Function observeElementOffset

packages/virtual-core/src/index.ts:141–190

(
  instance: Virtualizer<T, any>,
  cb: ObserveOffsetCallBack,
)

type ObserveOffsetCallBack = (offset: number, isScrolling: boolean) => void

export const observeElementOffset = <T extends Element>(
  instance: Virtualizer<T, any>,
  cb: ObserveOffsetCallBack,
) => {
  const element = instance.scrollElement
  if (!element) {
    return
  }
  const targetWindow = instance.targetWindow
  if (!targetWindow) {
    return
  }

  let offset = 0
  const fallback =
    instance.options.useScrollendEvent && supportsScrollend
      ? () => undefined
      : debounce(
          targetWindow,
          () => {
            cb(offset, false)
          },
          instance.options.isScrollingResetDelay,
        )

  const createHandler = (isScrolling: boolean) => () => {
    const { horizontal, isRtl } = instance.options
    offset = horizontal
      ? element['scrollLeft'] * ((isRtl && -1) || 1)
      : element['scrollTop']
    fallback()
    cb(offset, isScrolling)
  }
  const handler = createHandler(true)
  const endHandler = createHandler(false)
  endHandler()

  element.addEventListener('scroll', handler, addEventListenerOptions)
  const registerScrollendEvent =
    instance.options.useScrollendEvent && supportsScrollend
  if (registerScrollendEvent) {
    element.addEventListener('scrollend', endHandler, addEventListenerOptions)
  }
  return () => {
    element.removeEventListener('scroll', handler)
    if (registerScrollendEvent) {
      element.removeEventListener('scrollend', endHandler)
    }
  }
}

export const observeWindowOffset = (
  instance: Virtualizer<Window, any>,