Error when modifying a View from another View SwiftUI - swift

Question

I searched a lot about this error but it seems there's no solution...
UITableView was told to layout its visible cells and other contents without being in the
view hierarchy (the table view or one of its superviews has not been added to a window).
This may cause bugs by forcing views inside the table view to load and perform layout without
accurate information (e.g. table view bounds, trait collection, layout margins, safe area
insets, etc), and will also cause unnecessary performance overhead due to extra layout passes.
Make a symbolic breakpoint at UITableViewAlertForLayoutOutsideViewHierarchy to catch this in
the debugger and see what caused this to occur, so you can avoid this action altogether if
possible, or defer it until the table view has been added to a window
This is my actual code:
struct ContentView: View {
#Environment(\.managedObjectContext) var managedObjectContext
#FetchRequest(fetchRequest: FavoriteBooks.getAllFavoriteBooks()) var favoriteBooks:FetchedResults<FavoriteBooks>
#ObservedObject var bookData = BookDataLoader()
var body: some View {
NavigationView {
List {
Section {
NavigationLink(destination: FavoriteView()) {
Text("Go to favorites")
}
}
Section {
ForEach(0 ..< bookData.booksData.count) { num in
HStack {
Text("\(self.bookData.booksData[num].titolo)")
Button(action: {
**let favoriteBooks = FavoriteBooks(context: self.managedObjectContext)
favoriteBooks.titolo = self.bookData.booksData[num].titolo**
}) {
Image(systemName: "heart")
}
}
}
}
}
}
}
}
struct FavoriteView: View {
#Environment(\.managedObjectContext) var managedObjectContext
#FetchRequest(fetchRequest: FavoriteBooks.getAllFavoriteBooks()) var favoriteBooks:FetchedResults<FavoriteBooks>
var body: some View {
List {
**ForEach (self.favoriteBooks) { book in
Text("\(book.titolo!))")**
}
}
}
}
I just selected on bold what makes this error and I don't know how to avoid it because if I launch the app it doesn't crash but I cannot do anything.
Thanks in advance

Answer 1

You have a couple issues here. The first is, when you use ForEach, if your content is supposed to be able to change (which, with FetchRequest it is...) then either FavoriteBooks needs to be Identifiable or you need to pass in your id. You actually do this twice in the code:
ForEach(0 ..< bookData.booksData.count) { num in
// SwiftUI thinks this content never changes because it doesn't know how to resolve those changes. you didn't tell it
}
should be:
ForEach(0 ..< bookData.booksData.count, id: \.self) { ... }
Notice now you are telling it what the id is. If the count of bookData.booksData changes, now SwiftUI can resolve those changes. But really, why do you need the index in this case specifically? Why not just:
ForEach(bookData.booksData) { book in ... }
If you make this object type conform to Identifiable, you now have the book already.
Now on to another problem, your button action. Why are you re-executing the CoreData query here? You have the Set of the objects you want. This is another reason to just use ForEach(bookData.booksData), you don't have to resolve the index here. But in general, you should never need to re-execute your core data query to find a specific object. what this actually does is then trigger another update on your entire view hierarchy, which is likely why you get the error you're getting. you aren't supposed to be doing this.