HarmonyOS 通过uri获取到资源的Base64如何获取

一般情况下都是先拿到资源的data数据，然后Base64Helper转base64，以图片举例：

https://developer.huawei.com/consumer/cn/doc/harmonyos-references-V5/js-apis-util-V5#base64helper9

async pixelToBase64(data: PixelMap): Promise<string> {
  const imagePackerApi: image.ImagePacker = image.createImagePacker()
  let packOpts: image.PackingOption = { format: 'image/png', quality: 30 }
  try {
  let readBuffer: ArrayBuffer =  await imagePackerApi.packing(data, packOpts)
  let bufferArr = new Uint8Array(readBuffer)
  let help = new util.Base64Helper
  let base = help.encodeToStringSync(bufferArr)
  return base
  } catch (err) {
  return ''
}