掌握页面元素定位和移动
放大镜关键原理：鼠标在小图片上移动时，
通过捕捉鼠标在小图片上的位置定位大图片的相应位置
技术点：事件捕获、定位
offsetLeft与style.left的对比：
1）offsetLeft是与父级元素的距离，不包过滚动条的距离
2）style.left返回的是字符串，如30px，offsetLeft返回的是数值30
3）style.lft是可读写的，offsetLeft是只读的，所以要改变div的位置只能修改style.left
4）style.left的值需要事先定义，否则取到的值为空
难点：计算：如何让小图片的放大镜和大图片同步移动
距离定位图解：
1 <!DOCTYPE html> 2 <html> 3 <head lang="en"> 4 <meta charset="UTF-8"> 5 <title></title> 6 </head> 7 <style type="text/css"> 8 *{ 9 margin: 0; 10 padding: 0; 11 } 12 #demo{ 13 display: block; 14 400px; 15 height: 255px; 16 margin: 50px; 17 position: relative; 18 border: 1px solid #ccc; 19 } 20 #small-box{ 21 position: relative; 22 z-index: 1; 23 } 24 #float-box{ 25 display: none; 26 160px; 27 height: 120px; 28 position: absolute; 29 background: #ffffcc; 30 border: 1px solid #ccc; 31 filter: alpha(opacity=50); 32 opacity: 0.5; 33 } 34 #mark{ 35 position: absolute; 36 display: block; 37 400px; 38 height: 255px; 39 background-color: #fff; 40 filter: alpha(opacity=0); 41 opacity: 0; 42 z-index: 10; 43 } 44 #big-box{ 45 display: none; 46 position: absolute; 47 top: 0; 48 left: 460px; 49 400px; 50 height: 300px; 51 overflow: hidden; 52 border: 1px solid #ccc; 53 z-index: 1; 54 } 55 #big-box img{ 56 position: absolute; 57 z-index: 5; 58 } 59 </style> 60 <script> 61 window.onload=function(){ 62 var $=function(id){ 63 return document.getElementById(id); 64 } 65 var Demo = $("demo"); 66 var SmallBox = $("small-box"); 67 var Mark = $("mark"); 68 var FloatBox = $("float-box"); 69 var BigBox = $("big-box"); 70 var BigBoxImage = BigBox.getElementsByTagName("img")[0]; 71 Mark.onmouseover = function(){ 72 FloatBox.style.display = "block"; 73 BigBox.style.display = "block"; 74 } 75 Mark.onmouseout = function(){ 76 FloatBox.style.display = "none"; 77 BigBox.style.display = "none"; 78 } 79 Mark.onmousemove = function(e){ 80 var _event = e||window.event//兼容多个浏览器的参数模式 81 var left = _event.clientX - Demo.offsetLeft - SmallBox.offsetLeft - FloatBox.offsetWidth/2; 82 var top = _event.clientY - Demo.offsetTop - SmallBox.offsetTop - FloatBox.offsetHeight/2; 83 if(left < 0){ 84 left = 0; 85 }else if(left > Mark.offsetWidth - FloatBox.offsetWidth){ 86 left = Mark.offsetWidth - FloatBox.offsetWidth; 87 } 88 if(top < 0){ 89 top = 0; 90 }else if(top > Mark.offsetHeight - FloatBox.offsetHeight){ 91 top = Mark.offsetHeight - FloatBox.offsetHeight; 92 } 93 FloatBox.style.left = left + 'px'; 94 FloatBox.style.top = top + 'px'; 95 var percentX = left / (Mark.offsetWidth - FloatBox.offsetWidth); 96 var percentY = top / (Mark.offsetHeight - FloatBox.offsetHeight); 97 BigBoxImage.style.left = -percentX * (BigBoxImage.offsetWidth - BigBox.offsetWidth)+'px'; 98 BigBoxImage.style.top = -percentY * (BigBoxImage.offsetHeight - BigBox.offsetHeight)+'px'; 99 } 100 } 101 </script> 102 <body> 103 <div id="demo"> 104 <div id="small-box"> 105 <div id="mark"></div> 106 <div id="float-box"></div> 107 <img src="img/macbook-small.jpg" /> 108 </div> 109 <div id="big-box"> 110 <img src="img/macbook-big.jpg" /> 111 </div> 112 </div> 113 </body> 114 </html>