SwiftUI: ViewModel making app crash when loading a view - swift

Question

I want to rename an item in a ForEach list. When i try to load the EditListView for a selected list the entire app crashes.
This is a SwiftUI macOS app and the items are saved using CoreData.
The crash happens as soon as you click on "Edit List" for any of the list items.
It doesn't crash if i remove this view model var listVM: MyListViewModel from the EditListViewModel.
Here's the EditListView
struct EditListView: View {
let name: String
#Binding var isVisible: Bool
var list: MyListViewModel
#ObservedObject var editListVM: EditListViewModel
init(name: String,list: MyListViewModel, isVisible: Binding<Bool> ) {
self.list = list
editListVM = EditListViewModel(listVM: list)
_isVisible = isVisible
self.name = name
}
var body: some View {
VStack {
Text(name)
Button(action: {
editListItemVM.update()
}) {
Text("Update List Name")
}
Button(action: {
self.isVisible = false
}) {
Text("Cancel")
}
}......
EditListViewModel
class EditListViewModel: ObservableObject {
var listVM: MyListViewModel
#Published var name: String = ""
init(listVM: MyListViewModel) {
self.listVM = listVM
name = listVM.name
}
func update(){
....}
}
MyListViewModel
struct MyListViewModel: Identifiable {
private let myList: MyList
init(myList: MyList) {
self.myList = myList
}
var id: NSManagedObjectID {
myList.objectID
}
var name: String {
myList.name ?? ""
}
}
MyList Model
#objc(MyList)
public class MyList: NSManagedObject, BaseModel {
static var all: NSFetchRequest<MyList> {
let request: NSFetchRequest<MyList> = MyList.fetchRequest()
request.sortDescriptors = []
return request
}
}
extension MyList {
#nonobjc public class func fetchRequest() -> NSFetchRequest<MyList> {
return NSFetchRequest<MyList>(entityName: "MyList")
}
#NSManaged public var name: String?
}
extension MyList : Identifiable {
}
Here's the Main View
struct MyListsView: View {
#StateObject var vm: MyListsViewModel
#State private var showPopover: Bool = false
init(vm: MyListsViewModel) {
_vm = StateObject(wrappedValue: vm)
}
List {
Text("My Lists")
ForEach(vm.myLists) { myList in
NavigationLink {
MyListItemsHeaderView(name: myList.name)
.sheet(isPresented: $showPopover) {
EditListView(name: myList.name, list: MyListViewModel(myList: MyList()), isVisible: $showPopover)
}
}
}.contextMenu {
Button {
showPopover = true
// Show the EditListView
} label: {
Label("Edit List", systemImage: "pen.circle")
}......

Answer 1

First get rid of your view model objects we don't use those in SwiftUI. We use the View struct and the property wrappers like #FetchRequest make the struct behave like an object. It looks like this:
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
ItemView(item: item)
I recommend looking at Xcode's app template with core data checked.
Then for editing you can use .sheet(item: like this:
struct ItemEditor: View {
#ObservedObject var item: Item // this is the scratch pad item
#Environment(\.managedObjectContext) private var context
#Environment(\.dismiss) private var dismiss // causes body to run
let onSave: () -> Void
#State var errorMessage: String?
var body: some View {
NavigationView {
Form {
Text(item.timestamp!, formatter: itemFormatter)
if let errorMessage = errorMessage {
Text(errorMessage)
}
Button("Update Time") {
item.timestamp = Date()
}
}
.toolbar {
ToolbarItem(placement: .navigationBarLeading) {
Button("Cancel") {
dismiss()
}
}
ToolbarItem(placement: .navigationBarTrailing) {
Button("Save") {
// first save the scratch pad context then call the handler which will save the view context.
do {
try context.save()
errorMessage = nil
onSave()
} catch {
let nsError = error as NSError
errorMessage = "Unresolved error \(nsError), \(nsError.userInfo)"
}
}
}
}
}
}
}
struct ItemEditorConfig: Identifiable {
let id = UUID()
let context: NSManagedObjectContext
let item: Item
init(viewContext: NSManagedObjectContext, objectID: NSManagedObjectID) {
// create the scratch pad context
context = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
context.parent = viewContext
// load the item into the scratch pad
item = context.object(with: objectID) as! Item
}
}
struct EditItemButton: View {
let itemObjectID: NSManagedObjectID
#Environment(\.managedObjectContext) private var viewContext
#State var itemEditorConfig: ItemEditorConfig?
var body: some View {
Button(action: edit) {
Text("Edit")
}
.sheet(item: $itemEditorConfig, onDismiss: didDismiss) { config in
ItemEditor(item: config.item) {
do {
try viewContext.save()
} catch {
// 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.
let nsError = error as NSError
fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
}
itemEditorConfig = nil // dismiss the sheet
}
.environment(\.managedObjectContext, config.context)
}
}
func edit() {
itemEditorConfig = ItemEditorConfig(viewContext: viewContext, objectID: itemObjectID)
}
func didDismiss() {
// Handle the dismissing action.
}
}
struct ItemView: View {
#ObservedObject var item: Item
var body: some View {
Text("Item at \(item.timestamp!, formatter: itemFormatter)")
.toolbar {
ToolbarItem(placement: .navigationBarTrailing) {
EditItemButton(itemObjectID: item.objectID)
}
}
}
}

Answer 2

the params for EditListView in the main view were incorrect.
Fixed it with the following params:
.sheet(isPresented: $showPopover) {
EditListView(name: myList.name, list: myList, isVisible: $showPopover)
}