I'm new to programming and if someone could provide a basic explanation that would be sincerely appreciated.

I am trying to pass an NSManagedObject that I have fetched and is associated with the main view context to a child context for editing. However, nothing is showing up in my detail view. For some reason the managed object isn't being passed.

// Content View, everything is showing up fine here

struct ContentView: View {
    let persistenceController = PersistenceController.shared
    @Environment(\.managedObjectContext) private var viewContext
    @StateObject var viewModel = ContentViewModel()
    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Person.name, ascending: true)],
        animation: .default)
    private var people: FetchedResults<Person>
    
    var body: some View {
        NavigationView {
            List {
                ForEach(people) { person in
                    NavigationLink(destination: EditView(viewModel: EditViewModel(persistenceController: persistenceController, person: person))) {
                        Text(person.name ?? "Unknown")
                        Text("\(person.objectID)")
                    }
                }
            }

// EditViewModel 

struct EditViewModel {
    var person: Person
    let context: NSManagedObjectContext
    let persistenceController: PersistenceController
    
    init(persistenceController: PersistenceController, person: Person) {
        self.context = persistenceController.childViewContext()
        self.person = (persistenceController.childViewContext().object(with: person.objectID) as? Person)!
        self.persistenceController = persistenceController
    }

// EditView - all person properties are nil

struct EditView: View {
    @State var viewModel: EditViewModel
    var body: some View {
        GeometryReader { proxy in
            ScrollView(.vertical) {
                VStack {
                    TextField("Name", text: $viewModel.person.name ?? "")
                    Button("print") {
                        print(viewModel.person.name)
                    }
                }

Please let me know if I have failed to provide enough information to produce a minimum reproducible example and I will add more.

Thanks in advance.

CodePudding user response：

Since you are creating and injecting the view model into your edit view from your content view you don't need to use/pass your PersistenceController class and also since you are passing a Person object we don't need a NSManagedObjectContext either since the Person object has a reference to its context.

So I would rewrite the view model like this

struct EditViewModel {
    var person: Person
    let context: NSManagedObjectContext?
    let parentContext: NSManagedObjectContext

    init(person: Person) {
         guard let parentContext = person.managedObjectContext else {
             //no context is really bad and should never happen
             FatalError("Found a NSManagedObject that doesn't belong to a context")

         self.parentContext = parentContext 
         let childContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
         childContext.parent = parentContext

         guard let childPersopn = try? childContext.existingObject(with: person.objectID)) else { 
             self.childContext = nil        
             self.person = person
             return
         }
         self.context = childContext
         self.person = childPerson
    }
    //...
}

I find it a bit strange that the view model is a struct and not a class. You could make it into a class that conforms to ObservableObject and declare it as a @StateObject instead in your view.