I'm working on an iOS app using SwiftUI and CoreData and am running into a problem that I cannot seem to figure out.
To provide a little bit of information of what I am trying to do:
I have two CoreData entities that have a One-To-Many relationship:
Tarantula - A tarantula may molt many times (To Many) and when deleted, all of the molts shold also be removed (Cascade)
Molt - A molt belongs to a single Tarantula (To One) and when
deleted, should have the reference removed from the Tarantula (Nullify)
I then have a view that lists all of the molts for a given Tarantula that allows adding and deleting molts. It looks like this:
struct MoltListView: View {
		private static let DATE_FORMATTER: DateFormatter = {
						let d = DateFormatter()
						d.dateFormat = "MMM d, y"
						return d
				}()
		
		@Environment(\.managedObjectContext) private var viewContext
		
		@ObservedObject private var tarantula: Tarantula
		@FetchRequest private var molts: FetchedResults<Molt>
		@State private var userMessage: String = ""
		@State private var displayMessage: Bool = false
		
		init(tarantula: Tarantula) {
				self.tarantula = tarantula
				self._molts = FetchRequest(entity: Molt.entity(),
																	 sortDescriptors: [NSSortDescriptor(keyPath: \Molt.date, ascending: false)],
																	 predicate: NSPredicate(format: "tarantula = %@", tarantula))
		}
		
		var body: some View {
				List {
						Section(header: Text("Summary")) {
								Text("\(molts.count) Molt\(molts.count == 1 ? "" : "s")")
						}
						Section(header: Text("Molts")) {
								NavigationLink(destination: MoltView(tarantula: tarantula, molt: Molt.newModel())) {
										Text("Add Molt").foregroundColor(.blue)
								}
								ForEach(molts, id: \.self) { molt in
										NavigationLink(destination: MoltView(tarantula: tarantula, molt: molt)) {
												Text(MoltListView.DATE_FORMATTER.string(from: molt.modelDate))
										}
								}
								.onDelete(perform: deleteItems)
						}
				}.alert(isPresented: $displayMessage) {
						Alert(title: Text("Save Failure"), message: Text(userMessage), dismissButton: .default(Text("Ok")))
				}
		}
		
		private func deleteItems(offsets: IndexSet) {
				withAnimation {
						offsets.map { molts[$0] }.forEach(viewContext.delete)
						do {
								try viewContext.save()
						} catch {
								viewContext.rollback()
								userMessage = "\(error): \(error.localizedDescription)"
								displayMessage.toggle()
						}
				}
		}
}
The error I am experiencing comes from whenever I try to delete a molt from the list view. The app instantly crashes and the error is:
Simultaneous accesses to 0x7f92efc61cb8, but modification requires exclusive access
Find the complete error here:
error - https://developer.apple.com/forums/content/attachment/6d74dcde-d82b-4024-ade0-5936d8926488
I have tried removing the animation block and have played around with removing different UI components/restructuring.
The only way I have been able to prevent this error is to remove the delete rule on the Molt->Tarantula relationship from Nullify to No Action. However, this seems more like a hack to me instead of a fix.
Was hoping for some help on this issue.
TylerKenneth43
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