let res = fs.accessSync(filePath);
if (res) {
  //字体注册
  font.registerFont({
    familyName: $r('app.string.font_SourceHanSerif'),
    familySrc: DOWNLOADHEADER + filePath
  })
  this.targetFont = SOURCEHANSERIF;
  return;
}

try {
  // TODO ：知识点：下载网络文件
  request.downloadFile(getContext(), {
    url: URL,
    filePath: filePath
  }, (err, downloadTask) => {
    if (err) {
      logger.error('Failed to request the download. Cause: ' + JSON.stringify(err));
      return;
    }
    // 监听下载进度，赋值到状态变量
    downloadTask.on('progress', (receivedSize, totalSize) => {
      this.progressFlag = true;
      this.downloadNow = receivedSize;
      this.downloadTotal = totalSize;
      logger.info("Download receivedSize:" + receivedSize + " totalSize:" + totalSize);
    });

    // 监听下载完成事件
    downloadTask.on('complete', () => {
      this.progressFlag = false;
      this.downloadButtonText = SOURCE_HAN_SERIF;
      this.downloadButtonEnabled = true;
      try {
        promptAction.showToast({
          message: $r('app.string.toast_message'),
          duration: TOAST_DURATION
        });
      } catch (error) {
        const message = (error as BusinessError).message;
        const code = (error as BusinessError).code;
        logger.error(`showToast args error code is ${code}, message is ${message}`);
      }
      downloadTask.off('progress');
      downloadTask.off('fail');
      font.registerFont({
        familyName: $r('app.string.font_SourceHanSerif'),
        familySrc: DOWNLOAD_HEADER + filePath
      })
      this.targetFont = SOURCE_HAN_SERIF;
    })

    // 监听下载失败事件
    downloadTask.on('fail', (err: number) => {
      logger.info("DownloadTask failed");
      let res = fs.accessSync(filePath);
      if (res) {
        fs.unlink(filePath);
        downloadTask.off('fail');
      }
    })
  });
} catch (err) {
  logger.info("Download failed with error message: " + err.message + ", error code: " + err.code);
}

fontdynamicregistration                             // har类型
|---src/main/ets/components
|   |---FontDynamicRegistration.ets                 // 动态切换字体页面及具体逻辑