鸿蒙开发如何解码某一帧视频数据,将其保存成一个图片文件？

import media from '@ohos.multimedia.media';
import image from '@ohos.multimedia.image';
import { BusinessError } from '@ohos.base';
import picker from '@ohos.file.picker';
import fs from '@ohos.file.fs';
import photoAccessHelper from '@ohos.file.photoAccessHelper';

@Entry
@Component
struct Index {
  @State pixelMap: image.PixelMap | string | undefined = undefined; // pixelMap对象声明，用于图片显示

  build() {
    Column() {
      Button() {
        Text('select').fontSize(20).fontWeight(FontWeight.Bold)
      }.margin({ top: 20 }).width(100).height(40).onClick(() => this.showPicker())

      Image(this.pixelMap).width(300).height(300).margin({ top: 20 })
    }.width('100%').height('100%')
  }

  showPicker() { // 创建图库选择器对象实例
    const photoViewPicker = new photoAccessHelper.PhotoViewPicker(); // 图库选项配置，类型与数量
    let selectOptions = new photoAccessHelper.PhotoSelectOptions();
    selectOptions.MIMEType = photoAccessHelper.PhotoViewMIMETypes.VIDEO_TYPE;
    selectOptions.maxSelectNumber = 1; // 调用select()接口拉起图库界面进行文件选择,文件选择成功后，返回PhotoSelectResult结果集
    photoViewPicker.select(selectOptions).then((photoSelectResult: picker.PhotoSelectResult) => { // 用一个全局变量存储返回的uri
      if (photoSelectResult.photoUris?.[0]) {
        this.testFetchFrameByTime(photoSelectResult.photoUris[0]);
      }
    }).catch((err: BusinessError) => {
      console.error(`Invoke photoViewPicker.select failed, code is ${err.code}, message is ${err.message}`);
    })
  }

  async testFetchFrameByTime(filePath: string) { // 创建AVImageGenerator对象
    let avImageGenerator: media.AVImageGenerator = await media.createAVImageGenerator();
    let file = fs.openSync(filePath, fs.OpenMode.READ_ONLY);
    let avFileDescriptor: media.AVFileDescriptor = { fd: file.fd };
    avImageGenerator.fdSrc = avFileDescriptor; // 初始化入参
    let timeUs = 0;
    let queryOption = media.AVImageQueryOptions.AV_IMAGE_QUERY_NEXT_SYNC;
    let param: media.PixelMapParams = {
      width: 300, height: 400,
    } // 获取缩略图（promise模式）
    this.pixelMap = await avImageGenerator.fetchFrameByTime(timeUs, queryOption, param); // 释放资源（promise模式）
    avImageGenerator.release();
    console.info(`release success.`);
    fs.closeSync(file);
  }
}