frontend-tutorial-framework/src/app/services/queueing-subject.ts

// Based on https://github.com/ohjames/queueing-subject

import { Subject, Subscriber, Subscription } from 'rxjs';

export class QueueingSubject<T> extends Subject<T> {
  private queuedValues: T[] = [];

  next(value: T): void {
    if (this.closed || this.observers.length) {
      super.next(value);
    } else {
      this.queuedValues.push(value);
    }
  }

  _subscribe(subscriber: Subscriber<T>): Subscription {
    const subscription = super._subscribe(subscriber);

    if (this.queuedValues.length) {
      this.queuedValues.forEach(value => super.next(value));
      this.queuedValues.splice(0);
    }

    return subscription;
  }
}