Fabric.js 自由绘制椭圆

<canvas id="canvas" width="800" height="800"></canvas>

<script src="https://cdn.bootcdn.net/ajax/libs/fabric.js/521/fabric.js"></script>

<script>
  let canvas = null

  let currentType = 'default'
  let downPoint = null
  let upPoint = null

  let currentEllipse = null // 临时椭圆

  // 初始化画板
  function initCanvas() {
    canvas = new fabric.Canvas('canvas')

    canvas.on('mouse:down', canvasMouseDown)   // 鼠标在画布上按下
    canvas.on('mouse:move', canvasMouseMove)   // 鼠标在画布上移动
    canvas.on('mouse:up', canvasMouseUp)       // 鼠标在画布上松开
  }

  // 画布操作类型切换
  function typeChange(opt) {
    currentType = opt
    switch(opt) {
      case 'default':
        // 默认框选模式
        canvas.selection = true
        canvas.selectionColor = 'rgba(100, 100, 255, 0.3)'
        canvas.selectionBorderColor = 'rgba(255, 255, 255, 0.3)'
        canvas.skipTargetFind = false // 允许选中
        break
      case 'ellipse':
        // 椭圆模式
        // 将框选时，边框和背景设为透明
        canvas.selectionColor = 'transparent'
        canvas.selectionBorderColor = 'transparent'
        canvas.skipTargetFind = true // 禁止选中
        break
    }
  }

  // 鼠标在画布上按下
  function canvasMouseDown(e) {
    downPoint = e.absolutePointer

    if (currentType === 'ellipse') {
      currentEllipse = new fabric.Ellipse({
        top: downPoint.y,
        left: downPoint.x,
        rx: 0,
        ry: 0,
        fill: 'transparent',
        stroke: 'rgba(0, 0, 0, 0.2)'
      })
      canvas.add(currentEllipse)
    }
  }

  // 鼠标在画布上移动
  function canvasMouseMove(e) {
    if (currentType === 'ellipse' &amp;&amp; currentEllipse) {
      const currentPoint = e.absolutePointer

      let rx = Math.abs(downPoint.x - currentPoint.x) / 2
      let ry = Math.abs(downPoint.y - currentPoint.y) / 2

      let top = currentPoint.y > downPoint.y ? downPoint.y : downPoint.y - ry * 2
      let left = currentPoint.x > downPoint.x ? downPoint.x :  downPoint.x - rx * 2

      // 设置椭圆尺寸和所在位置
      currentEllipse.set('rx', rx)
      currentEllipse.set('ry', ry)
      currentEllipse.set('top', top)
      currentEllipse.set('left', left)
      // 刷新一下画布
      canvas.requestRenderAll()
    }
  }

  // 鼠标在画布上松开
  function canvasMouseUp(e) {
    upPoint = e.absolutePointer

    if (currentType === 'ellipse') {
      // 需要判断点击时和松开时鼠标的坐标点是否相等，相等的话就不创建椭圆了
      if (JSON.stringify(downPoint) === JSON.stringify(upPoint)) {
        canvas.remove(currentEllipse)
      } else {
        // 设置椭圆样式
        currentEllipse.set('stroke', '#000')
      }
      // 清除临时创建的椭圆
      currentEllipse = null
    }
  }

  window.onload = function() {
    initCanvas()
  }
</script>