How do I get notified (e.g., launch a function) when a Picker is used?

I'm guessing the didSet() doesn't work? Hm, I thought that should work as-is.

The alternative is to tie it into a publisher somehow. You can use a CurrentValueSubject<MyChoiceType, Never> to feed into your state, though, allowing you to attach other actions:

private var subscribers: Set = []
private var subject = CurrentValueSubject<int, never="">()
private var currentChoiceBinding = Binding(get: { self.subject.value }, set: { self.subject.send(newValue) })

init() {
    // assign state when the publisher gets a value
    subject.sink { self.currentChoice = $0 }
        .store(in: &subscribers)
    // call special method when the publisher gets a value
    subject.sink { doSomething() }
        .store(in: &subscribers)
}

var body: some View {
    Picker("Weapon", selection: currentChoiceBinding) {
        ForEach(0 ..< moves.count) {
            Text("\(self.moves[$0])")
        }
    }
}

The custom binding there uses the publisher's send() method to set new values, and its get() will return the current value from the publisher. Thus anything subscribed to the CurrentValueSubject will receive every value sent to the binding.

As you can see, I'm in the transition to event-handling to declarative logic.

Thanks for the suggestion... I'll pursue it.