SwiftUI：未调用@StateObject deinit？

Question

I have the following code:

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults<Item>

    var body: some View {
        NavigationView {
            List {
                ForEach(items) { item in
                    NavigationLink {
                        Text("Item at \(item.timestamp!, formatter: itemFormatter)")
                    } label: {
//                        Text(item.timestamp!, formatter: itemFormatter)
                        ItemCellView(model: ItemCellViewModel(item: item))
                    }
                }
                .onDelete(perform: deleteItems)
            }
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    EditButton()
                }
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
            Text("Select an item")
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()

            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 deleteItems(offsets: IndexSet) {
        withAnimation {
            offsets.map { items[$0] }.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)")
            }
        }
    }
}

private let itemFormatter: DateFormatter = {
    let formatter = DateFormatter()
    formatter.dateStyle = .short
    formatter.timeStyle = .medium
    return formatter
}()

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


struct ItemCellView: View {
    
    @StateObject var model:ItemCellViewModel
    
    var body: some View {
        
        Text(model.item.timestamp!, formatter: itemFormatter)
            .foregroundColor(.blue)
    }
    
}

class ItemCellViewModel: ObservableObject {
    
    @Published var item:Item
    
    init(item:Item) {
        self.item = item
    }
    
    deinit {
        print("ItemCellViewModel EDINIT \(self)")
    }
    
}

It draws this:

PROBLEM:

ItemCellViewModel deinit is NOT called after I swipe to delete the item. Can someone tell me why the ItemCellViewModel sticks around even after the ItemCellView is gone?

This is a simplified version of a codebase I am working in. I need that model to go away when the view is "deleted" by the user. Why is SwiftUI keeping ItemCellViewModel around??

Answer 1

View is not deleted in a fact (just removed from visible area) because List caches some number of views (visible area + ~2) and StateObject is persistent storage of view which keeps its state. So observed behavior is by-design.

Answer 2

Get rid of the view model object, in SwiftUI we use value types and the View struct is the view model that SwiftUI uses to create and update UIKit/AppKit views on our behalf. Learn this in SwiftUI Essentials WWDC 2019 . Also you can't nest an ObservableObject inside an ObservableObject either. To fix this change the ItemCellView to this:

struct ItemCellView: View {
    
    @ObservedObject var item: Item
    
    var body: some View {
        
        Text(item.timestamp!, formatter: itemFormatter)
            .foregroundColor(.blue)
    }
    
}