비동기 작업의 취소

type ViewState =
  | { kind: 'idle' | 'loading' | 'cancelled' }
  | { kind: 'ready'; value: string }
  | { kind: 'failed'; message: string };
type Load = (query: string, signal: AbortSignal) => Promise<string>;
type Outcome = 'applied' | 'superseded' | 'cancelled' | 'failed';

function createSearch(load: Load) {
  let revision = 0;
  let active: AbortController | null = null;
  let state: ViewState = { kind: 'idle' };

  async function run(query: string): Promise<Outcome> {
    const mine = ++revision;
    const previous = active;
    const controller = new AbortController();
    active = controller;
    state = { kind: 'loading' };
    previous?.abort();
    try {
      if (mine !== revision) return 'superseded';
      if (controller.signal.aborted) return 'cancelled';
      const value = await load(query, controller.signal);
      if (mine !== revision) return 'superseded';
      if (controller.signal.aborted) {
        state = { kind: 'cancelled' };
        return 'cancelled';
      }
      state = { kind: 'ready', value };
      return 'applied';
    } catch (cause) {
      if (mine !== revision) return 'superseded';
      if (controller.signal.aborted) {
        state = { kind: 'cancelled' };
        return 'cancelled';
      }
      state = { kind: 'failed', message: String(cause) };
      return 'failed';
    } finally {
      if (mine === revision) active = null;
    }
  }

  return {
    run,
    read: () => state,
    cancel() {
      const controller = active;
      if (controller) {
        active = null;
        state = { kind: 'cancelled' };
        controller.abort();
      }
    },
  };
}

async function main() {
  let finishOld!: (value: string) => void;
  const oldResult = new Promise<string>(resolve => { finishOld = resolve; });
  const search = createSearch((query, _signal) =>
    query === 'old' ? oldResult : Promise.resolve('NEW'));
  const oldRun = search.run('old');
  console.log(await search.run('new'));
  finishOld('OLD');
  console.log(await oldRun);
  console.log(search.read());
}
void main();