SwiftUI .sheet Unexpectedly found nil while unwrapping an Optional value - swiftui

Question

The Problem
The following example highlights my issue better than I can explain it. I explicitly give an optional variable a value before presenting a sheet. This sheet, which requires a non-optional variable to init, doesn't register the value and says it is nil. I can't understand why this would be if I only ever call the sheet after the optional has been given a value. Any help would be greatly appreciated. Thanks!
What I have tried
In the example I replaced:
.sheet(isPresented: $showModalView, content: {
EditBookView(book: editingBook!) //Fatal error here
})
with:
.sheet(isPresented: $showModalView, content: {
if let book = editingBook {
EditBookView(book: book)
}
})
However, this just shows an empty sheet (implying that editingBook is empty). But, interestingly when I close this empty sheet and select another item in the list, the view appears as intended.
Reproducible example
import SwiftUI
struct Book: Identifiable {
var id: UUID
var title: String
init(title: String){
self.title = title
self.id = UUID()
}
}
struct ContentView: View {
#State var books = [Book]()
#State var showModalView = false
#State var editingBook: Book? = nil
var body: some View {
List{
ForEach(books){ book in
VStack(alignment: .leading){
Text(book.title)
.font(Font.title.bold())
Text("id: \(book.id.uuidString)")
.foregroundColor(.gray)
Button(action: {
editingBook = book
showModalView = true
}){
Text("Edit")
.foregroundColor(.accentColor)
}
.buttonStyle(PlainButtonStyle())
.padding(.top)
}
}
}
.padding()
.onAppear{
for i in 0...50 {
books.append(Book(title: "Book #\(i)"))
}
}
.sheet(isPresented: $showModalView, content: {
EditBookView(book: editingBook!) //Fatal error here
})
}
}
struct EditBookView: View {
var book: Book
var body: some View {
Text(book.title)
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
Edit:
enum SheetChoice: Hashable, Identifiable {
case addContentView
case editContentView
var id: SheetChoice { self }
}
...
.sheet(item: $sheetChoice){ item in
switch item {
case .addContentView:
AddContentView()
.environmentObject(model)
case .editContentView:
//if let selectedContent = selectedContent {
ContentEditorView(book: selectedContent!, editingFromDetailView: false)
.environmentObject(model)
//}
}
}

Answer 1

Make sure you also use editingBook inside your body (not only sheet building block).
SwiftUI tracks which State variables are used in its body. When it’s not used, you might come into this weird situations when your body is called with ignored changes to that state variable.
So basically add this line at the beginning of your body:
var body: some View {
_ = editingBook
return <your view>
}
Alternatively, you can use this .sheet modifier version:
https://developer.apple.com/documentation/swiftui/view/sheet(item:ondismiss:content:)

Answer 2

Following the answer from #msmialko, I suspect this is a compiler problem.
_ = self.<your_variable>
inside body solves the problem.

Answer 3

One possible workaround is moving out the sheet content into another View, and pass the Binding to the #Stete to it:
struct Book: Identifiable {
var id: UUID
var title: String
init(title: String){
self.title = title
self.id = UUID()
}
}
enum SheetChoice: Hashable, Identifiable {
case addContentView
case editContentView
var id: SheetChoice { self }
}
class MyModel: ObservableObject {
}
struct ContentView: View {
#State var books = [Book]()
#State var selectedContent: Book? = nil
#State var sheetChoice: SheetChoice? = nil
#StateObject var model = MyModel()
var body: some View {
List{
ForEach(books){ book in
VStack(alignment: .leading){
Text(book.title)
.font(Font.title.bold())
Text("id: \(book.id.uuidString)")
.foregroundColor(.gray)
Button(action: {
selectedContent = book
sheetChoice = .editContentView
}){
Text("Edit")
.foregroundColor(.accentColor)
}
.buttonStyle(PlainButtonStyle())
.padding(.top)
}
}
}
.padding()
.onAppear{
for i in 0...50 {
books.append(Book(title: "Book #\(i)"))
}
}
.sheet(item: $sheetChoice){
item in
SheetContentView(item: item, selectedContent: $selectedContent)
.environmentObject(model)
}
}
}
struct SheetContentView: View {
var item: SheetChoice
var selectedContent: Binding<Book?>
var body: some View {
switch item {
case .addContentView:
AddContentView()
case .editContentView:
ContentEditorView(book: selectedContent.wrappedValue!,
editingFromDetailView: false)
}
}
}
struct ContentEditorView: View {
var book: Book
var editingFromDetailView: Bool
var body: some View {
Text(book.title)
}
}
struct AddContentView: View {
var body: some View {
Text("AddContentView")
}
}