<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
#all{
width: 500px;
height: 500px;
border: 2px solid sandybrown;
position: relative;
margin: 0 auto;
}
#div1{
width: 50px;
height: 50px;
background-color: red;
position: absolute;
}
#center{
width: 150px;
height: 150px;
background-color: black;
position: absolute;
margin: 175px;
}
</style>
</head>
<body>
<div id="all">
<div id="div1"></div>
<div id="center"></div>
</div>
<script type="text/javascript">
var oAll = document.getElementById("all");
var oDiv1 = document.getElementById("div1");
var oCenter = document.getElementById("center");
var maxL = oAll.clientWidth - oDiv1.clientWidth;
var maxT = oAll.clientHeight - oDiv1.clientHeight;
oDiv1.onmousedown = function(){
var ev = ev || window.event;
var lessX = ev.clientX - oDiv1.offsetLeft;
var lessY = ev.clientY - oDiv1.offsetTop;
document.onmousemove = function(){
var ev = ev || window.event;
var posL = ev.clientX - lessX;
var posT = ev.clientY - lessY;
if(oCenter.offsetLeft-oDiv1.offsetWidth<posL && posL<325 && oCenter.offsetLeft-oDiv1.offsetWidth<posT && posT<325 ){
oCenter.style.backgroundColor = "red"
}else{
oCenter.style.backgroundColor = "black"
}
if(posL<0){
posL = 0;
}
if(posT<0){
posT = 0;
}
if(posL>maxL){
posL = maxL;
}
if(posT>maxT){
posT = maxT;
}
oDiv1.style.left = posL + "px";
oDiv1.style.top = posT + "px";
}
}
document.onmouseup = function(){
document.onmousemove = null;
oDiv1.onmousemove = null;
}
</script>
</body>
