<div style="text-align: center">
  <pre id="code" style="font-size: 40px;font-family: Consolas;">code</pre>
</div>
<div id="container" class="C">
  <svg id="svg" viewBox="0 0 499 499" width="500" height="500"
       preserveAspectRatio="xMidYMid meet">
    <g id="main">
      <circle id="p1" cx="100" cy="250" r="16"/>
      <circle id="p2" cx="400" cy="250" r="16"/>

      <circle id="c1" cx="100" cy="100" r="8"/>
      <circle id="c2" cx="400" cy="100" r="8"/>

      <line id="l1" x1="100" y1="250" x2="100" y2="100"/>
      <line id="l2" x1="400" y1="250" x2="400" y2="100"/>

      <path id="path" d="M100,250 C100,100 400,100 400,250"/>
    </g>
  </svg>
</div>

   * {
      margin: 0;
      padding: 0;
    }

    #container {
      width: 100%;
      height: 500px;
      background-color: #fff;
      margin: 0 auto;
      text-align: center;
    }

    path {
      stroke-width: 8;
      stroke: #1E1935;
      stroke-linecap: round;
      fill: none;
    }

    circle {
      stroke-width: 2;
      stroke: #ef2d56;
      fill: #fff;
    }

    circle:hover {
      fill: #ef2d56;
      cursor: move;
    }

    line {
      stroke-width: 1;
      stroke: #999;
      stroke-linecap: round;
      stroke-dasharray: 5, 3;
    }

    path {
      stroke-width: 2;
      stroke: #1E1935;
      stroke-linecap: round;
      /* fill: none; */
    }

    path.fill {
      fill: #1E1935
    }

 let svg = document.getElementById('svg'),
    container = document.getElementById('container')
  let point = {}, line = {}
    , maxX = container.offsetWidth - 5
    , maxY = container.offsetHeight - 5,
    fill = false,
    code = document.getElementById('code'),
    // 状态
    drap = null,
    dPonit;

  function init() {
    let circle = svg.getElementsByTagName('circle');
    for (let i = 0; i < circle.length; i++) {
      point[circle[i].getAttributeNS(null, 'id')] = {
        x: +circle[i].getAttributeNS(null, 'cx'),
        y: +circle[i].getAttributeNS(null, 'cy'),
      }
    }
    line.l1 = svg.getElementById('l1')
    line.l2 = svg.getElementById('l2')
    line.path = svg.getElementById('path')
    svg.onmousedown = document.onmousemove = document.onmouseup = Drag
    svg.ontouchstart = document.ontouchmove = document.ontouchend = Drag
  }

  // 鼠标范围限制
  function PointM(x, y) {
    return {
      x: Math.max(0, Math.min(maxX, x)),
      y: Math.max(0, Math.min(maxY, y))
    }
  }


  function Drag(e) {
    e.stopPropagation()
    let t = e.target, id = t.id, et = e.type, m = PointM(e.pageX, e.pageY);
    // 鼠标按在path区域
    if (!drap && id === 'path' && et === 'mousedown') {
      fill = !fill;
      t.setAttributeNS(null, 'class', (fill ? 'fill' : ''))
      DragGet();
    }
    // 鼠标按在开始
    // typeof(point[id]!==undefined) 意思就是要按在这几个点上
    if (!drap && ['p1', 'p2', 'c1', 'c2'].includes(id) && (et === 'mousedown' || et === 'touchstart')) {
      drap = t;
      dPonit = m
    }
    // 移动的时候
    if (drap && ['p1', 'p2', 'c1', 'c2'].includes(id) && (et === 'mousemove' || et === 'touchmove')) {
      // 当前的坐标-开始的坐标等于差值
      point[id].x += m.x - dPonit.x;
      point[id].y += m.y - dPonit.y;
      if (point[id].x < maxX && point[id].y < maxY) {
        id = drap.id;
        dPonit = m;
        drap.setAttributeNS(null, "cx", point[id].x);
        drap.setAttributeNS(null, "cy", point[id].y);
      }
      DragGet()
    }

    // 松开的时候
    if (drap && (et === "mouseup" || et === "touchend")) {
      drap = null;
    }
  }

  // 更新数据
  function DragGet() {
    // line1
    line.l1.setAttributeNS(null, 'x1', point.p1.x);
    line.l1.setAttributeNS(null, 'y1', point.p1.y);
    line.l1.setAttributeNS(null, 'x2', point.c1.x);
    line.l1.setAttributeNS(null, 'y2', point.c1.y);
    // line2
    line.l2.setAttributeNS(null, 'x1', point.p2.x)
    line.l2.setAttributeNS(null, 'y1', point.p2.y)
    line.l2.setAttributeNS(null, 'x2', point.c2.x)
    line.l2.setAttributeNS(null, 'y2', point.c2.y)
    let d = `M ${point.p1.x},${point.p1.y} C ${point.c1.x},${point.c1.y} ${point.c2.x},${point.c2.y} ${point.p2.x},${point.p2.y}${fill ? 'Z' : ''}`
    line.path.setAttributeNS(null, 'd', d)
    code.textContent = d;
  }

  init()