放大镜

实现思路：
需要的事件：
鼠标移入和移出移动 onmouseover onmouseout onmousemove
鼠标移入：显示遮罩层mask和大图显示区
鼠标移出：隐藏遮罩层mask和大图显示区
鼠标移动：
1、操作mask的left和top 让mask动起来
2、操作bigImg的left和top 让大图动起来
3、mask移动方向和bigImg
<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title></title>
    </head>
    <style type="text/css">
        *{
            margin: 0;
            padding: 0;
        }
        #box{
             350px;
            height: 350px;
            border:1px solid #000;
            margin: 200px;
            position: relative;
        }
        #mask{
             175px;
            height: 175px;
            background: pink;
            opacity: .3;
            position: absolute;
            left: 0;
            top: 0;
            cursor: move;
            display: none;
        }
        #big{
             400px;
            height: 400px;
            border: 1px solid #000;
            position: absolute;
            top : 0;
            left :  355px;
            overflow: hidden;
            display: none;
        }
        #bigImg{
            position: absolute;
            top: 0;
            left: 0;
        }
    </style>
    <body>
        <div id="box">
            <div id="small">
                <img src="001.jpg" alt="" id="smallImg"/>
                <div id="mask"></div>
            </div>
            <div id="big">
                <img src="0001.jpg" alt="" id="bigImg"/>
            </div>
        </div>
    </body>
</html>
<script>
    function $id(id){
        return document.getElementById(id)
    }
    //涉及的事件 ： onmouseover  onmouseout onmousemove
    var box = $id("box"), //整个放大镜容器
        small = $id("small"),//小图区
        mask = $id("mask"),//遮罩
        smallImg = $id("smallImg"),//小图
        bigImg = $id("bigImg"), //大图
        big = $id("big"); //大图显示区
        
    //第一步 ： 鼠标移入移出到small上 显示和隐藏mask和big
    small.onmouseover = function(){
        big.style.display = "block";
        mask.style.display = "block";
    }
    small.onmouseout = function(){
        big.style.display = "none";
        mask.style.display = "none";
    }
    //第二步 ： 鼠标在small上移动  操作mask跟随鼠标移动
    //改变mask的left和top
    small.onmousemove = function(e){
        var e = e || event;
        var x = e.pageX - mask.offsetWidth/2-box.offsetLeft;
        var y = e.pageY - mask.offsetHeight/2-box.offsetTop;
        //得到mask移动时的最大的left和top
        var maxL = small.offsetWidth-mask.offsetWidth;
        var maxT = small.offsetHeight - mask.offsetHeight;
        //边界处理
        /*if( x<0 ){
            x = 0;
        }else if(x > maxL){
            x = maxL;
        }*/
        x = x < 0 ? 0 : ( x > maxL ? maxL : x);
        y = y < 0 ? 0 : ( y > maxT ? maxT : y);
        
        //设置mask的移动
        mask.style.left = x + "px";
        mask.style.top = y + "px";
        
        //分析大图向上或向左移动的距离
        //大图的left/mask的left = 大图宽度/小图宽度  = 大图显示区宽度/mask宽度
        /*var bigImgLeft = x*(bigImg.offsetWidth-big.offsetWidth)/(small.offsetWidth-mask.offsetWidth);
        var bigImgTop = y*(bigImg.offsetHeight-big.offsetHeight)/(small.offsetHeight-mask.offsetHeight);*/
        var bigImgLeft = x*(bigImg.offsetWidth)/(small.offsetWidth);
        var bigImgTop = y*(bigImg.offsetHeight)/(small.offsetHeight);
        
        //设置大图的移动
        bigImg.style.left = -bigImgLeft + "px";
        bigImg.style.top = -bigImgTop + "px";
    }
</script>