SwiftUI holding reference to deleted core data object causing crash

Question

Im finding it impossible to use core data with SwiftUI, because as I pass a core data to a view observed object variable, the navigation link view will hold a reference to the object even after the view has disappeared, so as soon as I delete the object from context the app crashes, with no error messages.

I have confirmed this by wrapping the core data object variable into a view model as an optional, then set the object to nil right after the context delete action and the app works fine, but this is not a solution because I need the core data object to bind to the swift ui views and be the source of truth. How is this suppose to work? I seriously cannot make anything remotely complex with SwiftUI it seems.

I have tried assigning the passed in core data object to a optional @State, but this does not work. I cannot use @Binding because it's a fetched object. And I cannot use a variable, as swiftui controls require bindings. It only makes sense to use a @ObservedObject, but this cannot be an optional, which means when the object assigned to it gets deleted, the app crashes, because i cannot set it to nil.

Here is the core data object, which is an observable object by default:

class Entry: NSManagedObject, Identifiable {

    @NSManaged public var date: Date
}

Here is a view that passes a core data entry object to another view.

struct JournalView: View {

    @Environment(\.managedObjectContext) private var context

    @FetchRequest(
        entity: Entry.entity(),
        sortDescriptors: [],
        predicate: nil,
        animation: .default
    ) var entries: FetchedResults<Entry>

    var body: some View {
        NavigationView {
            List {
                ForEach(entries.indices) { index in
                    NavigationLink(destination: EntryView(entry: self.entries[index])) {
                        Text("Entry")
                    }
                }.onDelete { indexSet in
                    for index in indexSet {
                        self.context.delete(self.entries[index])
                    }
                }
            }
        }
    }
}

Now here is the view that accesses all the attributes from the core data entry object that was passed in. Once, I delete this entry, from any view by the way, it is still referenced here and causes the app to crash immediately. I believe this also has something to do with the Navigation Link initializing all destination view before they are even accessed. Which makes no sense why it would do that. Is this a bug, or is there a better way to achieve this?

I have even tried doing the delete onDisappear with no success. Even if I do the delete from the JournalView, it will still crash as the NavigationLink is still referencing the object. Interesting it will not crash if deleting a NavigationLink that has not yet been clicked on.

struct EntryView: View {

    @Environment(\.managedObjectContext) private var context
    @Environment(\.presentationMode) private var presentationMode

    @ObservedObject var entry: Entry

    var body: some View {
        Form {

            DatePicker(selection: $entry.date) {
                Text("Date")
            }

            Button(action: {
                self.context.delete(self.entry)
                self.presentationMode.wrappedValue.dismiss()
            }) {
                Text("Delete")
            }
        }
    }
}

UPDATE

The crash is taking me to the first use of entry in the EntryView and reads Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).. thats the only message thrown.

The only work around I can think of is to add a property to the core data object "isDeleted" and set it to true instead of trying to delete from context. Then when the app is quit, or on launch, I can clean and delete all entries that isDeleted? Not ideal, and would prefer to figure out what it wrong here, as it appears I'm not doing anything different then the MasterDetailApp sample, which seems to work.

Answer 1

I basically had the same issue. It seems that SwiftUI loads every view immediately, so the view has been loaded with the Properties of the existing CoreData Object. If you delete it within the View where some data is accessed via @ObservedObject, it will crash.

My Workaround:

1. The Delete Action - postponed, but ended via Notification Center

    Button(action: {
      //Send Message that the Item  should be deleted
       NotificationCenter.default.post(name: .didSelectDeleteDItem, object: nil)

       //Navigate to a view where the CoreDate Object isn't made available via a property wrapper
        self.presentationMode.wrappedValue.dismiss()
      })
      {Text("Delete Item")}

You need to define a Notification.name, like:

extension Notification.Name {

    static var didSelectDeleteItem: Notification.Name {
        return Notification.Name("Delete Item")
    }
}

2. On the appropriate View, lookout for the Delete Message

// Receive Message that the Disease should be deleted
    .onReceive(NotificationCenter.default.publisher(for: .didSelectDeleteDisease)) {_ in

        //1: Dismiss the View (IF It also contains Data from the Item!!)
        self.presentationMode.wrappedValue.dismiss()

        //2: Start deleting Disease - AFTER view has been dismissed
        DispatchQueue.main.asyncAfter(deadline: .now() + TimeInterval(1)) {self.dataStorage.deleteDisease(id: self.diseaseDetail.id)}
    }

3. Be safe on your Views where some CoreData elements are accessed - Check for isFault!

    VStack{
         //Important: Only display text if the disease item is available!!!!
           if !diseaseDetail.isFault {
                  Text (self.diseaseDetail.text)
            } else { EmptyView() }
    }

A little bit hacky, but this works for me.

Answer 2

I encountered the same issue and did not really find a solution to the root problem. But now I "protect" the view that uses the referenced data like this:

var body: some View {
    if (clip.isFault) {
        return AnyView(EmptyView())
    } else {
        return AnyView(actualClipView)
    }
}

var actualClipView: some View {
    // …the actual view code accessing various fields in clip
}

That also feelds hacky, but works fine for now. It's less complex than using a notification to "defer" deletion, but still thanks to sTOOs answer for the hint with .isFault!

Answer 3

I have had the same issue for a while, the solution for me was pretty simple: In the View where the @ObservedObject is stored I simply put this !managedObject.isFault.

I experienced this class only with ManagedObjects with a date property, I don't know if this is the only circumstance the crash verifies.

import SwiftUI

struct Cell: View {
    
    @ObservedObject var managedObject: MyNSManagedObject
    
    var body: some View {
        if !managedObject.isFault {
           Text("\(managedObject.formattedDate)")
        } else {
            ProgressView()
        }
    }
}