用户上传的图片中第一张有可能是空白的图片，很奇怪，感觉是代码问题，ctx.drawImage后需要延迟调用canvasToTempFilePath吗?

wx.createSelectorQuery()
  .select('#canvas')
  .fields({node: true, size: true})
  .exec((res) => {
    const canvas = res[0].node
    let dpr = wx.getSystemInfoSync().pixelRatio;
    dpr = dpr > 2 ? 2 : dpr
    canvas.width = this.cWidth * dpr
    canvas.height = this.cHeight * dpr
    const ctx = canvas.getContext('2d')
    const img = canvas.createImage()
    img.src = tempFilePaths[0]
    img.onload = () => {
      ctx.drawImage(img, 0, 0, this.cWidth * dpr, this.cHeight * dpr)
      wx.canvasToTempFilePath({canvas, success: (res) => {
          ...
      }})
    }
  })