前提：有时候在项目中会有用到进度条的情况，使用css3也可以实现，但是对于性能不好的设备，或者网络不好的情况下，卡顿现象非常明显，避免出现不流畅的尴尬情况，所以记录一下，使用canvas来实现的方法。

效果图：  

dom中，首先定义canvas画板元素：  

<canvas id="canvas" width="500" height="500" style="background:#F7F7F7;">
    <p>you browser not support canvas!</p>
  </canvas>

对于不支持canvas的浏览器则会显示：you browser not support canvas!

接下来是js编写：
定义canvas.js并在页面引入

var canvas = document.getElementById('canvas'), //获取canvas元素
  context = canvas.getContext('2d'), //获取画图环境，指明为2d
  centerX = canvas.width / 2, //Canvas中心点x轴坐标
  centerY = canvas.height / 2, //Canvas中心点y轴坐标
  rad = Math.PI * 2 / 100, //将360度分成100份，那么每一份就是rad度
  speed = 0.1; //加载的快慢就靠它了

//绘制蓝色外圈
function blueCircle(n) {
  context.save();
  context.beginPath();
  context.strokeStyle = "#49f";
  context.lineWidth = 12;
  context.arc(centerX, centerY, 100, -Math.PI / 2, -Math.PI / 2 + n * rad, false);
  context.stroke();
  context.restore();
}

//绘制白色外圈
function whiteCircle() {
  context.save();
  context.beginPath();
  context.strokeStyle = "#A5DEF1";
  context.lineWidth = 12;
  context.arc(centerX, centerY, 100, 0, Math.PI * 2, false);
  context.stroke();
  context.closePath();
  context.restore();
}

//百分比文字绘制
function text(n) {
  context.save();
  context.fillStyle = "#F47C7C";
  context.font = "40px Arial";
  context.textAlign = "center";
  context.textBaseline = "middle";
  context.fillText(n.toFixed(0) + "%", centerX, centerY);
  context.restore();
}

//动画循环
(function drawFrame() {
  window.requestAnimationFrame(drawFrame, canvas);
  context.clearRect(0, 0, canvas.width, canvas.height);

  whiteCircle();
  text(speed);
  blueCircle(speed);

  if (speed > 100) speed = 0;
  speed += 0.1;
}());
window.requestAnimationFrame(drawFrame, canvas);

每行代码的注释标注非常清楚，如果还有不理解的可以去看canvas基础，应该就可以了。

链接: https://fly63.com/article/detial/3669