Preview in Canvas stops working as soon as I use an object passed by another view in SwiftUI - swift

Question

I have an app that uses Core Data, everything works fine when I compile to the simulator or a physical device, the issue is that for some reason the Preview in Canvas doesn't work. In the code below I'm passing an Item from ItemsView to ItemView through a NavigationLink, the issue starts as soon as I use the passed item anywhere in ItemView. Again, the issue is only in Canvas. I know it's not a big deal since it compiles fine but I got used to seeing the preview when designing the interface.
Error message:
ItemsApp crashed due to an uncaught exception NSInvalidArgumentException. Reason: - [Item name]: unrecognized selector sent to instance 0x600001d6c080.: The preview process appears to have crashed.
Items View: Preview works fine.
import SwiftUI
struct ItemsView: View {
#ObservedObject var coreDataViewModel:CoreDataViewModel
var body: some View {
NavigationView{
VStack{
List {
ForEach(coreDataViewModel.items) { item in
HStack{
VStack(alignment:.leading){
Text(item.name ?? "")
Text(item.price ?? "")
}
NavigationLink(destination: ItemView(coreDataViewModel: coreDataViewModel, selectedItem: item)){
}
}
}
}
}
}
}
}
Item View: Preview doesn't work. The issue starts when I call Text(selectedItem.name ?? "--")
import SwiftUI
struct ItemView: View {
#ObservedObject var coreDataViewModel: CoreDataViewModel
#State var selectedItem: Item
var body: some View {
VStack{
HStack{
Text(selectedItem.name ?? "--") // this causes the issue
}
}
.onAppear{
Text(selectedItem.name ?? "--") // this causes the issue
}
}
}
struct ItemView_Previews: PreviewProvider {
static var previews: some View {
ItemView(coreDataViewModel: CoreDataViewModel(), selectedItem: Item())
}
}
Any idea what could be wrong?
Am I passing the item correctly?
Thanks
EDIT:
Corrected view name from ServicesView to ItemView in NavigationLink and Previews. Also added the error message.
EDIT:
Added CoreDataManager and CoreDataViewModel
CoreDataManager
class CoreDataManager{
static let instance = CoreDataManager()
let container: NSPersistentContainer
let context: NSManagedObjectContext
init(){
container = NSPersistentContainer(name: "CoreDataContainer")
container.loadPersistentStores { (description, error) in
if let error = error{
print("Error loading Core Data. \(error)")
}
}
context = container.viewContext
}
func save(){
do{
try context.save()
}catch let error{
print("Error saving Core Data. \(error.localizedDescription)")
}
}
}
CoreDataViewModel
class CoreDataViewModel: ObservableObject{
let manager = CoreDataManager.instance
#Published var items: [Item] = []
init(){
getItems()
}
func addItem(name: String, price: String){
let item = Item(context: manager.context)
item.name = name
item.price = price
save()
getItems()
}
func getItems(){
let request = NSFetchRequest<Item>(entityName: "Item")
let sort = NSSortDescriptor(keyPath: \Item.name, ascending: true)
request.sortDescriptors = [sort]
do{
items = try manager.context.fetch(request)
}catch let error{
print("Error fetching businesses. \(error.localizedDescription)")
}
}
func save(){
self.manager.save()
}
}

Answer 1

Here are the steps to follow:
In your Persistance struct declare a variable preview with your preview Items:
static var preview: PersistenceController = {
let result = PersistenceController(inMemory: true)
let viewContext = result.container.viewContext
let newItem = Item(context: viewContext)
newItem.yourProperty = yourValue
do {
try viewContext.save()
} catch {
// error handling
}
return result
}()
Create 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 fetchedItem = (try! context.fetch(request).first)!
YourView(item: fetchedItem)
}
}
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)")
}
})
}
}