HarmonyOS如何将photoPicker拿到的uri转为base64？

async getImageBase64WithUri(uri:string): Promise<string> { 
  const file = await fs.open(uri, fs.OpenMode.READ_ONLY); 
  const imageSource : image.ImageSource = image.createImageSource(file.fd); 
  const imagePackerApi = image.createImagePacker(); 
  // 设置打包参数 
  // format：当前仅支持打包为JPEG、WebP 和 png 格式 
  // quality：JPEG 编码输出图片质量 
  // bufferSize：图片大小，默认 10M 
  const packOpts: image.PackingOption = { format: "image/jpeg", quality: 100 }; 
  let imageBuffer: ArrayBuffer = new ArrayBuffer(1); 
  let resultBase64Str = '' 
  try { 
  // 图片压缩或重新打包 
  imageBuffer = await imagePackerApi.packing(imageSource, packOpts); 
  let base64Str = buffer.from(imageBuffer).toString('base64') 
  resultBase64Str="data:image/jpeg;base64,"+base64Str 
} catch (err) { 
  console.error(`Invoke getImageArrayBufferWithUri failed, err: ${JSON.stringify(err)}`); 
} 
return resultBase64Str; 
}