npm install react-native-fs --save

react-native link react-native-fs

import RNFS from 'react-native-fs';

/*下載文件*/
  downloadFile() {
    // On Android, use "RNFS.DocumentDirectoryPath" (MainBundlePath is not defined)

    // 圖片
    // const downloadDest = `${RNFS.MainBundlePath}/${((Math.random() * 1000) | 0)}.jpg`;
    // const formUrl = 'http://img.kaiyanapp.com/c7b46c492261a7c19fa880802afe93b3.png?imageMogr2/quality/60/format/jpg';

    // 文件
    // const downloadDest = `${RNFS.MainBundlePath}/${((Math.random() * 1000) | 0)}.zip`;
    // const formUrl = 'http://files.cnblogs.com/zhuqil/UIWebViewDemo.zip';

    // 視頻
    // const downloadDest = `${RNFS.MainBundlePath}/${((Math.random() * 1000) | 0)}.mp4`;
    // http://gslb.miaopai.com/stream/SnY~bbkqbi2uLEBMXHxGqnNKqyiG9ub8.mp4?vend=miaopai&
    // https://gslb.miaopai.com/stream/BNaEYOL-tEwSrAiYBnPDR03dDlFavoWD.mp4?vend=miaopai&
    // const formUrl = 'https://gslb.miaopai.com/stream/9Q5ADAp2v5NHtQIeQT7t461VkNPxvC2T.mp4?vend=miaopai&';

    // 音頻
    const downloadDest = `${RNFS.MainBundlePath}/${((Math.random() * 1000) | 0)}.mp3`;
    // http://wvoice.spriteapp.cn/voice/2015/0902/55e6fc6e4f7b9.mp3
    const formUrl = 'http://wvoice.spriteapp.cn/voice/2015/0818/55d2248309b09.mp3';

    const options = {
      fromUrl: formUrl,
      toFile: downloadDest,
      background: true,
      begin: (res) => {
        console.log('begin', res);
        console.log('contentLength:', res.contentLength / 1024 / 1024, 'M');
      },
      progress: (res) => {

        let pro = res.bytesWritten / res.contentLength;

        this.setState({
          progressNum: pro,
        });
      }
    };
    try {
      const ret = RNFS.downloadFile(options);
      ret.promise.then(res => {
        console.log('success', res);

        console.log('file://' + downloadDest)

      }).catch(err => {
        console.log('err', err);
      });
    }
    catch (e) {
      console.log(error);
    }

  }

 /*將文本寫入本地 txt*/
  writeFile() {
    // create a path you want to write to
    const path = RNFS.MainBundlePath + '/test.txt';

    // write the file
    RNFS.writeFile(path, '這是一段文本，YES', 'utf8')
      .then((success) => {
        console.log('path', path);
      })
      .catch((err) => {
        console.log(err.message);
      });
  }

/*讀取txt文件內容*/
  readFile() {
    // create a path you want to delete
    const path = RNFS.MainBundlePath + '/test.txt';

    return RNFS.readFile(path)
      .then((result) => {
        console.log(result);

        this.setState({
          readTxtResult: result,
        })
      })
      .catch((err) => {
        console.log(err.message);

      });
  }

/*在已有的txt上添加新的文本*/
  appendFile() {
    const path = RNFS.MainBundlePath + '/test.txt';

    return RNFS.appendFile(path, '新添加的文本', 'utf8')
      .then((success) => {
        console.log('success');
      })
      .catch((err) => {
        console.log(err.message);

      });
  }

  /*刪除文件*/
  deleteFile() {
    // create a path you want to delete
    const path = RNFS.MainBundlePath + '/test.txt';

    return RNFS.unlink(path)
      .then(() => {
        console.log('FILE DELETED');
      })
      // `unlink` will throw an error, if the item to unlink does not exist
      .catch((err) => {
        console.log(err.message);
      });
  }

 /*上傳文件 only iOS*/
  uploadFile() {
    const uploadSrc = `${RNFS.MainBundlePath}/test.txt`;

    const uploadUrl = 'http://buz.co/rnfs/upload-tester.php';

    const options = {
      toUrl: uploadUrl,
      files: [{name: 'myfile', filename: 'test.txt', filepath: uploadSrc, filetype: 'text/plain'}],
      background: true,
      method: 'POST', // PUT
      begin: (res) => {
        console.log('begin', res);
      },
      progress: (res) => {
        console.log('progress', res);
      }
    };

    const ret = RNFS.uploadFiles(options);

    return ret.promise.then(res => {
      const response = JSON.parse(res.body);
      console.log(response);

    })
      .catch(err => {
        console.log('err', err);
      });
  }