我有以下代码，我刚刚修改了Xcode为您创建的SwiftUI项目示例。
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)
}
}
Item的定义如下：
extension Item {
@nonobjc public class func fetchRequest() -> NSFetchRequest<Item> {
return NSFetchRequest<Item>(entityName: "Item")
}
@NSManaged public var timestamp: Date?
}
extension Item : Identifiable {
}
当我通过滑动或按下删除按钮删除项目时，应用程序会崩溃。如果我去掉CellView，并将所有代码直接放在导航链接中。。。它不会崩溃。
如何在保留单独的CellView的同时解决此问题？
到目前为止我学到了什么：
如果我更改：
@ObservedObject var item:Item
至：
@State var item:Item
撞车事故消失了。知道吗？
尽量避免强制打开选项和使用条件(视图生成器现在允许这样的构造(
HStack {
if let timestamp = item.timestamp {
Text(timestamp, formatter: itemFormatter)
}