<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>百分比加载</title>
</head>
<body>
<canvas id="canvas" width="500" height="500" style="background:#000;"></canvas>
<script>
window.onload = function(){
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
//绘制外圈
function blueCircle(n){
context.save();//保存当前环境的状态(save和restore可以保证样式属性只运用于该段canvas元素)
context.strokeStyle = "#49f"; //设置描边样式
context.lineWidth = 5; //设置线宽
context.beginPath(); //路径开始
context.arc(centerX, centerY, 100 , -Math.PI/2, -Math.PI/2 +n*rad, false); //用于绘制圆弧context.arc(x坐标，y坐标，半径，起始角度，终止角度，顺时针/逆时针)
context.stroke(); //绘制
context.closePath(); //路径结束
context.restore();//返回之前保存过的路径状态和属性
}
//绘制白色外圈
function whiteCircle(){
context.save();
context.beginPath();
context.strokeStyle = "#fff";
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.strokeStyle = "#fff"; //设置描边样式
context.font = "40px Arial"; //设置字体大小和字体
//绘制字体，并且指定位置
context.strokeText(n.toFixed(0)+"%", centerX-25, centerY+10);
context.stroke(); //执行绘制
context.restore();
}
//动画循环
(function drawFrame(){
window.requestAnimationFrame(drawFrame, canvas);//requestAnimationFrame作用与setTimeInterval一样，不同的是它会根据浏览器的刷新频率自动调整动画的时间间隔。
context.clearRect(0, 0, canvas.width, canvas.height);
whiteCircle();
text(speed);
blueCircle(speed);
if(speed > 100){
speed = 0;
}else{
speed += 0.1;
}
}());
}
</script>
</body>
</html>