HTML5 Canvas实现圆形并显示百分比的进度条实例详解

<canvas id="canvas" width="500" height="500" style="background:#000;"></canvas>

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5像素宽的运动外圈
        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.lineWidth = 2; //设置线宽
            context.strokeStyle = "red";
            context.arc(centerX, centerY, 100 , 0, Math.PI*2, false);
            context.stroke();
            context.closePath();
            context.restore();
        }

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);
            context.clearRect(0, 0, canvas.width, canvas.height);
            whiteCircle();
            text(speed);
            blueCircle(speed);
            if(speed > 100) speed = 0;
            speed += 0.1;
        }());

<canvas id="canvas" width="500" height="500" style="background:#000;"></canvas>
<script>
    window.onload = function(){
        var canvas = document.getElementById(&amp;#39;canvas&amp;#39;),  //获取canvas元素
            context = canvas.getContext(&amp;#39;2d&amp;#39;),  //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5像素宽的运动外圈
        function blueCircle(n){
            context.save();
            context.strokeStyle = &quot;#fff&quot;; //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.lineWidth = 2; //设置线宽
            context.strokeStyle = &quot;red&quot;;
            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 = &quot;#fff&quot;; //设置描边样式
            context.font = &quot;40px Arial&quot;; //设置字体大小和字体
            //绘制字体，并且指定位置
            context.strokeText(n.toFixed(0)+&quot;%&quot;, centerX-25, centerY+10);
            context.stroke(); //执行绘制
            context.restore();
        } 
        //动画循环
        (function drawFrame(){
            window.requestAnimationFrame(drawFrame);
            context.clearRect(0, 0, canvas.width, canvas.height);
            whiteCircle();
            text(speed);
            blueCircle(speed);
            if(speed &gt; 100) speed = 0;
            speed += 0.1;
        }());
    }
</script>