createCanvas () {
  wx.createSelectorQuery()
    .select('#myCanvas')
    .fields({
      node: true,
      size: true,
    })
    .exec((res) => {
      console.log(res)
      this.initCanvas(res)
    })
},
initCanvas(res) {
  const that = this
  const width = res[0].width
  const height = res[0].height
  const canvas = res[0].node
  const ctx = canvas.getContext('2d')
  const dpr = wx.getSystemInfoSync().pixelRatio
  canvas.width = width * dpr
  canvas.height = height * dpr
  ctx.scale(dpr, dpr)
  var img = canvas.createImage()
  img.src =  'https://mediabook.oss-cn-shanghai.aliyuncs.com/bangdivr/img/posterbg.jpg'
  img.onload = (res) => {
    console.log(res)
    ctx.drawImage(img ,0, 0, 750, 1448, 0, 0, 750, 1448)
   }
  img.onerror = (err) => {
    console.log(err)
  }
  
},