until

until<T>(service: Service<T>, predicate: (props: T) => boolean): Promise<T>

A one-shot wait on a service.

Usage

import { until, useScroll } from '@studiometa/js-toolkit';

async function afterScroll() {
  const props = await until(useScroll(), ({ isScrolling }) => !isScrolling);
  return props.y;
}

Parameters

• service — a Service<T>, and nothing else.
• predicate — called with each update.

Return value

• a promise resolving with a copy of the props on the first update that matches.

The contract

• It releases the subscription before it resolves, so an awaiting caller never holds a source open.
• It resolves with a copy, because the props object belongs to its service and is only valid for the duration of the call that received it.
• It resolves on the current props when they already match, through the service's hasProps() — so await until(useScroll(), ({ isScrolling }) => !isScrolling) on a still page returns immediately rather than waiting for a scroll that never comes.

When to reach for it

When the code reads as a step in a sequence rather than a reaction:

await until(useScroll(), ({ isScrolling }) => !isScrolling);
this.measure();

For a reaction, subscribe. For an ongoing condition, subscribe and act on each delivery.