js代码

<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>

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