Proposal: pass invalidate function to derived function handler

[btakita]

This feature would allow async derived handlers to determine when they are invalid:

derived([a, b], async ([$a, $b], set, invalidate) => {
  invalidate()
  const [$a_, $b_] = await Promise.all([a_fn($a), b_fn($b)])
  set($a_ + $b_)
})

This would also enable multi-step derived state machines without diamond dependency issues:

derived([a, b], async ([$a, $b], set, invalidate) => {
  invalidate()
  const $a_ = await a_fn($a)
  set({ step: 1, $a_ })
  invalidate()
  const $b_ = await b_fn($b)
  set({ step: 2, $a_, $b_ }
})

A con against this is that it would add an additional api item & will only be useful for async derived stores.

This idea originated from #3219 (comment)

[btakita]

#3575

[PaulMaly]

@btakita I'm not sure my #3575 related to this proposal. Seems, both about different areas of the store. Actually, we already have invalidate callback in writable store right now, but I'm not sure it's useful without my fix. Btw, I like your proposal too.

[Conduitry]

I don't think we want more complication in the built-in store implementations. Everything you can import from svelte/... is blessed code that could just as easily exist in userland. If you have more specific needs for the derived store, you can fork it and have your own version.