SwiftUI: Dismissed view causes Index out of Range error - swiftui

Question

This one has been tearing my hair out, I'm hoping someone can show me where I'm going wrong.
The goal is I need to be able to manage a dynamic list of items (i.e. the list could grow or shrink), where each item's properties can be adjusted (edited), and the items are related to a given parent.
In the abridged code below I've called the item Cell, which belongs to a Row. A Row can have many Cells, and each Cell has an amount which can be changed by the user. This is an abridged version of my code, which I hope makes the relationship labelling a little clearer but, crucially, it gives me the same error which is:
If the number of Cells is less than the original amount (i.e. 3 or less in this example) then the application crashes with an 'Index out of Range' error whenever the view is dismissed. Up to that point cells can be added or removed without any issue whatsoever and I don't get this error when changing the amount of cell items. I've looked all over SO and various blogs and can't find anyone who has encountered this particular issue - it seems most of the Index out of Range posts occur when actually modifying the list, my error only happens once the list is shrunk and then dismissed.
I've attached some sample code below - you should be able to cut/paste and try it out.
PS. I know calculateTotals() is sketch; please don't add letters or punctuation to your totals - it's just to test the bindings are bubbling correctly :)
Cell
struct Cell: Identifiable {
var amount: String
var id = UUID()
init(_ amount: String = "0.00"){
self.amount = amount
}
}
Row
class Row: ObservableObject {
#Published var cells: [Cell]
init(){
self.cells = [
Cell("10"),
Cell("15"),
Cell("20"),
Cell("25")]
}
}
ContentView
struct ContentView: View {
#State private var displayAmounts = false
var body: some View {
NavigationView {
Button(action: {
self.displayAmounts.toggle()
}) {
Text("View amounts")
}
}
.sheet(isPresented: self.$displayAmounts) {
CellSheet()
}
}
}
CellSheet
struct CellSheet: View {
#ObservedObject var row: Row = Row()
private func deleteCell(at offsets: IndexSet) {
self.row.cells.remove(atOffsets: offsets)
}
private func calculateTotals() -> String {
var total = Double("0.00")!
for cell in self.row.cells {
if( "" != cell.amount ) {
total += Double(cell.amount)!
}
}
return String("\(total)")
}
var body: some View {
VStack{
List {
ForEach(row.cells.indices, id: \.self){ i in
CellItem(amount: self.$row.cells[i].amount)
}.onDelete(perform: deleteCell)
}
Button(action: {
self.row.cells.append(Cell())
}) {
Text("Add new cell")
}
Text(calculateTotals())
}
}
}
CellItem - some oddness here: if I wrap the TextField with an HStack then the app crashes immediately if remove a cell -- even if the total number of cells is greater than the original amount (4).
struct CellItem: View {
#Binding var amount: String
var body: some View {
// Uncomment HStack and deleting rows immediately causes index out of range.
// HStack {
TextField("Amount: ", text: $amount)
// }
}
}
I'm really at a loss as to why/how this is happening. Clearly Swift is trying to access an index that doesn't exist (if I remove the binding and just output the value there's no problem), but I don't understand why that would cause issues when the view is dismissed. My guess is that Swift is caching some things in memory? The HStack wrapping issue is also peculiar.
Anyway, I'm relatively new to Swift so it's possible I'm overlooking something obvious. For additional context, I'm running XCode 11.4.1 and targeting iOS 13.4.
You should be able to lift all of this code straight into a new project and it will compile. Any help will be gratefully received :)

Answer 1

Ok, literally minutes after creating this post (and I've been on this for a couple of days), I think I have a solution. I've modified my List() in ContentView as follows:
List {
ForEach(Array(row.cells.enumerated()), id:\.1.id) { (i, cell) in
CellItem(amount: self.$row.cells[i].amount)
}.onDelete(perform: deleteCell)
}
This was based on this answer as I think (?) enumerated() is better suited when the array is of a variable length. The HStack crashing is also resolved with the above implementation. Maybe someone can add some more context to this.