SwiftUI: Safe subscript for binding array

Question

We have the practice of using safe subscript when accessing any element in a collection. Below is the extension we have.

extension Collection {
    subscript(safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

But when I try to use this with a binding object, it gives me an error saying

Extraneous argument label 'safe:' in subscript

Below is the problematic code

struct MyView: View {
    @ObservedObject var service: service

    var body: some View {
        List {
            ForEach(service.items.indices) { index in
                Toggle(isOn: self.$service.items[safe: index]?.isOn ?? false) {  // Error: Extraneous argument label 'safe:' in subscript
                    Text("isOn")
                }
            }
        }
    }
}

Answer 1

Two issues:

1. You don't need to use items[safe: index], because you are given only valid indices by items.indices. You will never have an index that is outsides the bounds of the array.

2. You can't use items[safe: index], because self.$service.items is a Binding<[Item]>, which is not a Collection, so your extension to Collection doesn't apply.

Just remove the safe: and you're good to go.

See the end of this answer for more detail.

Answer 2

The diagnostic message is confusing, but the problem is that your subscript returns an optional, but you're treating it like a non-optional. You're going to have to handle the case where it returns nil.

Personally I think this approach is fighting the system. You'd be better off using ForEach(service.items). Rather than "safe" subscripts, avoid subscripts entirely.