新版

<!-- canvas.wxml -->
<canvas type="2d" id="myCanvas"></canvas>

// canvas.js
    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 dpr = wx.getSystemInfoSync().pixelRatio
        canvas.width = res[0].width * dpr
        canvas.height = res[0].height * dpr
        ctx.scale(dpr, dpr)

        ctx.fillRect(0, 0, 100, 100)
      })

• 画图片

       var imgUrl = 'https://ss2.bdstatic.com/70cFvnSh_Q1YnxGkpoWK1HF6hhy/it/u=1446459463,4116825017&fm=26&gp=0.jpg';
        //将网络图片转成本地路径     
        wx.getImageInfo({
          src: imgUrl,
          success: function (res) {
            // cav.drawImage(res.path, 0, 0, 375, 375)
            console.log(res)
            const img = canvas.createImage()
            // img.src = '/assets/img/home-no-followed-icon.png'
            img.src = res.path
            img.onload = () => ctx.drawImage(img, 100, 100);
          }
        })

旧版

<!-- canvas.wxml -->
<canvas style="width: 300px; height: 200px;" canvas-id="firstCanvas"></canvas>

// canvas.js
// 在page获取
var ctx = wx.createCanvasContext('firstCanvas');
// 在组件中获取
var ctx = wx.createCanvasContext('firstCanvas', this);



    context.setStrokeStyle("#00ff00")
    context.setLineWidth(5)
    context.rect(0, 0, 200, 200)
    context.stroke()
    context.setStrokeStyle("#ff0000")
    context.setLineWidth(2)
    context.moveTo(160, 100)
    context.arc(100, 100, 60, 0, 2 * Math.PI, true)
    context.moveTo(140, 100)
    context.arc(100, 100, 40, 0, Math.PI, false)
    context.moveTo(85, 80)
    context.arc(80, 80, 5, 0, 2 * Math.PI, true)
    context.moveTo(125, 80)
    context.arc(120, 80, 5, 0, 2 * Math.PI, true)
    context.stroke()
    context.draw()

• 画图

var imgUrl = 'https://ss2.bdstatic.com/70cFvnSh_Q1YnxGkpoWK1HF6hhy/it/u=1446459463,4116825017&fm=26&gp=0.jpg';
ctx.drawImage(imgUrl, 0, 0, 65, 114)
ctx.draw(true,function(){

})