Why is withAnimation changing how these references are managed? - ios

Question

In the following code, including one less line in a withAnimation block triggers an unowned reference fault. Dismissing the view shown by button A and B should trigger the addition of a new element in both case. However, B causes the reference to the items list to be invalid:
struct ListItem: Identifiable, Equatable {
var id = UUID()
var name: String
}
class MyListClass: ObservableObject {
#Published var items = [ListItem]()
}
struct DetailView: View {
#Binding var item: ListItem
var body: some View {
VStack(alignment: .leading) {
TextField("Name", text: self.$item.name)
}
}
}
struct MyListView: View {
#StateObject var myList = MyListClass()
#State private var itemDraft = ListItem(name: "Foo")
#State private var showingA = false
#State private var showingB = false
var body: some View {
NavigationView {
List(myList.items) { item in
// Pass binding to item into DetailsView
Text(item.name)
}
.navigationBarItems(
leading: Button(action: {
showingA.toggle()
}) {
Text("A")
},
trailing: Button(action: {
showingB.toggle()
}) {
Text("B")
}
)
.sheet(isPresented: $showingA) {
DetailView(item: $itemDraft)
.onDisappear { // Closure A
let newItem = itemDraft
withAnimation {
itemDraft = ListItem(name: "Bar")
myList.items.append(newItem) // <- here
}
}
}
.sheet(isPresented: $showingB) {
DetailView(item: $itemDraft)
.onDisappear { // Closure B
let newItem = itemDraft
itemDraft = ListItem(name: "Bar")
withAnimation {
myList.items.append(newItem) // <- and here
}
}
}
}
}
}
The error is
Attempted to read an unowned reference but the object was already deallocated
What is the difference in references management between closures A (annotation above) and B? (Bonus points: what tool of xcode is the most useful to find the source of that bug?)