Hello everyone,

I'm building a SwiftUI app (iOS 15+) using Core Data. My post is simply out of interest, as I already found a workaround for the issue I had, but if someone could explain me why I had to do it this way, I'd be more than happy to learn why!

(For the sake of simplicity I show you a simplistict version of my code with only the interesting parts.)

After being able to add and list items (using Paul's awesome articles), I of course wanted to edit items! This is where I ended up watching Donny Wals' talk about this and followed his approach (https://youtu.be/P8rqjs_CNsk?t=1700) of using a viewModel which stores a temporary NSManagedObject in a child context, before persisting it to the main context when we are done. Clever.

In his video the viewModel is a struct, which is stored as a @State variable in the editing View. In my case I have some controls in this editing View which appear/disappear/change depending of other values the user changes (switchs, fields, whatever…).

My initial viewModel based on Donny Wals (persistenceController functions can be found in his talk, but I don't belive it really matters for my problem):

struct EditViewModel {
    var item: Item // This is a NSManagedObject

    private let childContext: NSManagedObjectContext
    private let persistenceController = PersistenceController.shared

    init(item: Item? = nil) {
        self.childContext = persistenceController.childViewContext()

        if let item = item {
            self.item = persistenceController.copyForEditing(of: item, in: childContext)
        } else {
            self.item = persistenceController.newTemporaryInstance(in: childContext)
        }
    }

    // More functions below to persist changes, etc.
}

And my EditView:

struct EditView: View {
    @State private var vm: EditViewModel

    init(item: Item? = nil) {
        _vm = State(initialValue: EditViewModel(item: item))
    }

    var body: some View {
        Form {
            Text(vm.item.text) // Should update when we change text in the TextField below.
            TextField("", text: $vm.item.text)
        }
    }
}

With this code, the issue I've faced is the Text view doesn't seem to reflect changes when we edit the TextField.

Changing my viewModel for a class does the trick, and it works fine:

class EditViewModel: ObservableObject {
    @Published var item: Item // This is a NSManagedObject

    // ...
}

The big question is why! Why do I have to use an ObservableObject. From what I understood, a struct as a @State should behaves the same, no? Is this related to the NSManagedObject under-the-hood architecture?

Thanks for enlightening me on this weird thing!

Is this related to the NSManagedObject under-the-hood architecture?

Yep. Since an NSManagedObject is a reference type, changing a property on the object doesn't change the value of the object, which means your struct, which is a value type, doesn't change either, which means SwiftUI doesn't recognize that your state has changed and won't refresh the View.