scrollup.ts

/**
 * @param scrollable The element being scrolled
 * @param debounceMS The number of milliseconds to debounce scroll events
 * @param sp The function returning the scroll position coordinates.
 * @return A boolean valued observable indicating whether the element is scrolling up or down
 */
export function scrollingUp(
  scrollable: any, 
  debounceMS: number, 
  sp: scrollPosition): Observable<boolean> {
  return fromEvent(scrollable, 'scroll').pipe(
    debounceTime(debounceMS), 
    distinctUntilChanged(), 
    map(v => sp()), 
    pairwise(), 
    switchMap(p => {
    const y1 = p[0][1]
    const y2 = p[1][1]
    return y1 - y2 > 0 ? of(false) : of(true)
  }))
}
export type scrollPosition = ()=>[number, number]