//.wxml文件如下

<canvas type="2d" id="myCanvas" style="width:98%;height:700rpx;border:1rpx solid #999"></canvas>

//.js文件中在onReady内写的内容如下

 onReady: function () {

    const query = wx.createSelectorQuery()

    query.select('#myCanvas')

      .fields({ node: true, size: true })

      .exec((res) => {

        const canvas = res[0].node

        const ctx = canvas.getContext('2d')

        const systemInfo =  wx.getSystemInfoSync()

        const dpr = systemInfo.pixelRatio

        canvas.width = res[0].width * dpr

        canvas.height = res[0].height * dpr

        ctx.scale(dpr, dpr)

        this.ctx = ctx

        ctx.strokeStyle='#FF7F00'

        ctx.lineWidth=2

        ctx.beginPath()

        ctx.moveTo(100,100) 

        ctx.lineTo(110,50)

        ctx.lineTo(112,110)

        ctx.lineTo(120,120)

        ctx.lineTo(150,10)

        ctx.stroke()

      })

  },