// 元素的 position 一定要为 absolute

window.onload = function () {
    var oDiv = document.getElementById(‘div1‘);
    oDiv.onmousedown = function (ev)    {
        var oEvent = ev||event;
        var disX= oEvent.clientX - oDiv.offsetLeft;
        var disY= oEvent.clientY - oDiv.offsetTop;
        document.onmousemove = function (ev) {
            var oEvent = ev||event;
            oDiv.style.left = oEvent.clientX - disX + ‘px‘;
            oDiv.style.top = oEvent.clientY - disY + ‘px‘;
        };
        document.onmouseup  =  function () {
            document.onmousemove = null;
            document.onmouseup = null;
        };
    };
};