View is not rerendered in Nested ForEach loop

Question

I have the following component that renders a grid of semi transparent characters:

    var body: some View {
               


        

Answer 1

ForEach should only be used for constant data. So it is only evaluated once by definition. Try wrapping it in a List and you will see errors being logged like:

ForEach, Int, TupleView<(Spacer, HStack, Int, TupleView<(Spacer, Text, Spacer)>>>, Spacer)>> count (7) != its initial count (0). ForEach(_:content:) should only be used for constant data. Instead conform data to Identifiable or use ForEach(_:id:content:) and provide an explicit id!

I was surprised by this as well, and unable to find any official documentation about this limitation.

As for why it is not possible for you to access the i in the inner ForEach-loop, I think you probably have a misleading compiler error on your hands, related to something else in the code that is missing in your snippets. It did compile for me after completing the missing parts with a best guess (Xcode 11.1, Mac OS 10.14.4).

Here is what I came up with to make your ForEach go over something Identifiable:

struct SettingsElement: Identifiable {
    var id: Int { value }

    let value: Int

    init(_ i: Int) { value = i }
}

class Settings: ObservableObject {
    @Published var rows = [SettingsElement(0),SettingsElement(1),SettingsElement(2)]
    @Published var columns = [SettingsElement(0),SettingsElement(1),SettingsElement(2)]
}

struct ContentView: View {
    @EnvironmentObject var settings: Settings

    func getSymbol(index: Int) -> Text { Text("\(index)") }

    var body: some View {
        VStack{
            Text("\(self.settings.rows.count) x \(self.settings.columns.count)")
            ForEach(self.settings.rows) { i in
                VStack {
                    HStack {
                        ForEach(self.settings.columns) { j in
                            Text("\(i.value) \(j.value)")
                        }
                    }
                }
            }
        }
    }
}