SwiftUI list references deleted NSManagedObjects - ios

Question

When using #FetchRequest to populate a List, SwiftUI will attempt to access a deleted NSManagedObject even after it has been deleted from the store (for example, via swipe action).
This bug is very common and easy to reproduce: simply create a new Xcode project with the default SwiftUI + CoreData template. Then replace ContentView with this code:
struct ContentView: View {
#Environment(\.managedObjectContext) private var moc
#FetchRequest(sortDescriptors: []) private var items: FetchedResults<Item>
var body: some View {
List {
ForEach(items) { item in
ItemRow(item: item)
}
.onDelete {
$0.map{items[$0]}.forEach(moc.delete)
try! moc.save()
}
}
.toolbar {
Button("Add") {
let newItem = Item(context: moc)
newItem.timestamp = Date()
try! moc.save()
}
}
}
}
struct ItemRow: View {
#ObservedObject var item: Item
var body: some View {
Text("\(item.timestamp!)")
}
}
Add a few items to the list, then swipe to delete a row: the app will crash. An ItemRow is attempting to draw with the now-deleted Item.
A common workaround is to wrap the whole subview in a fault check:
struct ItemRow: View {
#ObservedObject var item: Item
var body: some View {
if !item.isFault {
Text("\(item.timestamp!)")
}
}
}
But this is a poor solution and has side effects (objects can be faults when not deleted).
This answer suggests that wrapping the deletion in viewContext.perform{} would work, but the crash still occurs for me.
Any better solutions/workarounds out there?

Answer 1

This does seem to be the result of the ObservedObject switching to a fault as part of the delete operation, while timestamp! is force unwrapped. The force unwrap works for the full object but not the fault.
An approach which does seem to work is to remove the .onDelete action at the ForEach level, and replace it with a swipeAction at the row level:
ForEach(items) { item in
ItemRow(item: item)
.swipeActions {
Button("Delete", role: .destructive) {
viewContext.delete(item)
try? viewContext.save()
}
}
}
In any respect, it shows that even with a simple NSManagedObject like Item, relying on force unwrapping of attributes comes with risks.