I am working on a project to develop a iOS app and widget extension(iOS 17+ and Xcode 15), and using coreData to manage user data, I have a 'lastModify: Date' derived attribute for my user entity, however the in-memory persistent store is not allow to have derived properties has argument, it leads app to crash when I trying to create a dummy data for preview provider, the error message: UserInfoWidgetExtension crashed due to an uncaught exception 'NSInvalidArgumentException'. Reason: Core Data provided atomic stores do not support derived
.
And my code:
import WidgetKit
import SwiftUI
import Intents
import AppIntents
import CoreData
struct WidgetDetailsProvider: AppIntentTimelineProvider {
func placeholder(in context: Context) -> SimpleEntry {
return SimpleEntry(date: .now)
}
func snapshot(for configuration: UserInfoWidgetIntent, in context: Context) async -> SimpleEntry {
return SimpleEntry(date: .now)
}
func timeline(for configuration: UserInfoWidgetIntent, in context: Context) async -> Timeline<SimpleEntry> {
let entry = SimpleEntry(date: Date())
let timeline = Timeline(entries: [entry], policy: .never)
return timeline
}
}
struct UserInfoWidgetIntent: WidgetConfigurationIntent {
static var title: LocalizedStringResource = "Test"
static var description: IntentDescription = IntentDescription("Test description")
@Parameter(title: "Display in full name")
var isDisplayFullname: Bool
init(isDisplayFullname: Bool) {
self.isDisplayFullname = isDisplayFullname
}
init() {}
}
struct SimpleEntry: TimelineEntry {
let date: Date
let isDisplayFullname: Bool
}
struct UserInfoWidgetEntryView : View {
var entry: WidgetDetailsProvider.Entry
@FetchRequest(sortDescriptors: [SortDescriptor(\.lastModify, order: .reverse)]) var userInfo: FetchedResults<UserInfo>
var body: some View {
VStack {
Text(entry.date, style: .time)
.frame(alignment: .leading)
HStack {
if userInfo.isEmpty {
Text("The list is Empty")
}
ForEach(userInfo) {user in
if entry.isDisplayFullname {
Text(user.fullname)
} else {
Text(user.name)
}
}
}
}.containerBackground(for: .widget) {
Color.white
}
}
}
struct UserInfoWidget: Widget {
let kind: String = "UserInfoWidgetIntent"
var body: some WidgetConfiguration {
AppIntentConfiguration(
kind: kind,
intent: UserInfoWidgetIntent.self,
provider: WidgetDetailsProvider()) { entry in
UserInfoWidgetEntryView(entry: entry)
}
.configurationDisplayName("My Widget")
.description("This is an example widget.")
.supportedFamilies([.systemSmall, .systemMedium, .systemLarge])
}
}
struct UserInfoWidget_Previews: PreviewProvider {
static var previews: some View {
let dummyContainer: NSPersistentContainer = {
let container = NSPersistentContainer(name: "UserModel")
let description = NSPersistentStoreDescription()
description.type = NSInMemoryStoreType
container.persistentStoreDescriptions = [description]
container.loadPersistentStores {description, error in
if let error = error {
print("Failed to load: \(error)")
}
}
let dummy = Barcode(context: container.viewContext)
dummy.username = "foo"
do {
try container.viewContext.save()
} catch {
fatalError("Failed to save entity.")
}
return container
}()
UserInfoWidgetEntryView(entry: SimpleEntry(date: Date()))
.previewContext(WidgetPreviewContext(family: .systemMedium))
.environment(\.managedObjectContext, dummyContainer.viewContext)
}
}
Eventually I remove the derived attribute and it work just fine, but I am wondering it is have other way to avoid this problem? Is there any way to preview coreData entity with derived attribute? Thank you.
Update 12/1:
The error message of crash is:
UserInfoWidgetExtension crashed due to an uncaught exception 'NSInvalidArgumentException'. Reason: Core Data provided atomic stores do not support derived
.
And update the code of UserInfoWidget.swift.
Update 15/1: Thanks to @loremipsum suggestion and it work well with following code:
let dummyContainer: NSPersistentContainer = {
let container = NSPersistentContainer(name: "UserInfoModel")
let description = NSPersistentStoreDescription()
//description.type = NSInMemoryStoreType
description.url = URL(fileURLWithPath: "/dev/null")
container.persistentStoreDescriptions = [description]
container.loadPersistentStores {description, error in
if let error = error {
print("Failed to load: \(error)")
}
}
let dummy = UserInfo(context: container.viewContext)
dummy.name = "foo"
dummy.fullname = "dummy foo"
do {
try container.viewContext.save()
} catch {
fatalError("Failed to save entity.")
}
return container
}()
USerInfoWidgetEntryView(entry: SimpleEntry(date: Date()))
.previewContext(WidgetPreviewContext(family: .systemSmall))
.environment(\.managedObjectContext, dummyContainer.viewContext)
There are 2 ways to have an in-memory container.
One is by using NSInMemoryStoreType
description.type = NSInMemoryStoreType
And the other is by setting the url to null
description.url = URL(fileURLWithPath: "/dev/null")
The first way is known to have many limitations such as cascading deletion, the second way is what Apple uses to create a preview container in the standard PersistentController
that is created by Xcode.