JS实现圆形及半圆进度条拖拽交互

<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
  <title> 圆形进度条拖拽滑动 </title>
</head>
<body>
<canvas id="canvasId" width="400" height="400"></canvas>
<script type="text/javascript">
    var canvas = document.getElementById("canvasId");
    var ctx = canvas.getContext("2d");
    var ox = 200;
    var oy = 200;
    var or = 180;
    var br = 15;
    var moveFlag = false;

    function offset(r,d) {//根据弧度与距离计算偏移坐标
        return {x: -Math.sin(r)*d, y: Math.cos(r)*d};
    };

    function draw(n) {
        ctx.clearRect(0,0,canvas.width,canvas.height);
        ctx.strokeStyle = "#99a";
        ctx.lineWidth = 5;
        ctx.beginPath();
        ctx.arc(ox,oy,or,0,Math.PI,true);//半圆
        // ctx.arc(ox,oy,or,0,2*Math.PI,true);//整圆
        ctx.stroke();
        ctx.strokeStyle = "#69f";
        ctx.lineWidth = 5;
        ctx.beginPath();
        ctx.arc(ox,oy,or,Math.PI,(n*2+0.5)*Math.PI,false);
        // ctx.arc(ox,oy,or,0.5*Math.PI,(n*2+0.5)*Math.PI,false);
        ctx.stroke();
        ctx.fillStyle = "#69f";
        ctx.font = "80px Arial";
        ctx.textAlign = "center";
        ctx.textBaseline = "middle";
        ctx.fillText(Math.round(n*100-25)+"%",ox,oy);
        ctx.fillStyle = "#00f";
        ctx.beginPath();
        var d =  offset(n*2*Math.PI,or);
        ctx.arc(ox+d.x,oy+d.y,br,0,2*Math.PI,true);
        ctx.fill();
    }

    var on = ("ontouchstart" in document)? {
        start: "touchstart", move: "touchmove", end: "touchend"
    } : {
        start: "mousedown", move: "mousemove", end: "mouseup"
    };

    function getXY(e,obj) {
        var et = e.touches? e.touches[0] : e;
        var x = et.clientX;
        var y = et.clientY;
        return {
            x : x - obj.offsetLeft + (document.body.scrollLeft || document.documentElement.scrollLeft),
            y : y - obj.offsetTop  + (document.body.scrollTop || document.documentElement.scrollTop)
        }
    }

    canvas.addEventListener(on.start, function(e) {
        moveFlag = true;
    }, false);

    canvas.addEventListener(on.move, function(e) {
        if (moveFlag) {
            var k = getXY(e,canvas);
            var r = Math.atan2(k.x-ox, oy-k.y);
            var hd = (Math.PI+r)/(2*Math.PI);

            // 半圆的滑动范围判断
            if (hd <= 0.75 && hd >= 0.25) {
                draw(hd);
            }
        }
    }, false);

    canvas.addEventListener(on.end, function(e) {
        moveFlag = false;
    }, false);

    draw(0.25);
</script>

</body>
</html>