Using FetchRequest in a SwiftUI view model

Question

I'm trying to follow the MVVM pattern in SwiftUI, and I’m running into a problem with the core data and the 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.

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()
        }
    }
}

Constantine · Accepted Answer · 2020-02-17 09:15:33Z

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()
}

XcodeNOOB · Accepted Answer · 2021-03-02 12:24:48Z

0

I've end up giving up with the idea of EnvironmentObject and use a wrapper service with protocol around the singleton of the core data storage. I don't think there's other option of doing this if you still want to keep the MVVM principles.

answered Mar 2, 2021 at 12:24

XcodeNOOB

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1 Comment

Richard Witherspoon Over a year ago

This is more or less what I've ended up doing as well.

dbDev · Accepted Answer · 2022-06-21 03:54:58Z

0

CoreData can be thought of as a ViewModel itself. We use @FetchRequest in a view to "observe" changes happening with the model CoreData describes. If you're also using CloudKit with CoreData, then while your view is active, your @FetchRequest is also active and any inbound changes from iCloud will be "observed" and will cause your view to update.

If you want to fetch data from CoreData from a ViewModel you built yourself though, just use the non-SwiftUI functions. I literally just wrote this in a ViewModel today:

let request = API.fetchRequest()
let sort = NSSortDescriptor(keyPath: \API.priority, ascending: true)
request.sortDescriptors = [sort]
do {
  if let apis = try container.viewContext.fetch(request) as? [API] {
    logger.debug("APIs fetched: \(apis.count)")
    self.apis = apis
  }
} catch {
  logger.debug("Fetch failed 😭")
}

So just like that, I'm fetching data into my ViewModel where I can now assign it to some of the @Published properties. Happy coding!

answered Jun 21, 2022 at 3:54

dbDev

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1 Comment

MJ Studio Over a year ago

This seems anti-pattern. Can we convince that publisher emit changes from any update from iCloud or local?

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