Scroller.as mouse wheel scrolling

private function skin_mouseWheelHandler(event:MouseEvent):void
    {
        const vp:IViewport = viewport;
        if (event.isDefaultPrevented() || !vp || !vp.visible)
            return;

        // Dispatch the "mouseWheelChanging" event. If preventDefault() is called
        // on this event, the event will be cancelled.  Otherwise if  the delta
        // is modified the new value will be used.
        var changingEvent:FlexMouseEvent = MouseEventUtil.createMouseWheelChangingEvent(event);
        if (!dispatchEvent(changingEvent))
        {
            event.preventDefault();
            return;
        }

        const delta:int = changingEvent.delta;

        var nSteps:uint = Math.abs(event.delta);
        var navigationUnit:uint;

        // Scroll delta "steps".  If the VSB is up, scroll vertically,
        // if -only- the HSB is up then scroll horizontally.

        // TODO: The problem is that viewport.validateNow() doesn’t necessarily
        // finish the job, see http://bugs.adobe.com/jira/browse/SDK-25740.
        // Since some imprecision in mouse-wheel scrolling is tolerable this is
        // ok for now.  For 4.next we should add Scroller API for (reliably)
        // scrolling in different increments and refactor code like this to
        // depend on it.  Also applies to VScroller and HScroller mouse
        // handlers.

        if (verticalScrollBar && verticalScrollBar.visible)
        {
            navigationUnit = (delta < 0) ? NavigationUnit.DOWN : NavigationUnit.UP;
            for (var vStep:int = 0; vStep < nSteps; vStep++)
            {
                var vspDelta:Number = vp.getVerticalScrollPositionDelta(navigationUnit);
                if (!isNaN(vspDelta))
                {
                    vp.verticalScrollPosition += vspDelta;
                    if (vp is IInvalidating)
                        IInvalidating(vp).validateNow();
                }
            }
            event.preventDefault();
        }
        else if (horizontalScrollBar && horizontalScrollBar.visible)
        {
            navigationUnit = (delta < 0) ? NavigationUnit.RIGHT : NavigationUnit.LEFT;
            for (var hStep:int = 0; hStep < nSteps; hStep++)
            {
                var hspDelta:Number = vp.getHorizontalScrollPositionDelta(navigationUnit);
                if (!isNaN(hspDelta))
                {
                    vp.horizontalScrollPosition += hspDelta;
                    if (vp is IInvalidating)
                        IInvalidating(vp).validateNow();
                }
            }
            event.preventDefault();
        }
    }