@thi.ng/transducers-async/mult.js

Async versions of various highly composable transducers, reducers and iterators

import { illegalState } from "@thi.ng/errors/illegal-state";
const mult = (src) => new Mult(src);
class Mult {
  constructor(src) {
    this.src = src;
  }
  subs = [];
  isActive = false;
  /**
   * Creates a new subscription (aka custom `AsyncIterable`) which will
   * receive any future values from `src`. The returned subscription can be
   * removed again via {@link Mult.unsubscribe}.
   */
  subscribe() {
    const sub = new MSub();
    this.subs.push(sub);
    if (!this.isActive) {
      this.isActive = true;
      (async () => {
        for await (let val of this.src) {
          for (const s of this.subs) s.resolve(val);
          if (val === void 0) this.subs.length = 0;
          if (!this.subs.length) break;
          await Promise.all(this.subs.map((x) => x.notifyP));
        }
        for (const s of this.subs) s.resolve(void 0);
        this.subs.length = 0;
        this.isActive = false;
      })();
    }
    return sub;
  }
  /**
   * Attempts to remove given child subscription (presumably created via
   * {@link Mult.subscribe}). Returns true if removal was successful.
   *
   * @param sub
   */
  unsubscribe(sub) {
    const idx = this.subs.findIndex((x) => x === sub);
    if (idx >= 0) {
      this.subs.splice(idx, 1);
      sub.resolve(void 0);
      return true;
    }
    return false;
  }
}
class MSub {
  valueP;
  notifyP;
  resolve;
  notify;
  active = false;
  constructor() {
    this.$await();
  }
  async *[Symbol.asyncIterator]() {
    if (this.active) illegalState("multiple consumers unsupported");
    this.active = true;
    while (true) {
      const res = await this.valueP;
      if (res === void 0) break;
      yield res;
      this.notify();
      this.$await();
    }
    this.active = false;
  }
  $await() {
    this.notifyP = new Promise((res) => this.notify = res);
    this.valueP = new Promise((res) => this.resolve = res);
  }
}
export {
  MSub,
  Mult,
  mult
};