📄 Custom RxJS operators 5/7: "pollWhile operator"

#angular #rxjs #pollWhile

The good old polling mechanism is still quite relevant in applications which rely on data that changes over a period of time. Let’s consider a scenario when an application has to make http requests in a certain time interval until an http response body fulfils a given condition, e.g. analysis status is set to completed. The built-in repeat operator combined with a limiting operator allows to accomplish the goal with a few lines of code.

operator

interface PollWhileConfig<T> {
  predicateFn: (v: T) => boolean;
  delay: number;
  count?: number;
  lastOnly?: boolean;
}

function pollWhile<T>({
  predicateFn,
  delay,
  count = Infinity,
  lastOnly = false,
}: PollWhileConfig<T>): MonoTypeOperatorFunction<T> {
  const limiter = lastOnly
    ? pipe(
        filter((v: T) => !predicateFn(v)),
        take(1)
      )
    : takeWhile(predicateFn, true);

  return pipe(repeat({ delay, count }), limiter);
}