Add onWheel callback for swap axes

[bragovo]

To create behaviour like on this site https://canals-amsterdam.nl/ (mouse wheel vertical but scroll by Y axis), I need add deltaY to scrollOffset.x
This advise #19 didn't help me, because position.x always 0.

Instead of adding an option for this, I created callback function on wheel event. So in my code i can replace it like this:

SBRef.current = new ScrollBooster({
  viewport: rootRef.current,
  content: contentRef.current,
  direction: 'horizontal',
  emulateScroll: true,
  scrollMode: 'native',
  onWheel: (state, event) => {
    SBRef.current.scrollOffset.x = -event.deltaY
  }
})

I'm not sure about how to set offset, like example above or:

...
this.props.onWheel(state, event, this);
...
onWheel: (state, event, object) => {
  object.scrollOffset.x = -event.deltaY
}

I don't see problems in PR approach.

[bragovo]

Example for apple mouse, working this hoz and ver scroll

        onWheel: (state, event) => {
          let offsetX = 0
          if (event.deltaY >= 0 && event.deltaX >= 0) offsetX = Math.max(event.deltaY, event.deltaX)
          if (event.deltaY <= 0 && event.deltaX <= 0) offsetX = Math.min(event.deltaY, event.deltaX)
          SBRef.current.scrollOffset.x = -offsetX
          SBRef.current.scrollOffset.y = -event.deltaY
        }

[ilyashubin]

Maybe move it to start of the function, before condition, so it can be used without emulateScroll option?

[bragovo]

I'm not sure, but in this case my modifications this.scrollOffset will be override by default, correct? Callback function must be after:

            this.scrollOffset.x = -event.deltaX;
            this.scrollOffset.y = -event.deltaY;

or it will be useless.

[ilyashubin]

Oh you right, thanks :) Merging it then