flex设置scroller的速度

protected function mainScroller _mouseWheelChangingHandler(event:FlexMouseEvent):void
{
	event.preventDefault();
	var modifier:int       = 5;
	var delta:Number       = Number(event.delta) * modifier;
	var viewport:IViewport = mainScroller.verticalScrollBar.viewport;
	var vPos:Number        = viewport.verticalScrollPosition;
	var maximum:Number     = mainScroller.verticalScrollBar.maximum;
	
	if (delta < 0) 
	{
		mainScroller.verticalScrollBar.viewport.verticalScrollPosition = Math.min(vPos - delta, maximum) ;        
	}
	else
	{
		mainScroller.verticalScrollBar.viewport.verticalScrollPosition = Math.max(vPos - delta, 0);
	}
}