我正在使用SwiftUI和CoreData开发iOS应用程序，遇到了一个似乎无法解决的问题。
提供一点关于我正在尝试做什么的信息：
我有两个具有一对多关系的CoreData实体：
- 狼蛛-狼蛛可能会蜕皮很多次(To many(，删除后，所有蜕皮也会被删除(Cascade(
- 蜕皮-蜕皮属于一只狼蛛(to One(，删除后，应该将引用从狼蛛中删除(Nullify(
然后我有一个视图，列出了给定狼蛛的所有蜕皮，允许添加和删除蜕皮。它看起来像这样：
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()
}
}
}
}
每当我试图从列表视图中删除蜕皮时，我就会遇到错误。应用程序立即崩溃，错误为：
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
我能够防止这个错误的唯一方法是删除Molt->从无效到无行动的狼蛛关系。然而，在我看来，这更像是破解而不是修复。
我想知道是否有人可能以前经历过这个问题，或者可能能够帮助我解决它
已确认此问题是由系统错误引起的。
解决方法是将删除逻辑封装在NSManagedObjectContext.perform中。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()
}
}
}
}
以下是与此问题相关的苹果开发者论坛帖子，供参考：https://developer.apple.com/forums/thread/668299