我发布了这个问题：
SwiftUI：从单元格视图中删除托管对象会使应用程序崩溃？ https://stackoverflow.com/questions/73159270/swiftui-deleting-managed-object-from-cell-view-crashes-app
当我试图理解它崩溃的原因时，我尝试更改模型Item具有timestamp非可选：
extension Item {
@nonobjc public class func fetchRequest() -> NSFetchRequest<Item> {
return NSFetchRequest<Item>(entityName: "Item")
}
@NSManaged public var timestamp: Date
}
extension Item : Identifiable {
}
正如 Asperi 指出的，使用这个：
if let timestamp = item.timestamp {
Text(timestamp, formatter: itemFormatter)
}
确实修复了崩溃时timestamp是可选的。
然而，这只是我正在测试的一些代码，以了解如何正确构建我的视图。我需要使用没有可选属性的模型，因此我无法使用为上面链接的问题提供的答案。
所以这个问题是为了解决我的情况CellView使用的属性是不可选在托管对象上。
如果我把这段代码直接放在ContentView不使用CellView它不会崩溃。这不会崩溃：
struct ContentView: View {
@Environment(\.managedObjectContext) private var viewContext
@FetchRequest(
sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
animation: .default)
private var items: FetchedResults<Item>
var body: some View {
NavigationView {
List {
ForEach(items) { item in
NavigationLink {
Text(item.timestamp, formatter: itemFormatter)
} label: {
// CellView(item: item)
HStack {
Text(item.timestamp, formatter: itemFormatter) // <<- CRASH ON DELETE
Button {
withAnimation {
viewContext.delete(item)
try? viewContext.save()
}
} label: {
Text("DELETE")
.foregroundColor(.red)
}
.buttonStyle(.borderless)
}
}
}
.onDelete(perform: deleteItems)
}
.toolbar {
ToolbarItem(placement: .navigationBarTrailing) {
EditButton()
}
ToolbarItem {
Button(action: addItem) {
Label("Add Item", systemImage: "plus")
}
}
}
Text("Select an item")
}
}
private func addItem() {
withAnimation {
let newItem = Item(context: viewContext)
newItem.timestamp = Date()
do {
try viewContext.save()
} catch {
let nsError = error as NSError
fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
}
}
}
private func deleteItems(offsets: IndexSet) {
withAnimation {
offsets.map { items[$0] }.forEach { item in
viewContext.delete(item)
}
do {
try viewContext.save()
} catch {
let nsError = error as NSError
fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
}
}
}
}
但是，我需要知道如何保持CellView, use @ObservedObject并使这项工作成功。在这种情况下，这样做没什么大不了的，但在实际情况下，CellView更大时，这种方法不能很好地扩展。无论如何，为什么要使用@ObservedObject单独来看是错的吗？
那么，为什么应用程序崩溃时timestamp模型中不是可选的吗？
为什么视图试图为已删除的项目重绘 CellView ？如何解决这个问题？
为了清楚起见，我在此处发布了非可选案例的新代码，因此您不必返回并查看链接的问题，然后将其更改为非可选。这是整个崩溃的代码：
struct ContentView: View {
@Environment(\.managedObjectContext) private var viewContext
@FetchRequest(
sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
animation: .default)
private var items: FetchedResults<Item>
var body: some View {
NavigationView {
List {
ForEach(items) { item in
NavigationLink {
Text(item.timestamp, formatter: itemFormatter)
} label: {
CellView(item: item)
}
}
.onDelete(perform: deleteItems)
}
.toolbar {
ToolbarItem(placement: .navigationBarTrailing) {
EditButton()
}
ToolbarItem {
Button(action: addItem) {
Label("Add Item", systemImage: "plus")
}
}
}
Text("Select an item")
}
}
private func addItem() {
withAnimation {
let newItem = Item(context: viewContext)
newItem.timestamp = Date()
do {
try viewContext.save()
} catch {
let nsError = error as NSError
fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
}
}
}
private func deleteItems(offsets: IndexSet) {
withAnimation {
offsets.map { items[$0] }.forEach { item in
viewContext.delete(item)
}
do {
try viewContext.save()
} catch {
let nsError = error as NSError
fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
}
}
}
}
struct CellView: View {
@Environment(\.managedObjectContext) private var viewContext
@ObservedObject var item:Item
var body: some View {
HStack {
Text(item.timestamp, formatter: itemFormatter) // <<- CRASH ON DELETE
Button {
withAnimation {
viewContext.delete(item)
try? viewContext.save()
}
} label: {
Text("DELETE")
.foregroundColor(.red)
}
.buttonStyle(.borderless)
}
}
}
private let itemFormatter: DateFormatter = {
let formatter = DateFormatter()
formatter.dateStyle = .short
formatter.timeStyle = .medium
return formatter
}()
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
}
}
