大家好，我按照RFFS npm指导原则使用它来实现
https://www.npmjs.com/package/react-native-fs
对于进度条，我使用了库
React Native Progress Circle
以下代码行的链接

downloadFile(options: DownloadFileOptions): { jobId: number, promise: Promise<DownloadResult> }

type DownloadFileOptions = {
   fromUrl: string;          // URL to download file from
   toFile: string;           // Local filesystem path to save the file to
  headers?: Headers;        // An object of headers to be passed to the server
  background?: boolean;     // Continue the download in the background after the app terminates (iOS only)
  discretionary?: boolean;  // Allow the OS to control the timing and speed of the download to improve perceived performance  (iOS only)
  cacheable?: boolean;      // Whether the download can be stored in the shared NSURLCache (iOS only, defaults to true)
  progressDivider?: number;
  begin?: (res: DownloadBeginCallbackResult) => void;
  progress?: (res: DownloadProgressCallbackResult) => void;
  resumable?: () => void;    // only supported on iOS yet
  connectionTimeout?: number // only supported on Android yet
  readTimeout?: number       // supported on Android and iOS
};

在这里我实现了上面的代码行

RNFS.downloadFile({
     fromUrl: encodedfileURL,
     toFile: downloadfilePath,
     //headers
     background: true, **// Continue the download in the background after the app terminates (iOS only)**
     discretionary: true, **// Allow the OS to control the timing and speed of the download to improve perceived performance  (iOS only)**
     cacheable: true, **// Whether the download can be stored in the shared NSURLCache (iOS only, defaults to true)**
  
     begin: (res: DownloadBeginCallbackResult) => {
       console.log("Response begin ===\n\n");
       console.log(res);
     },
     progress: (res: DownloadProgressCallbackResult) => {
      //here you can calculate your progress for file download

       console.log("Response written ===\n\n");
       let progressPercent = (res.bytesWritten / res.contentLength)*100; // to calculate in percentage
       console.log("\n\nprogress===",progressPercent)
       this.setState({ progress: progressPercent.toString() });
       item.downloadProgress = progressPercent;
       console.log(res);
     }
   })
     .promise.then(res => {
       console.log("res for saving file===", res);
       return RNFS.readFile(downloadfilePath, "base64");
})

RFFS在下载完成时提供回调，请尝试以下操作：

RNFS.downloadFile(DownloadFileOptions).promise.then((r) => {
               //call another function here
  });

您需要在DownloadFileOptions中将其作为进度选项传递：

progress: (data) => this._downloadFileProgress(data)

实际上，react-native-fs库中有一个bug。当你传递progress函数并想使用下载进度在UI上显示一些东西时，你也必须传递begin函数。
我说这是一个bug，因为react-native-fs开发人员可以检查它，如果消费者开发人员传递了progress而没有传递begin，他们可以传递noop函数作为默认属性。

const { jobId, promise } = FileSystem.downloadFile({
  fromUrl: resource,
  toFile: `${DOWNLOAD_DIR}/${utils.nameMaker()}.mp3`,
  background: true,
  discretionary: true,
  cacheable: true,
  begin: noop,  // <===== this line I meant
  progress: onProgress,
});

您可以阅读此处了解更多信息