canvas 绘制圆形进度条

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>canvas 圆形进度条效果</title>
    <style>
    *{margin:0;padding:0;}
    body{text-align:center;background-color:#000;}
    </style>
</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; //加载的快慢就靠它了 
            //绘制蓝色外圈
            function blueCircle(n){
                context.save();
                context.strokeStyle = "#fff"; //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();
            }
            //绘制白色外圈
            function whiteCircle(){
                context.save();
                context.beginPath();
                context.strokeStyle = "white";
                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(); //save和restore可以保证样式属性只运用于该段canvas元素
                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);
                context.clearRect(0, 0, canvas.width, canvas.height);
                whiteCircle();
                text(speed);
                blueCircle(speed);
                if(speed > 100) speed = 0;
                speed += 0.1;
            }());
        }
    </script>

</body>
</html>
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