canvas 绘制环形进度条

结果：
代码：
<!DOCTYPE html>
<html>
<head lang="en">
  <meta charset="UTF-8">
  <title></title>
  <style>
    body {
      text-align: center;
    }
    canvas {
      background: #ddd;
    }
  </style>
</head>
<body>
  <h3>绘制路径——圆形</h3>
  <canvas id="c13" width="500" height="400"></canvas>
  <script>
    var ctx = c13.getContext('2d');

    //绘制灰色的背景圆环
    ctx.beginPath();
    ctx.arc(250,200,100,0, 2*Math.PI);
    ctx.lineWidth = 20;
    ctx.strokeStyle = '#aaa';
    ctx.stroke();

    //绘制圆形进度条
    var start = -90;  //进度条的起始角度
    var end = -90;    //进度条的终止角度
    var t = setInterval(function(){
      //此练习中可以省略“清除画布”步骤
      ctx.beginPath();
      ctx.arc(250,200,100,start*Math.PI/180,end*Math.PI/180);
      ctx.strokeStyle = '#0a0';
      ctx.stroke();
      
      end += 3;
      if(end>270){
        clearInterval(t);
      }
    }, 50)
  </script>
</body>
</html>