Function useAtomQuery

packages/vue/src/query.ts:287–312

(
  atom: () => Atom.Atom<AsyncResult.AsyncResult<A, E>>
)

export const withDataFallback = <A, E>(
  // Structural read so both Ref and Readonly<Ref> from useAtomValue are accepted.
  rawResult: { readonly value: AsyncResult.AsyncResult<A, E> },
  options?: unknown
): ComputedRef<AsyncResult.AsyncResult<A, E>> => {
  const opts = options as
    | { readonly initialData?: A | (() => A); readonly placeholderData?: A | ((prev: A | undefined) => A) }
    | undefined
  const initialData = opts?.initialData
  const placeholderData = opts?.placeholderData
  if (initialData === undefined && placeholderData === undefined) {
    // Identity when no fallback options. Callers that need a real ComputedRef
    // (e.g. useAtomValue's Readonly<Ref>) wrap before calling this helper.
    return rawResult as ComputedRef<AsyncResult.AsyncResult<A, E>>
  }

  let lastData: A | undefined
  return computed(() => {
    const r = rawResult.value
    const concrete = AsyncResult.value(r)
    if (Option.isSome(concrete)) {
      lastData = concrete.value
    }
    // Fallback is pending-only. Failure (even without previousSuccess) and Success pass through.
    if (!AsyncResult.isInitial(r)) {
      return r
    }
    if (initialData !== undefined) {
      const v = typeof initialData === "function" ? (initialData as () => A)() : initialData
      return AsyncResult.success(v, { waiting: r.waiting })
    }