feat: support custom scroll options (#95)

This adds two new options which make it easier to customize the scroll
behavior.

`onScrollElement` which makes it possible to use a different
element to bind the `onScroll` event too.

`scrollOffsetFn` allows a custom function to calculate the offset rather
than using the `parentRef`'s width or height.

README.md

@@ -278,6 +278,12 @@ const {
 - `paddingEnd: Integer`
   - Defaults to `0`
   - The amount of padding in pixels to add to the end of the virtual list
+  - `onScrollElement: React.useRef(DOMElement)`
+   - Optional
+   - Allows using a different element to bind the `onScroll` event to
+ - `scrollOffsetFn: Function() => number`
+   - Optional
+   - This function, if passed, is called on scroll to get the scroll offest rather than using `parentRef`'s `width` or `height`
 
 ### Returns
 

src/index.js

@@ -15,6 +15,8 @@ export function useVirtual({
   horizontal,
   scrollToFn,
   useObserver,
+  onScrollElement,
+  scrollOffsetFn,
 }) {
   const sizeKey = horizontal ? 'width' : 'height'
   const scrollKey = horizontal ? 'scrollLeft' : 'scrollTop'
@@ -69,12 +71,12 @@ export function useVirtual({
 
   const [range, setRange] = React.useState({ start: 0, end: 0 })
 
+  const element = onScrollElement ? onScrollElement.current : parentRef.current
   useIsomorphicLayoutEffect(() => {
-    const element = parentRef.current
     if (!element) { return }
 
     const onScroll = () => {
-      const scrollOffset = element[scrollKey]
+      const scrollOffset = scrollOffsetFn ? scrollOffsetFn() : element[scrollKey]
       latestRef.current.scrollOffset = scrollOffset
       setRange(prevRange => calculateRange(latestRef.current, prevRange))
     }
@@ -90,7 +92,7 @@ export function useVirtual({
     return () => {
       element.removeEventListener('scroll', onScroll)
     }
-  }, [parentRef.current, scrollKey, size /* required */, outerSize /* required */])
+  }, [element, scrollKey, size /* required */, outerSize /* required */])
 
   const virtualItems = React.useMemo(() => {
     const virtualItems = []
@@ -185,8 +187,8 @@ export function useVirtual({
         align === 'center'
           ? measurement.start + measurement.size / 2
           : align === 'end'
-          ? measurement.end
-          : measurement.start
+            ? measurement.end
+            : measurement.start
 
       scrollToOffset(toOffset, { align, ...rest })
     },