HarmonyOS 如何将沙盒图片转位base64

请参考：

import picker from '@ohos.file.picker';
import fs from '@ohos.file.fs'
import { image } from '@kit.ImageKit';
import buffer from '@ohos.buffer';
import { BusinessError } from '@ohos.base'

@Entry
@Component
export struct IRDemoIndex {
  @State message: string = 'Hello World';
  context = getContext()
  @State pixelMap: PixelMap | null = null
  @State resultBase64Str: string | null = null

  build() {
    Row() {
      Column() {
        Text(this.message)
          .fontSize(50)
          .fontWeight(FontWeight.Bold)

        Button('沙盒图片转base64').onClick((event: ClickEvent) => {
          const resourceManager = this.context.resourceManager
          const imageBuffer = resourceManager.getRawFileContentSync('app_icon.png')

          const pathDir = this.context.filesDir
          let filePath = pathDir + "/icon.png";
          let file = fs.openSync(filePath, fs.OpenMode.READ_WRITE | fs.OpenMode.CREATE);
          let writeLen = fs.writeSync(file.fd, imageBuffer.buffer);
          fs.closeSync(file);
          console.log(`图片写入沙箱成功`)

          const imageSource: image.ImageSource = image.createImageSource(imageBuffer.buffer as ArrayBuffer);
          let decodingOptions: image.DecodingOptions = {
            editable: true,
            desiredPixelFormat: 3,
            rotate: 90
          }

          imageSource.createPixelMap(decodingOptions).then((pixelMap: PixelMap) => {

            this.pixelMap = pixelMap
            // pixelmap 转base64 先要有一个packing的过程
            let imagePackerApi = image.createImagePacker();
            let packOpts: image.PackingOption = { format: "image/jpeg", quality: 98 };
            imagePackerApi.packing(pixelMap, packOpts).then((data: ArrayBuffer) => {
              let base64Str = buffer.from(data).toString('base64')
              let resultBase64Str = "data:image/png;base64," + base64Str
              this.resultBase64Str = resultBase64Str
              console.log(`resultBase64Str = ${resultBase64Str}`)
            }).catch((error: BusinessError) => {
              console.error('Failed to pack the image. And the error is: ' + error);
            })
            pixelMap.release();
          })

        })
        Image(this.resultBase64Str).width(100)
      }
      .width('100%')
    }
    .height('100%')
  }
}