scheduleIterable.ts

import { Observable } from '../Observable';
import { SchedulerLike } from '../types';
import { iterator as Symbol_iterator } from '../symbol/iterator';
import { isFunction } from '../util/isFunction';
import { executeSchedule } from '../util/executeSchedule';

/**
 * Used in {@link scheduled} to create an observable from an Iterable.
 * @param input The iterable to create an observable from
 * @param scheduler The scheduler to use
 */
export function scheduleIterable<T>(input: Iterable<T>, scheduler: SchedulerLike) {
  return new Observable<T>((subscriber) => {
    let iterator: Iterator<T, T>;

    // Schedule the initial creation of the iterator from
    // the iterable. This is so the code in the iterable is
    // not called until the scheduled job fires.
    executeSchedule(subscriber, scheduler, () => {
      // Create the iterator.
      iterator = (input as any)[Symbol_iterator]();

      executeSchedule(
        subscriber,
        scheduler,
        () => {
          let value: T;
          let done: boolean | undefined;
          try {
            // Pull the value out of the iterator
            ({ value, done } = iterator.next());
          } catch (err) {
            // We got an error while pulling from the iterator
            subscriber.error(err);
            return;
          }

          if (done) {
            // If it is "done" we just complete. This mimics the
            // behavior of JavaScript's `for..of` consumption of
            // iterables, which will not emit the value from an iterator
            // result of `{ done: true: value: 'here' }`.
            subscriber.complete();
          } else {
            // The iterable is not done, emit the value.
            subscriber.next(value);
          }
        },
        0,
        true
      );
    });

    // During teardown, if we see this iterator has a `return` method,
    // then we know it is a Generator, and not just an Iterator. So we call
    // the `return()` function. This will ensure that any `finally { }` blocks
    // inside of the generator we can hit will be hit properly.
    return () => isFunction(iterator?.return) && iterator.return();
  });
}