my scrollbar

scrollBar.addEventListener(MouseEvent.MOUSE_DOWN,downBar);        //滚动条鼠标按下
scrollBar.addEventListener(MouseEvent.MOUSE_MOVE,moveBar);        //滚动条滚动
scrollBar.addEventListener(MouseEvent.MOUSE_UP,upBar);            //滚动条鼠标弹起
scrollBar.height = txt.height * barLine.height / txt.textHeight; //初始化滚动条的高度
var isDown:Boolean;        //滚动条鼠标是否按下
var mousey:Number;        //鼠标y坐标
function downBar(e:MouseEvent):void{
    isDown = true;        //鼠标按下
    mousey = mouseY;    //鼠标y坐标初始值
}
function upBar(e:MouseEvent):void{
    isDown = false;        //鼠标弹起
}
function moveBar(e:MouseEvent):void{
    if(isDown){
        if(scrollBar.y < barLine.y){        //滚动条到最顶端时
            scrollBar.y = barLine.y;
        }else if(scrollBar.y > (barLine.y + barLine.height - scrollBar.height)){ 
            //滚动到最底端时
            scrollBar.y = barLine.y + barLine.height - scrollBar.height;
        }else{
            scrollBar.y += (mouseY-mousey); //滚动条滚动的幅度
            var hh = scrollBar.height*(mouseY-mousey) / txt.height; 
            //计算textfield应该滚动的幅度
            txt.scrollV += hh;                //滚动textfield的内容;
            mousey = mouseY;                //重新定义鼠标y坐标
        }
    }
}