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Introduction to the method of generating a circle that will become larger and smaller using JS

巴扎黑
巴扎黑Original
2017-09-14 11:52:322444browse

本文实例讲述了JS实现生成会变大变小的圆环。分享给大家供大家参考。具体如下：

这里使用javascript生成圆环，会变大变小，对于研究如何利用JavaScript生成动画效果，这是个很好的范例。

运行效果如下图所示：

具体代码如下：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style>
#p1 {width:2px; height:2px; position:absolute; background:red; left:150px; top:200px;}
p {width:3px; height:3px; position:absolute; background:black;}
</style>
<title>JS圆环</title>
<script type="text/javascript">
var n=30;
var r=100;
var a=true;
window.onload=function ()
{
 var op1=document.getElementById("p1");
 var ap=[];
 var op=null;
 var j=0;
 var i=0;
 for(i=0;i<n;i++)
 {
  op=document.createElement("p");
  ap.push(op);
  document.body.appendChild(op);
 }
 calcDrg();
 function calcDrg()
 {
  for(i=0;i<n;i++)
  {
   var degress=360*i/n+j;
   var a=Math.sin(degress*Math.PI/180)*r;
   var b=Math.cos(degress*Math.PI/180)*r;
   ap[i].style.left=op1.offsetLeft+b+"px";
   ap[i].style.top=op1.offsetTop-a+"px";
  }
 }
 setInterval(function (){
  j++;
  var s=0.3;
  a?r-=s:r+=s;
  if(r<=0 || r>=100)
  {
   a=!a;
  }
  calcDrg();
 }, 10);
};
</script>
</head>
<body>
<p id="p1">
</p>
</body>
</html>

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