拖动滚动条，scrollTop数值没有变化

function checkscrollSlide(){
var oparent=document.getElementById("main");
var oboxs= getClassObj(oparent,"box");
var lastboxH=oboxs[oboxs.length-1].offsetTop+Math.floor(oboxs[oboxs.length-1].offsetHeight/2);
var myscroll=document.body.scrollTop||document.documentElement.scrollTop;
var height=document.body.clientHeight||document.documentElement.clientHeight;
console.log(myscroll);
}