How to edit a list of subclass objects - swiftui

Question

I have a list of items of different classes derived from the same class.
The goal: editing any object using a different view
The model:
class Paper: Hashable, Equatable {
var name: String
var length: Int
init() {
name = ""
length = 0
}
init(name: String, length: Int) {
self.name = name
self.length = length
}
static func == (lhs: Paper, rhs: Paper) -> Bool {
return lhs.length == rhs.length
}
func hash(into hasher: inout Hasher) {
hasher.combine(length)
}
}
class ScientificPaper: Paper {
var biology: Bool
override init(name: String, length: Int) {
biology = false
super.init(name: name, length: length)
}
}
class TechnicalPaper: Paper {
var electronics: Bool
override init(name: String, length: Int) {
electronics = false
super.init(name: name, length: length)
}
}
The main view containing the list.
struct TestView: View {
#Binding var papers: [Paper]
#State private var edit = false
#State private var selectedPaper = Paper()
var body: some View {
let scientificBinding = Binding<ScientificPaper>(
get: {selectedPaper as! ScientificPaper},
set: { selectedPaper = $0 }
)
VStack {
List {
ForEach(papers, id: \.self) { paper in
HStack {
Text(paper.name)
Text("\(paper.length)")
Spacer()
Button("Edit") {
selectedPaper = paper
edit = true
}
}
}
}
}
.sheet(isPresented: $edit) {
VStack {
if selectedPaper is ScientificPaper {
ScientificForm(paper: scientificBinding)
}
if selectedPaper is TechnicalPaper {
TechnicalForm(paper: technicalBinding)
}
}
}
}
}
The custom view for each class.
struct ScientificForm: View {
#Binding var paper: ScientificPaper
var body: some View {
Form {
Text("Scientific")
TextField("Name: ", text: $paper.name)
TextField("Length: ", value: $paper.length, formatter: NumberFormatter())
TextField("Biology: ", value: $paper.biology, formatter: NumberFormatter())
}
}
}
struct TechnicalForm: View {
#Binding var paper: TechnicalPaper
var body: some View {
Form {
Text("Technical")
TextField("Name: ", text: $paper.name)
TextField("Length: ", value: $paper.length, formatter: NumberFormatter())
TextField("Electronics: ", value: $paper.electronics, formatter: NumberFormatter())
}
}
}
Problem is that at run time I get the following:
Could not cast value of type 'Paper' to 'ScientificPaper'.
maybe because the selectedPaper is already initialized as Paper.
What is the right strategy to edit list items belonging to different classes?

Answer 1

The error is due to creating binding in body, which calculates on every refresh, so binding is invalid.
The solution is to make binding as computable property, so it is requested only after validation in correct flow.
Tested with Xcode 12.1 / iOS 14.1 (demo is for scientificBinding only for simplicity)
struct TestView: View {
#Binding var papers: [Paper]
#State private var edit = false
#State private var selectedPaper = Paper()
var scientificBinding: Binding<ScientificPaper> { // << here !!
return Binding<ScientificPaper>(
get: {selectedPaper as! ScientificPaper},
set: { selectedPaper = $0 }
)
}
var body: some View {
VStack {
List {
ForEach(papers, id: \.self) { paper in
HStack {
Text(paper.name)
Text("\(paper.length)")
Spacer()
Button("Edit") {
selectedPaper = paper
edit = true
}
}
}
}
}
.sheet(isPresented: $edit) {
VStack {
if selectedPaper is ScientificPaper {
ScientificForm(paper: scientificBinding)
}
// if selectedPaper is TechnicalPaper {
// TechnicalForm(paper: technicalBinding)
// }
}
}
}
}