@lyra/form-builder/lib/lyra/utils/withMaxConcurrency.js

Lyra form builder

'use strict';

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.withMaxConcurrency = withMaxConcurrency;
exports.createThrottler = createThrottler;

var _rxjs = require('rxjs');

const DEFAULT_CONCURRENCY = 4;

// Takes a observable-returning function and returns a new function that limits on the number of
// concurrent observables.

function remove /*:: <T>*/(array /*: Array<T>*/, item /*: T*/) /*: Array<T>*/ {
  const index = array.indexOf(item);
  if (index > -1) {
    array.splice(index, 1);
  }
  return array;
}

function withMaxConcurrency(func /*: any => Observable<*>*/, concurrency /*: number*/ = DEFAULT_CONCURRENCY) {
  const throttler = createThrottler(concurrency);
  return (...args /*: Array<any>*/) => (0, _rxjs.from)(throttler(func(...args)));
}

function createThrottler(concurrency /*: number*/ = DEFAULT_CONCURRENCY) {
  const currentSubscriptions /*: Array<Subscription>*/ = [];
  const pendingObservables /*: Array<Observable<*>>*/ = [];
  const ready$ = new _rxjs.Subject();

  return request;

  function request(observable /*: Observable<*>*/) {
    return new _rxjs.Observable(observer => {
      if (currentSubscriptions.length >= concurrency) {
        return scheduleAndWait(observable).mergeMap(request).subscribe(observer);
      }
      const subscription = observable.subscribe(observer);
      currentSubscriptions.push(subscription);
      return () => {
        remove(currentSubscriptions, subscription);
        remove(pendingObservables, observable);
        subscription.unsubscribe();
        check();
      };
    });
  }

  function scheduleAndWait(observable) {
    pendingObservables.push(observable);
    return ready$.asObservable().first(obs => obs === observable);
  }

  function check() {
    while (pendingObservables.length > 0 && currentSubscriptions.length < concurrency) {
      ready$.next(pendingObservables.shift());
    }
  }
}