.self in ForEach

I’m on Day 25 of Hacking With SwiftUI, and Paul is making a point about how SwiftUI can loop over an array to build a view. He starts with this:

let agents = ["Cyril", "Lana", "Pam", "Sterling"]
VStack {
    ForEach(0..<agents.count) {
        Text(agents[$0])
    }
}

But then proposes an alternative:

let agents = ["Cyril", "Lana", "Pam", "Sterling"]
VStack {
    ForEach(agents, id: \.self) {
        Text($0)
    }
}

He explains the use of \.self here by saying

So, we come back to how Swift can identify values in our array. When we were using a range such as 0..<5 or 0..<agents.count, Swift knew for sure that each item was unique because it would use the numbers from the range – each number was used only once in the loop, so it would definitely be unique.

In our array of strings that’s no longer possible, but we can clearly see that each value is unique: the values in ["Cyril", "Lana", "Pam", "Sterling"] don’t repeat. So, what we can do is tell SwiftUI that the strings themselves – “Cyril”, “Lana”, etc – are what can be used to identify each view in the loop uniquely.

I’m having a couple of problems with this.

Grumble One

The first is that Swift can’t really use the “strings themselves”. So this doesn’t work:

ForEach(agents, String($0)) {
    Text($0)
}

It complains that there’s no initialiser to take those arguments. So \.self must be something fancier. This is something I can presumably investigate by looking at the initialisers for ForEach.

Grumble Two

And the second grumble is that the first formulation ForEach(0..<agents.count) throws a compiler warning “Non-constant range: argument must be an integer literal“

Which is true, if I swap it for the number ‘4’ it stops complaining. I guess in the initialiser for ForEach there’s something specifying this although it’s not clear to me why. Again it’s an initialiser mystery because I can make the warning go away by adding the id reference to self.

Googling for the warning finds a page on the Hacking with Swift forums that throws a little bit of light on the issue by looking at the inits.

The difference between these two that we’re interested in is that in the first, the type is Range, and the second Data. @RoosterBoy‘s explanation for why we should use literal ints for the range is that it’s to be safe from the array changing size during the loop – which is slightly unsatisfying because that could still happen with the literal int range.

Grumble One again

These initialisers also shed some light on my first problem, the references used by SwiftUI should be KeyPaths. Sarun’s explanation of them makes them sound like actual references – ie memory addresses for the properties. That would make sense since SwiftUi wants them to keep track of things.

So I tried this:

ForEach(agents, id: \String.$0) {
    Text($0)
}

but the compiler is still unhappy – so clearly I am not knowledgeable enough yet to solve this one.