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JavaScript Taobao main image magnifying glass function

高洛峰Original: 2016-12-09 15:39:571441browse

If a worker wants to do his job well, he must first sharpen his tools. To achieve a certain effect, we must first understand its principles.
The function of the magnifying glass is to obtain the position of the mouse in the small image, and then calculate the part of the large image that needs to be displayed based on the size ratio of the large and small images, and then use positioning to make the part to be displayed in the large image appear in the right border.
Then look at the code. It will be easier to understand based on the code.

html part

<!DOCTYPE html> 
<html lang="en"> 
<head> 
 <meta charset="UTF-8"> 
 <title>放大镜效果</title> 
 <link rel="stylesheet" href="magnifier.css"> 
</head> 
<body> 
 <div id="wrapper"> 
 <!--小图-->
  <div id="img_min"> 
  <!--图片-->
  <img src="test.jpg" alt="min"> 
  <!--跟随鼠标的白块-->
  <p id="mousebg"></p> 
  </div> 
  <!--大图-->
  <div id="img_max"><img id="img2_img" src="test.jpg" alt="max"></div> 
 </div> 
 <script type="text/javascript" src="magnifier.js"></script> 
</body> 
</html>

css part

*{ 
 margin: 0; 
 padding: 0; 
} 
div{ 
 position: relative; 
} 
div>div{ 
 width: 300px; 
 height: 300px; 
 float: left; 
 margin: 100px; 
 overflow: hidden; 
} 
#img_min>img{ 
 /*display: block;*/
 width: 300px; 
} 
#img_max{ 
 display: none; 
   
} 
#img_max>img{ 
 position: absolute; 
 top: 0; 
 left: 0; 
 display: block; 
 width: 1500px; 
} 
#mousebg{ 
 display: none; 
 position: absolute; 
 width: 60px; 
 height: 60px; 
 background-color: rgba(255,255,255,.7); 
 top: 0; 
 left: 0; 
}

The most important javascript part

window.onload = function () { 
 var img1 = document.getElementById(&#39;img_min&#39;);//小图盒子 
 var img2 = document.getElementById(&#39;img_max&#39;);//大图盒子 
 var img2_img = document.getElementById(&#39;img2_img&#39;);//大图图片 
 var wrap = document.getElementById(&#39;wrapper&#39;); 
 var mousebg = document.getElementById(&#39;mousebg&#39;);//鼠标白块 
 var mul = 5; 
 //当某一个模块dispaly:none的时候不能使用offsetWidth获取它的宽高 
 img1.onmouseover = function () { 
  //鼠标进入 
  img2.style.display = &#39;block&#39;; 
  mousebg.style.display = &#39;block&#39;; 
    
 } 
 img1.onmouseout = function () { 
  //鼠标离开 
  img2.style.display = &#39;none&#39;; 
  mousebg.style.display = &#39;none&#39;; 
 } 
 img1.onmousemove = function (event) { 
  var _event = event||window.event;//兼容性处理 
  var mouseX = _event.clientX - wrap.offsetLeft - img1.offsetLeft; 
  //计算鼠标相对与小图的位置 
  var mouseY = _event.clientY - wrap.offsetTop - img1.offsetTop; 
  
  //特殊情况处理，分别靠近四条边的时候 
  if(mouseX<mousebg.offsetWidth/2){ 
   mouseX = mousebg.offsetWidth/2; 
  } 
  if(mouseX>img1.offsetWidth-mousebg.offsetWidth/2){ 
   mouseX = img1.offsetWidth-mousebg.offsetWidth/2; 
  } 
  if(mouseY<mousebg.offsetHeight/2){ 
   mouseY = mousebg.offsetHeight/2; 
  } 
  if(mouseY>img1.offsetHeight-mousebg.offsetHeight/2){ 
   mouseY = img1.offsetHeight-mousebg.offsetHeight/2; 
  } 
  //计算大图的显示范围 
  img2_img.style.left = -mul*mouseX+img2.offsetWidth/2+"px"; 
  img2_img.style.top = -mul*mouseY+img2.offsetHeight/2+"px"; 
  //使鼠标在白块的中间 
  mousebg.style.left = mouseX-mousebg.offsetWidth/2+"px"; 
  mousebg.style.top = mouseY-mousebg.offsetHeight/2+"px"; 
    
 } 
}

If you understand it after reading the code and comments, use Li Yunlong's words to say: "Oh, you kid tnd is really a genius." Then you can quickly browse through the following analysis part and it will be OK.

Analysis part:

The html and css parts are simple layout codes and will not be explained any more. The js part code is also relatively simple. Let’s directly explain the code of the mouse movement event part.
First, use a picture to explain the principle of obtaining the position of the mouse relative to the small picture:

JavaScript Taobao main image magnifying glass function

You can see that through the operation in the code, the value we obtain is the value of the mouse relative to the upper left corner of img1.
After understanding this step, we can actually say that half of our work is done.
Then, we skip the processing of special cases and directly perform the basic operation of positioning the image on the right.
Because the offsetWidth, offsetHeight, style.width, and style.height properties are used. The ranges of style.width, style.height and offsetWidth and offsetHeight are the same. I will describe the other differences in detail in another blog. Let’s first use a picture to understand these attributes, and compare them with the above attributes (pictures are from the Internet, deleted)

JavaScript Taobao main image magnifying glass function

Then we explain the code:

Use of pictures in the big picture frame on the right style.left is positioned in the large picture frame. The negative sign is because the movement direction of our mouse is exactly opposite to the movement direction of the picture in our large picture frame. mul is calculated based on the size of the large picture and the small picture. Proportion, what is calculated by -mul*mouseX is actually the relative position of the picture in the large frame, but at this time you will find that the position of your mouse on the right is in the upper left corner of the frame, so we need to add an img2 .offsetWidth/2 to center the image. Similarly, we can perform the same processing on the ordinate.

//计算大图的显示范围
  img2_img.style.left = -mul*mouseX+img2.offsetWidth/2+"px";
  img2_img.style.top = -mul*mouseY+img2.offsetHeight/2+"px";

Now we are going to deal with special situations. When you do the previous step, you will find that when the mouse moves to the edge, the small white block of the mouse sometimes runs out of the scope of the picture. So we have to process it and limit it to the scope of the picture. Because the mouse is in the middle of the white transparent block, we just limit the mouse to a position that is half the length/width of the white block above, below, left, and right of the picture border. .

//特殊情况处理，分别靠近四条边的时候
  if(mouseX<mousebg.offsetWidth/2){
   mouseX = mousebg.offsetWidth/2;
  }
  if(mouseX>img1.offsetWidth-mousebg.offsetWidth/2){
   mouseX = img1.offsetWidth-mousebg.offsetWidth/2;
  }
  if(mouseY<mousebg.offsetHeight/2){
   mouseY = mousebg.offsetHeight/2;
  }
  if(mouseY>img1.offsetHeight-mousebg.offsetHeight/2){
   mouseY = img1.offsetHeight-mousebg.offsetHeight/2;
  }

When the distance to the left is less than half the width, we make mouseX equal to half the width, so that the white block will not continue to move. The same applies to the other three directions.
After completing this step, our effects are complete.
ps: Abstract places can be understood by drawing pictures.

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