I'm trying to load items from the database into a list, it works fine on device and simulator but Preview will always crash with the following message:
"Cannot preview in this file — Connection interrupted: send message to agent"
import SwiftUI
import CoreData
struct SettingsView: View {
#Environment(\.managedObjectContext) var moc
#FetchRequest(entity: ChildProfile.entity(), sortDescriptors: []) var children: FetchedResults<ChildProfile>
var body: some View {
VStack {
List {
Section(header: Text("Children")) {
ForEach(children, id: \.id) { child in
ChildRow(child: child)
}
}
}
}
}
}
struct SettingsView_Previews: PreviewProvider {
static let moc = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
static var previews: some View {
let defaultChild = ChildProfile(context: moc)
defaultChild.id = UUID()
defaultChild.name = "Dale"
return SettingsView().environment(\.managedObjectContext, moc)
}
}
Looking at the diagnostics of thew preview crash it shows:
Error Domain=NSCocoaErrorDomain Code=516
"“ChildProfile+CoreDataClass.swift” couldn’t be linked to
“Intermediates” because an item with the same name already exists."
UserInfo={NSSourceFilePathErrorKey=/Users/james/Library/Developer/Xcode/DerivedData/BedtimeClock-hehxrojiiepyadgitseedusiyozd/Build/Intermediates.noindex/Previews/Project
One/Intermediates.noindex/Project
One.build/Debug-iphonesimulator/Project
One.build/DerivedSources/CoreDataGenerated/Model/ChildProfile+CoreDataClass.swift,
NSUserStringVariant=(
> Link
> ),
NSDestinationFilePath=/var/folders/dp/hztzhf572q26jwx6vb9nqpw0000gn/T/previews-diagnostics-20200713-200707/Intermediates/ChildProfile+CoreDataClass.swift,
NSFilePath=/Users/james/Library/Developer/Xcode/DerivedData/Bedtime_Clock-hehxrojiiepyadgitseedusiyozd/Build/Intermediates.noindex/Previews/Project
One/Intermediates.noindex/Project
One.build/Debug-iphonesimulator/Project
One.build/DerivedSources/CoreDataGenerated/Model/ChildProfile+CoreDataClass.swift,
NSUnderlyingError=0x7fb78a150620 {Error Domain=NSPOSIXErrorDomain
Code=17 "File exists"}}
I can successfully preview ChildRow(), using the same preview code. So is this a bug or am I doing something wrong?
Related
G'day everyone,
I'm trying to work out how CoreData relationships can work with UI elements like pickers.
At the moment I have a 3 view app (based on the Xcode boilerplate code) which displays a list of parent entities, which have children which have children. I want a picker to select which grandchild a child entity should refer to.
At the moment I have two funny side effects:
When I run the app as a preview (so there is pre-populated data... this sample code will break without the data in place),
the selected grandchild in the picker is the grandchild of the first
child, irrespective of which child you're dropped into in the first
view.
When I drop back and pick another child, now the picked grabs the correct initial selection from the child entity
When I select a child and "save" that, the value in the child summary does not change, until I click another child at which point the value changes before the transition to the modal view.
I am clearly missing something in my understanding of the sequence of events when presenting modals in SwiftUI... can any what shed any light on what I've done wrong?
Here's a video to make this more clear:
https://github.com/andrewjdavison/Test31/blob/main/Test31%20-%20first%20click%20issue.mov?raw=true
Git repository of the sample is https://github.com/andrewjdavison/Test31.git, but in summary:
Data Model:
View Source:
import SwiftUI
import CoreData
struct LicenceView : View {
#Environment(\.managedObjectContext) private var viewContext
#Binding var licence: Licence
#Binding var showModal: Bool
#State var selectedElement: Element
#FetchRequest private var elements: FetchedResults<Element>
init(currentLicence: Binding<Licence>, showModal: Binding<Bool>, context: NSManagedObjectContext) {
self._licence = currentLicence
self._showModal = showModal
let fetchRequest: NSFetchRequest<Element> = Element.fetchRequest()
fetchRequest.sortDescriptors = []
self._elements = FetchRequest(fetchRequest: fetchRequest)
_selectedElement = State(initialValue: currentLicence.wrappedValue.licenced!)
}
func save() {
licence.licenced = selectedElement
try! viewContext.save()
showModal = false
}
var body: some View {
VStack {
Button(action: {showModal = false}) {
Text("Close")
}
Picker(selection: $selectedElement, label: Text("Element")) {
ForEach(elements, id: \.self) { element in
Text("\(element.desc!)")
}
}
Text("Selected: \(selectedElement.desc!)")
Button(action: {save()}) {
Text("Save")
}
}
}
}
struct RegisterView : View {
#Environment(\.managedObjectContext) private var viewContext
#State var showModal: Bool = false
var currentRegister: Register
#State var currentLicence: Licence
init(currentRegister: Register) {
currentLicence = Array(currentRegister.licencedUsers! as! Set<Licence>)[0]
self.currentRegister = currentRegister
}
var body: some View {
VStack {
List {
ForEach (Array(currentRegister.licencedUsers! as! Set<Licence>), id: \.self) { licence in
Button(action: {currentLicence = licence; showModal = true}) {
HStack {
Text("\(licence.leasee!) : ")
Text("\(licence.licenced!.desc!)")
}
}
}
}
}
.sheet(isPresented: $showModal) {
LicenceView(currentLicence: $currentLicence, showModal: $showModal, context: viewContext )
}
}
}
struct ContentView: View {
#Environment(\.managedObjectContext) private var viewContext
#FetchRequest(
sortDescriptors: [NSSortDescriptor(keyPath: \Register.id, ascending: true)],
animation: .default)
private var registers: FetchedResults<Register>
var body: some View {
NavigationView {
List {
ForEach(registers) { register in
NavigationLink(destination: RegisterView(currentRegister: register)) {
Text("Register id \(register.id!)")
}
}
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
}
}
[1]: https://i.stack.imgur.com/AfaNb.png
I didn't really understand this
• selected grandchild in the picker is the grandchild of the first child, irrespective of which child you're dropped into in the first view.
• When I drop back and pick another child, now the picked grabs the correct initial selection from the child entity
Could you attach a video that represents a problem?
But I can give you a solution to the preview problem and the second one.
Preview
If you use preview with Core Data, you need to use a viewContextcreated with MockData and pass it to your View. Here I provide a generic code, that can be modified for each of your views:
In your Persistance struct (CoreData Manager) declare a variable preview with your preview Items:
static var preview: PersistenceController = {
let result = PersistenceController(inMemory: true)
let viewContext = result.container.viewContext
// Here you create your Mock Data
let newItem = Item(context: viewContext)
newItem.yourProperty = yourValue
do {
try viewContext.save()
} catch {
// error handling
}
return result
}()
Make sure it has inMemory: Bool in its init, as it is responsible for separating real viewContext and previewContext:
init(inMemory: Bool = false) {
container = NSPersistentContainer(name: "TestCD")
if inMemory {
container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
}
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
}
Create Mock Item from your viewContext and pass it to preview:
struct YourView_Previews: PreviewProvider {
static var previews: some View {
let context = PersistenceController.preview.container.viewContext
let request: NSFetchRequest<Item> = Item.fetchRequest()
let fetchedItems = try! context.fetch(request)
YourView(item: fetchedItems)
}
}
If you use #FetchRequest and #FetchedResults it makes it easier, as they will do creating and fetching objects for you. Just implement a preview like this:
struct YourView_Previews: PreviewProvider {
static var previews: some View {
YourView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
}
}
Here is Persistence struct created by Xcode at the moment of the project initialization:
import CoreData
struct PersistenceController {
static let shared = PersistenceController()
static var preview: PersistenceController = {
let result = PersistenceController(inMemory: true)
let viewContext = result.container.viewContext
let item = Item(context: viewContext)
item.property = yourProperty
do {
try viewContext.save()
} catch {
}
return result
}()
let container: NSPersistentContainer
init(inMemory: Bool = false) {
container = NSPersistentContainer(name: "TestCD")
if inMemory {
container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
}
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
}
}
Second problem
Core Data objects are built with classes, so their type is a reference. When you change a property is a class it doesn't notifiy the view struct to redraw with a new value. (exception is classes, that are created to notify about changes.)
You need to explicitly tell your RegisterView struct to redraw itself after you dismiss your LicenceView. You can do it by creating one more variable in your RegisterView - #State var id = UUID(). Then attach an .id(id) modifier at the end of your VStack
VStack {
//your code
}.id(id)
Finally, create a function viewDismissed which will change the id property in your struct:
func viewDismissed() {
id = UUID()
}
Now, attach this function to your sheet with an optional parameter onDismiss
.sheet(isPresented: $showModal, onDismiss: viewDismissed) {
LicenceView(currentLicence: $currentLicence, showModal: $showModal, context: viewContext )
}
OK. Huge vote of thanks to Lorem for getting me to the answer. Thanks too for Roma, but it does turn out that his solution, whilst it worked to resolve one of my key problems, does introduce inefficiencies - and didn't resolve the second one.
If others are hitting the same issue I'll leave the Github repo up, but the crux of it all was that #State shouldn't be used when you're sharing CoreData objects around. #ObservedObject is the way to go here.
So the resolution to the problems I encountered were:
Use #ObservedObject instead of #State for passing around the CoreData objects
Make sure that the picker has a tag defined. The documentation I head read implied that this gets generated automatically if you use ".self" as the id for the objects in ForEach, but it seems this is not always reliable. so adding ".tag(element as Element?)" to my picker helped here.
Note: It needed to be an optional type because CoreData makes all the attribute types optional.
Those two alone fixed the problems.
The revised "LicenceView" struct is here, but the whole solution is in the repo.
Cheers!
struct LicenceView : View {
#Environment(\.managedObjectContext) private var viewContext
#ObservedObject var licence: Licence
#Binding var showModal: Bool
#FetchRequest(
sortDescriptors: [NSSortDescriptor(keyPath: \Element.desc, ascending: true)],
animation: .default)
private var elements: FetchedResults<Element>
func save() {
try! viewContext.save()
showModal = false
}
var body: some View {
VStack {
Button(action: {showModal = false}) {
Text("Close")
}
Picker(selection: $licence.licenced, label: Text("Element")) {
ForEach(elements, id: \.self) { element in
Text("\(element.desc!)")
.tag(element as Element?)
}
}
Text("Selected: \(licence.licenced!.desc!)")
Button(action: {save()}) {
Text("Save")
}
}
}
}
In my ContentView I have a FetchRequest<Project>. I navigate to ProjectView using a NavigationLink. From ProjectView I navigate to AddItemView using another NavigationLink. In AddItemView when I add an Item to the Project and call container.viewContext.save() the AddItemView automatically dismisses back to the ContentView.
My guess is that saving to CoreData updates the FetchRequest<Project> list which in turn updates the views, but I am not sure.
How can I save a new Item to the Project in CoreData and only navigate back to ProjectView and not ContentView?
To reproduce:
Create new Single View App and check Core Data and Host in CloudKit
In the .xcdatamodel delete the default Entity and replace it with an Entity called Project which has attributes date: Date and title: String and an Entity called Item which has an attribute name: String. Give the Project a relationship called items (to type Item) and choose “to many” on the right. Give the Item a relationship called project that is the inverse of items.
Replace the code in Persistence.swift with this:
// Persistence.swift
import CoreData
struct PersistenceController {
static let shared = PersistenceController()
let container: NSPersistentCloudKitContainer
init(inMemory: Bool = false) {
container = NSPersistentCloudKitContainer(name: "CoreDataBug")
if inMemory {
container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
}
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
// Replace this implementation with code to handle the error appropriately.
// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
/*
Typical reasons for an error here include:
* The parent directory does not exist, cannot be created, or disallows writing.
* The persistent store is not accessible, due to permissions or data protection when the device is locked.
* The device is out of space.
* The store could not be migrated to the current model version.
Check the error message to determine what the actual problem was.
*/
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
}
}
Copy ContentView
// ContentView.swift
import SwiftUI
struct ContentView: View {
#Environment(\.managedObjectContext) var moc
let projects: FetchRequest<Project>
init() {
projects = FetchRequest<Project>(entity: Project.entity(), sortDescriptors: [
NSSortDescriptor(keyPath: \Project.date, ascending: false)
])
}
var body: some View {
NavigationView {
List {
ForEach(projects.wrappedValue) { project in
NavigationLink(destination: ProjectView(project: project)) {
Text(project.title ?? "Title")
}
}
}
.navigationTitle("Projects")
.toolbar {
ToolbarItem(placement: ToolbarItemPlacement.navigationBarTrailing) {
Button {
withAnimation {
let project = Project(context: moc)
let now = Date()
project.date = now
project.title = now.description
try? moc.save()
}
} label: {
Label("Add Project", systemImage: "plus")
}
}
}
}
}
}
Create ProjectView.swift and copy this:
// ProjectView.swift
import SwiftUI
struct ProjectView: View {
#ObservedObject var project: Project
var items: [Item] {
project.items?.allObjects as? [Item] ?? []
}
var body: some View {
List {
ForEach(items) { item in
Text(item.name ?? "")
}
}
.toolbar {
ToolbarItem(placement: ToolbarItemPlacement.navigationBarTrailing) {
NavigationLink(destination: AddItemView(project: project)) {
Label("Add Item", systemImage: "plus")
}
}
}
}
}
Create AddItemView.swift and copy this:
import SwiftUI
// AddItemView.swift
import SwiftUI
struct AddItemView: View {
#Environment(\.presentationMode) var presentationMode
#Environment(\.managedObjectContext) var moc
let project: Project
#State private var selectedName: String = ""
var body: some View {
TextField("Type name here", text: $selectedName)
.navigationTitle("Add Item")
.navigationBarItems(trailing: Button("Add") {
let ingestion = Item(context: moc)
ingestion.project = project
ingestion.name = selectedName
try? moc.save()
presentationMode.wrappedValue.dismiss()
})
}
}
Run the app. Click the plus on the top right. Click the project that just slid in. In the ProjectView click the plus on the top right again. Type a name in the TextField and click add on the top right. When the AddItemView is dismissed it probably went back to ContentView. If not add another item to the project.
I have a parent view which does a #FetchRequest and passes the FetchedResults<T> to a child view. Everything works, and the child view is able to parse through the FetchedResults. However, I can't figure out how to set up the data so that the child's Preview struct will work. What's the proper way to set up some constant data in Preview struct so that I can instantiate the child view and pass in FetchedResults<T>?
As FetchedResults<T> is a RandomAccessCollection and swift array also is a RandomAccessCollection, here is possible solution.
Update: verified with Xcode 13.3 / iOS 15.4
struct ContentView: View {
#Environment(\.managedObjectContext) var context
#FetchRequest(entity: Person.entity(), sortDescriptors: [])
var result: FetchedResults<Person>
var body: some View {
VStack(alignment: .leading) {
Text("Persons").font(.title)
PersonsView(results: result) // FetchedResults<Person> is a collection
}
}
}
// generalize PersonsView to depend just on collection
struct PersonsView<Results:RandomAccessCollection>: View where Results.Element == Person {
let results: Results
var body: some View {
ForEach(results, id: \.self) { person in
Text("Name: \(person.name ?? "<unknown>")")
}
}
}
// Tested with Xcode 11.4 / iOS 13.4
// DOES NOT WORK ANYMORE !!
// struct ChildView_Previews: PreviewProvider {
// static var previews: some View {
// PersonsView(results: [Person()]) // << use regular array //to test
// }
}
Update: fixed & tested part for Xcode 12 / iSO 14 (due to crash of above PreviewProvider)
It appears entity now should be read & specified explicitly:
struct ChildView_Previews: PreviewProvider {
static let entity = NSManagedObjectModel.mergedModel(from: nil)?.entitiesByName["Person"]
static var previews: some View {
let person = Person(entity: entity!, insertInto: nil)
person.name = "Test Name"
return PersonsView(results: [person])
}
}
Use the preview PersistenceController within your pre-generated PersistenceController struct (inside the "Persistence.swift" file).
so if you pass an item from a Core Data "Item"-entity:
struct ContentView: View {
...
private var items: FetchedResults<Item>
..
ForEach(items) { item in
DetailView(item: item)
}
..
In the Detail-View go like this:
struct DetailView: View {
var item: FetchedResults<Item>.Element
var body: some View {
Text("Items text = \(item.text ?? "")")
}
}
struct Detail_Previews: PreviewProvider {
static var previews: some View {
let viewContext = PersistenceController.preview.container.viewContext
let previewItem = Item(context: viewContext)
previewItem.text = "Text4preview"
return Detail(item: previewItem)
}
}
A very simple CoreData app: All code provided below.
Start up with CoreData template single view app.
2 entities with a string attribute each: Message(title) and Post(name)
A NavigationView containing
NavigationLink to a list of messages
NavigationLink to a list of posts
Each linked ListView (Message/Post) has
a button to add an item to the list
a button to remove all items from the list
Now, when you run this app on a simulator (any iOS 13.x version) all runs as expected from the description above.
But on a DEVICE running iOS 13.4
Tap "Messages"
Creating/deleting messages works fine, SwiftUi view updates immediately.
Tap "back"
Tap "Messages" again. While still creating/deleting messages works fine: The debugger now shows a warning: "Context in environment is not connected to a persistent store coordinator: NSManagedObjectContext: 0x280ed72c0
Tap "Posts"
==> App crashes with EXC_BREAKPOINT (code=1, subcode=0x1f3751f08)
You can start the process with Posts first, too. Then the same crash occurs on the messages list view.
I strongly believe this is an iOS 13.4 bug because similar code ran fine on Xcode 11.3 / iOS 13.3.
Does anyone know a fix or workaround for this?
Here is a link to the full project: Full Xcode Project
The ContentView:
import SwiftUI
import CoreData
struct MessageList: View {
#Environment(\.managedObjectContext) var moc
#FetchRequest(entity: Message.entity(), sortDescriptors: [])
var messages: FetchedResults<Message>
var body: some View {
List() {
ForEach(messages, id: \.self) { message in
Text(message.title ?? "?")
}
}
.navigationBarItems(trailing:
HStack(spacing: 16) {
Button(action: deleteMessages) {
Image(systemName: "text.badge.minus")
}
Button(action: addMessage) {
Image(systemName: "plus.app")
}
}
)
}
func addMessage() {
let m = Message(context: moc)
m.title = "Message: \(Date())"
try! moc.save()
}
func deleteMessages() {
messages.forEach {
moc.delete($0)
}
}
}
struct PostList: View {
#Environment(\.managedObjectContext) var moc
#FetchRequest(entity: Post.entity(), sortDescriptors: [])
var posts: FetchedResults<Post>
var body: some View {
List {
ForEach(0..<posts.count, id: \.self) { post in
Text(self.posts[post].name ?? "?")
}
}
.navigationBarItems(trailing:
HStack(spacing: 16) {
Button(action: deletePosts) {
Image(systemName: "text.badge.minus")
}
Button(action: addPost) {
Image(systemName: "plus.app")
}
}
)
}
func addPost() {
let p = Post(context: moc)
p.name = "Post \(UUID().uuidString)"
try! moc.save()
}
func deletePosts() {
posts.forEach {
moc.delete($0)
}
try! moc.save()
}
}
struct ContentView: View {
#Environment(\.managedObjectContext) var moc
var body: some View {
NavigationView {
VStack(alignment: .leading){
NavigationLink(destination: MessageList()) {
Text("Messages")
}.padding()
NavigationLink(destination: PostList()) {
Text("Posts")
}.padding()
Spacer()
}
}.navigationViewStyle(StackNavigationViewStyle())
}
}
struct ContentView_Previews: PreviewProvider {
static let moc = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
static var previews: some View {
ContentView()
.environment(\.managedObjectContext, moc)
}
}
Screenshot of the Model:
The SceneDelegate (unaltered from template, provided for completeness):
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
var window: UIWindow?
func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
let contentView = ContentView().environment(\.managedObjectContext, context)
if let windowScene = scene as? UIWindowScene {
let window = UIWindow(windowScene: windowScene)
window.rootViewController = UIHostingController(rootView: contentView)
self.window = window
window.makeKeyAndVisible()
}
}
func sceneDidDisconnect(_ scene: UIScene) {}
func sceneDidBecomeActive(_ scene: UIScene) {}
func sceneWillResignActive(_ scene: UIScene) {}
func sceneWillEnterForeground(_ scene: UIScene) {}
func sceneDidEnterBackground(_ scene: UIScene) {
(UIApplication.shared.delegate as? AppDelegate)?.saveContext()
}
}
The AppDelegate (unaltered from template, provided for completeness):
import UIKit
import CoreData
#UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
return true
}
func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
}
func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {}
// MARK: - Core Data stack
lazy var persistentContainer: NSPersistentCloudKitContainer = {
let container = NSPersistentCloudKitContainer(name: "Coredata134")
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
return container
}()
func saveContext () {
let context = persistentContainer.viewContext
if context.hasChanges {
do {
try context.save()
} catch {
let nserror = error as NSError
fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
}
}
}
}
Update iOS 14.0 (beta 1):
This issue seems to have been resolved on iOS 14.
I also believe this is a bug.
You can workaround for now by setting the environment variable again within the NavigationLinks in ContentView:
NavigationLink(destination: MessageList().environment(\.managedObjectContext, moc)) {
Text("Messages")
}.padding()
NavigationLink(destination: PostList().environment(\.managedObjectContext, moc)) {
Text("Posts")
}.padding()
EDIT:
Just noticed that this workaround has at least one serious negative side effect: in case the #FetchRequest in the destination View uses a sortDescriptor and the destination View itself contains a NavigationLink, (e.g. to a DetailView), then modifying an attribute contained in the sortDescriptor in the DetailView will cause the DetailView to be popped and pushed again as soon as the new attribute value leads to a new sort order.
To demonstrate this:
a) add a new attribute of type Integer 16 named "value" to the Message entity in the Core Data model.
b) update func addMessage() as follows:
func addMessage() {
let m = Message(context: moc)
m.title = "Message: \(Date())"
m.value = 0
try! moc.save()
}
c) add the following struct to ContentView.swift
struct MessageDetailList: View {
#ObservedObject var message: Message
var body: some View {
Button(action: {
self.message.value += 1
}) {
Text("\(message.title ?? "?"): value = \(message.value)")
}
}
}
d) Update the ForEach in struct MessageList as follows:
ForEach(messages, id: \.self) { message in
NavigationLink(destination: MessageDetailList(message: message).environment(\.managedObjectContext, self.moc)) {
Text("\(message.title ?? "?"): value = \(message.value)")
}
}
e) replace #FetchRequest in MessageList with:
#FetchRequest(entity: Message.entity(), sortDescriptors: [NSSortDescriptor(key: "value", ascending: false)])
Run the code and tap on "Messages". Create three messages, then tap on the third one. In the DetailView, tap on the Button. This will increase the value attribute of this message to 1 and thus resort the fetch results on MessageList, which will trigger a pop and push again of the detail list.
After completing a delete of a row in a SwiftUI List I am getting a "Thread 1: EXC_BAD_INSTRUCTION" error. It seems the Core Data delete works as after I restart that data has been removed. So maybe something to do with SwiftUI trying to update it's view after the Core Data delete is performed????
Code:
import SwiftUI
import CoreData
struct ContentView: View {
#Environment(\.managedObjectContext) var context
#FetchRequest(entity: GCItem.entity(), sortDescriptors: []) var gcItems: FetchedResults<GCItem>
private func addItem(title:String) {
let newItem = GCItem(context: context)
newItem.id = UUID()
newItem.title = title
do {
try context.save()
} catch let e as NSError {
fatalError("Unresolved error \(e), \(e.userInfo)")
}
}
private func deleteItem(at offsets:IndexSet) {
self.context.perform {
// Delete Item
for index in offsets {
let item = self.gcItems[index]
self.context.delete(item)
}
// Persist
do {
try self.context.save()
} catch let e as NSError {
// TODO: How to undelete list???
print("ERROR : Can not save GCItem items: \(e.description)")
}
}
}
var body: some View {
NavigationView {
VStack {
List() {
ForEach(gcItems) { gcItem in
HStack {
Text("test")
}
}
.onDelete(perform: self.deleteItem)
}
Button(action: { self.addItem(title: "Testing 123") }) {
Text("ADD ITEM")
}
}
}
}
}
#if DEBUG
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
return ContentView().environment(\.managedObjectContext, context)
}
}
#endif
Screen Shot
The fix was NOT to use attributes in Core Data named "id". So I changed this to "myId" and then things worked fine.