changeImgPx(url: string, width: number, height: number): Promise<string> {
    return new Promise((resolve, reject) => {
      const query = wx.createSelectorQuery()
      query.select('#js-img-ctrl-canvas')
        .fields({
          node: true,
          size: true
        })
        .exec((res) => {
          const canvas = res[0].node
          const ctx = canvas.getContext('2d')
          let img = canvas.createImage()
          img.src = url
          img.onload = () => {
            ctx.drawImage(img, 0, 0, width, height);
            setTimeout(() => {
              wx.canvasToTempFilePath({
                canvas,
                destWidth: width,
                destHeight: height,
                fileType: 'jpg',
                quality: 1,
                success: (outRes) => {
                  resolve(outRes.tempFilePath)
                },
                fail: (err) => {
                  reject(err)
                }
              })
            }, 2000)
          }
        })
    })
  },

先使用canvas2d的drawImage, 然后使用wx.canvasToTempFilePath, 就是白图. 安卓一切正常, ios不行是白图