let box1=document.querySelector(".box1");
 let box2=document.querySelector(".box2");
 let son=document.querySelector(".son");
 let ceng=document.querySelector(".ceng");
 //鼠标移入：son，box2出现
 ceng.onmouseenter=function(){
  son.style.display="block"
  box2.style.display="block"
 }
 //鼠标移出：son，box2消失
 ceng.onmouseleave=function(){
  son.style.display="none"
  box2.style.display="none"
 }
 //鼠标移动：son的位置随鼠标移动，box2背景图的位置变化
 ceng.onmousemove=function(e){
  let sw=e.clientX-son.offsetWidth/2
  let sh=e.clientY-son.offsetHeight/2  
  if(sw>ceng.offsetWidth-son.offsetWidth){
   sw=ceng.offsetWidth-son.offsetWidth
  }
  else if(sw<=0){
   sw=0;
  }
  if(sh>ceng.offsetHeight-son.offsetHeight){
   sh=ceng.offsetHeight-son.offHeight
  }
  else if(sh<=0){
   sh=0;
  }
  son.style.left=sw+"px"
  son.style.top=sh+"px"
  box2.style.backgroundPosition=`${-2*sw}px ${-2*sh}px`
 }