function is
    /// executed, after updating the values of any dynamic properties
    /// stored in `self`.
    mutating func update()
}