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 o

Answer 1

I had the same issue recently. Adding an entity property to the view fixed it.

ForEach(entities, id: \.self) { entity in
    Button(action: {

    }) {
        MyCell(entity: entity)
    }
}

To

ForEach(entities, id: \.self) { entity in
    Button(action: {

    }) {
        MyCell(entity: entity, property: entity.property)
    }
}

I suspect that the nullable Core Data entity is the cause of the issue, where as adding a non-nil property as a var (e.g, var property: String) fixed it

Answer 2

I have tried all previous solutions, none worked for me.

This one, worked.

I had my list like this:

List {
  ForEach(filteredItems, id: \.self) { item in
    ListItem(item:item)
  }
.onDelete(perform: deleteItems)


private func deleteItems(offsets: IndexSet) {
  //deleting items

This was crashing.

I modified the code to this one

List {
  ForEach(filteredItems, id: \.self) { item in
    ListItem(item:item)
  }
  .onDelete { offsets in
     // delete objects
  }

This works fine without crashing.

For heaven's sake, Apple!

Answer 3

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 4

A view modifier for this (based on conditional view modifiers):

import SwiftUI
import CoreData

extension View {
    @ViewBuilder
    func `if`<Transform: View>(
        _ condition: Bool,
        transform: (Self) -> Transform
    ) -> some View {
        if condition {
            transform(self)
        } else {
            self
        }
    }
}

extension View {
    func hidingFaults(_ object: NSManagedObject) -> some View {
        self.if(object.isFault) { _ in EmptyView() }
    }
}

Having said that, it's worth checking you're performing CoreData operations asynchronously on the main thread, doing it synchronously can be a source of grief (sometimes, but not always).

Answer 5

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 6

After some research online, it's clear to me that this crash can be caused by many things related to optionals. For me, I realized that declaring a non-optional Core Data attribute as an optional in the NSManagedObject subclass was causing the issue.

Specifically, I have a UUID attribute id in Core Data that cannot have a default value, but is not optional. In my subclass, I declared @NSManaged public var id: UUID. Changing this to @NSManaged public var id: UUID? fixed the problem immediately.