WritableResource composition #71089

export function withPreviousValue<T>(
  input: ResourceRef<T>,
  options?: {
    when?: (previous: T) => boolean;
  }
): ResourceRef<T> {
  const derived = linkedSignal<ResourceSnapshot<T>, ResourceSnapshot<T>>({
    source: input.snapshot,
    computation: (snap, previous) => {
      if (
        snap.status === 'loading' &&
        previous &&
        previous.value.status !== 'error' &&
        (!options?.when || options.when(previous.value.value))
      ) {
        // When the input resource enters loading state, we keep the value
        // from its previous state, if any.
        return { status: 'loading' as const, value: previous.value.value };
      }
      // Otherwise we simply forward the state of the input resource.
      return snap;
    },
  });

  const readOnly = resourceFromSnapshots(derived);
  const value = linkedSignal(readOnly.value);
  value.set = input.set.bind(input);
  value.update = input.update.bind(input);
  value.asReadonly = () => readOnly.value;
  return {
    value,
    status: readOnly.status,
    error: readOnly.error,
    isLoading: readOnly.isLoading,
    snapshot: readOnly.snapshot,
    hasValue: readOnly.hasValue.bind(readOnly) as ResourceRef<T>['hasValue'],
    set: input.set.bind(input),
    update: input.update.bind(input),
    reload: input.reload.bind(input),
    destroy: input.destroy.bind(input),
    asReadonly: () => readOnly,
  };
}