SwiftUI with Core Data and MVVM doesn't update

Question

I am trying to use Core Data with MVVM in SwiftUI, but I can't manage to make the List in my view update whenever there are changes to Core Data.

Please check this example. Why my List doesn't update if items is a Published variable?

QUESTION: How to make the List update every time I add or delete items?

Right now I need to force quit the app and open it again to see changes. I've searched for many similar questions and I wasn't able to make it work. Maybe I'm missing something obvious.

EXAMPLE CODE:

ContentView.swift

import SwiftUI
import CoreData

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext
    @StateObject var vm = ContentViewModel()

    var body: some View {
        NavigationView {
            List {
                ForEach(vm.items) { item in
                    Text(item.text!)
                }
            }
            .toolbar {
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
                ToolbarItem(placement: .destructiveAction){
                    Button(action: deleteAll) {
                        Image(systemName: "trash")
                            .foregroundColor(.red)
                    }
                }
            }
            Text("Select an item")
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.text = "List item"

            do {
                try viewContext.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
    
    private func deleteAll() {
        vm.items.forEach(viewContext.delete)
        
        do {
            try viewContext.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            let nsError = error as NSError
            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
    }
}

ContentViewModel.swift

import Foundation
import CoreData

final class ContentViewModel: ObservableObject {
    // Why my List doesn't update if items is a Published variable?
    @Published var items: [Item] = []
    let container: NSPersistentContainer = PersistenceController.shared.container
    
    init() {
        let fetchRequest = NSFetchRequest<Item>(entityName: "Item")
        
        do {
            items = try container.viewContext.fetch(fetchRequest)
        } catch let error {
            print("Error fetching. \(error)")
        }
    }
}

Persistence.swift (it's almost the same as default Core Data example file in Xcode 14)

import CoreData

struct PersistenceController {
    static let shared = PersistenceController()

    static var preview: PersistenceController = {
        let result = PersistenceController(inMemory: true)
        let viewContext = result.container.viewContext
        for _ in 0..<10 {
            let newItem = Item(context: viewContext)
            newItem.text = "Abc"
        }
        do {
            try viewContext.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            let nsError = error as NSError
            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
        }
        return result
    }()

    let container: NSPersistentContainer

    init(inMemory: Bool = false) {
        container = NSPersistentContainer(name: "CoreDataMVVM")
        if inMemory {
            container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
        }
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.

                /*
                 Typical reasons for an error here include:
                 * The parent directory does not exist, cannot be created, or disallows writing.
                 * The persistent store is not accessible, due to permissions or data protection when the device is locked.
                 * The device is out of space.
                 * The store could not be migrated to the current model version.
                 Check the error message to determine what the actual problem was.
                 */
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
        container.viewContext.automaticallyMergesChangesFromParent = true
    }
}

xcdatamodeld has 1 entity: Item with 1 attribute: text: String

Answer 1

We don't need MVVM in SwiftUI because the View struct is a view model. If you use a view model object instead, you'll have this consistency problem. To make the list update, use @FetchRequest in the View struct.

FYI FetchRequest is a DynamicProperty struct and gets the Environment's managedObjectContext in its update func.