SwiftUI/CoreData:同时访问0x7f92efc61cb8，但修改需要独占访问

Question

我正在使用SwiftUI和CoreData开发一个SwiftUI应用程序，遇到了一个我似乎搞不懂的问题。

为了提供一些我想要做的事情的信息：

我有两个CoreData实体，它们有一对多的关系：

1. Tarantula -一只狼蛛可能多次蜕皮(对多只)，当被删除时，所有的蜕皮也被移除(级联)
2. molt -A蜕皮属于单一的tarantula (对一只)，当被删除时，应该从Tarantula (无效)

中删除引用。

然后，我有一个观点，列出所有的蜕皮为一个给定的塔兰杜拉，允许添加和删除蜕皮。看起来是这样的：

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()
            }
        }
    }
}

当我试图从列表视图中删除一个molt时，我正在经历的错误来自于。该应用程序立即崩溃，错误是：

Simultaneous accesses to 0x7ffb68d2c518, but modification requires exclusive access.
Previous access (a modification) started at SwiftUI`closure #2 in ListCoreCoordinator.tableView(_:commit:forRowAt:) + 53 (0x7fff56544a85).
Current access (a read) started at:
0    libswiftCore.dylib                 0x00007fff2f41fe90 swift_beginAccess + 568
1    SwiftUI                            0x00007fff56540670 ListCoreCoordinator.dataSource.getter + 50
2    SwiftUI                            0x00007fff56541140 ListCoreCoordinator.updateUITableView(_:to:transaction:) + 608
3    SwiftUI                            0x00007fff5653ff40 ListRepresentable.updateUIView(_:context:) + 892
4    SwiftUI                            0x00007fff569acc60 PlatformViewRepresentableAdaptor.updateViewProvider(_:context:) + 263
5    SwiftUI                            0x00007fff565e8ac0 closure #1 in closure #1 in closure #4 in closure #1 in PlatformViewChild.updateValue() + 229
6    SwiftUI                            0x00007fff565e89a0 closure #1 in closure #4 in closure #1 in PlatformViewChild.updateValue() + 195
7    SwiftUI                            0x00007fff56899b40 ViewRendererHost.performExternalUpdate(_:) + 186
8    SwiftUI                            0x00007fff565e8930 closure #4 in closure #1 in PlatformViewChild.updateValue() + 73
9    SwiftUI                            0x00007fff565e71e0 closure #1 in PlatformViewChild.updateValue() + 2363
10   SwiftUI                            0x00007fff565e6c40 PlatformViewChild.updateValue() + 650
11   SwiftUI                            0x00007fff562fa640 partial apply for implicit closure #2 in implicit closure #1 in closure #1 in closure #1 in Attribute.init<A>(_:) + 26
12   AttributeGraph                     0x00007fff4be8952a AG::Graph::UpdateStack::update() + 505
13   AttributeGraph                     0x00007fff4be89a6a AG::Graph::update_attribute(AG::data::ptr<AG::Node>, bool) + 335
14   AttributeGraph                     0x00007fff4be91884 AG::Subgraph::update(unsigned int) + 781
15   SwiftUI                            0x00007fff5693a5f0 GraphHost.runTransaction() + 186
16   SwiftUI                            0x00007fff5693e110 GraphHost.runTransaction(_:) + 79
17   SwiftUI                            0x00007fff5693cba0 GraphHost.flushTransactions() + 211
18   SwiftUI                            0x00007fff5693bd40 specialized GraphHost.asyncTransaction<A>(_:mutation:style:) + 464
19   SwiftUI                            0x00007fff56509ac0 AttributeInvalidatingSubscriber.invalidateAttribute() + 236
20   SwiftUI                            0x00007fff56509a40 AttributeInvalidatingSubscriber.receive(_:) + 105
21   SwiftUI                            0x00007fff5650a0d0 protocol witness for Subscriber.receive(_:) in conformance AttributeInvalidatingSubscriber<A> + 16
22   SwiftUI                            0x00007fff5669a7e0 SubscriptionLifetime.Connection.receive(_:) + 89
23   Combine                            0x00007fff4ab32ec0 ObservableObjectPublisher.Inner.send() + 122
24   Combine                            0x00007fff4ab324b0 ObservableObjectPublisher.send() + 801
25   libswiftCoreData.dylib             0x00007fff53ebc900 @objc NSManagedObject._willChange_Swift_Trampoline() + 114
26   CoreData                           0x00007fff251b7e4f -[NSManagedObjectContext _postObjectsDidChangeNotificationWithUserInfo:] + 773
27   CoreData                           0x00007fff251c6b5b -[NSManagedObjectContext _createAndPostChangeNotification:deletions:updates:refreshes:deferrals:wasMerge:] + 1825
28   CoreData                           0x00007fff251b8c0d -[NSManagedObjectContext _processRecentChanges:] + 1041
29   CoreData                           0x00007fff2511c241 -[NSManagedObjectContext save:] + 356
30   Tarantulas                         0x0000000101a14060 closure #1 in MoltListView.deleteItems(offsets:) + 1165
31   Tarantulas                         0x0000000101a07080 thunk for @callee_guaranteed () -> (@error @owned Error) + 15
32   Tarantulas                         0x0000000101a16b80 partial apply for thunk for @callee_guaranteed () -> (@error @owned Error) + 20
33   SwiftUI                            0x00007fff569b77e0 closure #1 in withTransaction<A>(_:_:) + 83
34   SwiftUI                            0x00007fff569b7860 partial apply for closure #1 in withTransaction<A>(_:_:) + 25
35   libswiftCore.dylib                 0x00007fff2f22b160 withExtendedLifetime<A, B>(_:_:) + 12
36   SwiftUI                            0x00007fff569b7690 withTransaction<A>(_:_:) + 66
37   SwiftUI                            0x00007fff569b7580 withAnimation<A>(_:_:) + 140
38   Tarantulas                         0x0000000101a13720 MoltListView.deleteItems(offsets:) + 375
39   Tarantulas                         0x0000000101a136e0 implicit closure #2 in implicit closure #1 in closure #2 in closure #1 in MoltListView.body.getter + 42
40   SwiftUI                            0x00007fff560c0700 partial apply for thunk for @escaping @callee_guaranteed (@in_guaranteed IndexSet) -> () + 17
41   SwiftUI                            0x00007fff56713c50 partial apply for thunk for @escaping @callee_guaranteed (@in_guaranteed IndexSet) -> (@out ()) + 17
42   SwiftUI                            0x00007fff560c0890 DeleteInteraction.delete() + 125
43   SwiftUI                            0x00007fff564a0090 SystemListDataSource.deleteCell(forRowAt:) + 64
44   SwiftUI                            0x00007fff56660d40 ShadowListDataSource.commitUpdates() + 963
45   SwiftUI                            0x00007fff56544a50 closure #2 in ListCoreCoordinator.tableView(_:commit:forRowAt:) + 76
46   SwiftUI                            0x00007fff56544730 ListCoreCoordinator.tableView(_:commit:forRowAt:) + 455
47   SwiftUI                            0x00007fff56544ab0 @objc ListCoreCoordinator.tableView(_:commit:forRowAt:) + 131
48   UIKitCore                          0x00007fff248ade86 -[UITableView _animateDeletionOfRowAtIndexPath:] + 188
49   UIKitCore                          0x00007fff248b68d7 __82-[UITableView _contextualActionForDeletingRowAtIndexPath:usingPresentationValues:]_block_invoke + 60
50   UIKitCore                          0x00007fff247fe817 -[UIContextualAction executeHandlerWithView:completionHandler:] + 148
51   UIKitCore                          0x00007fff2480c7b4 -[UISwipeOccurrence _executeLifecycleForPerformedAction:sourceView:completionHandler:] + 656
52   UIKitCore                          0x00007fff2480cf2d -[UISwipeOccurrence _performSwipeAction:inPullView:swipeInfo:] + 621
53   UIKitCore                          0x00007fff2480ec53 -[UISwipeOccurrence swipeActionPullView:tappedAction:] + 92
54   UIKitCore                          0x00007fff24816a96 -[UISwipeActionPullView _tappedButton:] + 148
55   UIKitCore                          0x00007fff2467b5db -[UIApplication sendAction:to:from:forEvent:] + 83
56   UIKitCore                          0x00007fff23fa488d -[UIControl sendAction:to:forEvent:] + 223
57   UIKitCore                          0x00007fff23fa4b43 -[UIControl _sendActionsForEvents:withEvent:] + 332
58   UIKitCore                          0x00007fff23fa3384 -[UIControl touchesEnded:withEvent:] + 500
59   UIKitCore                          0x00007fff241a546a _UIGestureEnvironmentUpdate + 8849
60   UIKitCore                          0x00007fff241a4c77 -[UIGestureEnvironment _updateForEvent:window:] + 887
61   UIKitCore                          0x00007fff246b893d -[UIWindow sendEvent:] + 4752
62   UIKitCore                          0x00007fff2469367d -[UIApplication sendEvent:] + 633
63   UIKitCore                          0x00007fff24720d55 __processEventQueue + 13895
64   UIKitCore                          0x00007fff2471aca7 __eventFetcherSourceCallback + 104
65   CoreFoundation                     0x00007fff2038c369 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
66   CoreFoundation                     0x00007fff2038c1be __CFRunLoopDoSource0 + 180
67   CoreFoundation                     0x00007fff2038b65c __CFRunLoopDoSources0 + 248
68   CoreFoundation                     0x00007fff20385bb1 __CFRunLoopRun + 878
69   CoreFoundation                     0x00007fff2038548f CFRunLoopRunSpecific + 567
70   GraphicsServices                   0x00007fff2b76ad28 GSEventRunModal + 139
71   UIKitCore                          0x00007fff24674df7 -[UIApplication _run] + 912
72   UIKitCore                          0x00007fff24679fd3 UIApplicationMain + 101
73   SwiftUI                            0x00007fff568cfc60 closure #1 in KitRendererCommon(_:) + 119
74   SwiftUI                            0x00007fff568cfbc0 runApp<A>(_:) + 143
75   SwiftUI                            0x00007fff56407980 static App.main() + 61
76   Tarantulas                         0x0000000101a17940 static TarantulasApp.$main() + 33
77   Tarantulas                         0x0000000101a17c20 main + 20
78   libdyld.dylib                      0x00007fff20256408 start + 1

我能够防止此错误的唯一方法是删除Molt->Tarantula关系中的删除规则，使其从无效变为不采取任何行动。然而，对我来说，这似乎更像是黑客，而不是修复。

我想知道是否有人曾经经历过这个问题，或者也许能够帮助我解决这个问题。

Answer 1

此问题已被证实是由于系统错误造成的。

解决方法是在NSManagedObjectContext.perform中封装删除逻辑。例如：

private func deleteItems(offsets: IndexSet) {
    withAnimation {
        viewContext.perform { // HERE
            offsets.map { molts[$0] }.forEach(viewContext.delete)
            do {
                try viewContext.save()
            } catch {
                viewContext.rollback()
                userMessage = "\(error): \(error.localizedDescription)"
                displayMessage.toggle()
            }
        }
    }
}

作为参考，这里是苹果开发者论坛上与这个问题相关的帖子：https://developer.apple.com/forums/thread/668299