actions.js

// TODO: Run epic on app start if geolocation was already explicitly requested.
const watchAppPositionEpic = (action$, { geolocation }) => {
  const geolocation$ = Observable.create(observer => {
    const onNext = (position: Position) => {
      const action = watchAppPositionNext(geopositionToObject(position));
      observer.next(action);
    };
    const onError = ({ code, message }: PositionError) => {
      const action = watchAppPositionFail({ code, message });
      observer.next(action);
    };
    const watchID = geolocation.watchPosition(onNext, onError);
    return () => {
      geolocation.clearWatch(watchID);
    };
  });
  return action$.ofType(WATCH_APP_POSITION)
    .switchMap(() =>
      geolocation$.takeUntil(action$.ofType(APP_STOP))
    );
}