使用react-native-fs

react native(保存图片,下载图片)

### 插件使用
```javascript
yarn add react-native-fs
react-native link react-native-fs
```
### 封装保存图片方法
```javascript
/**
 * Created by vision on 2018/8/27.
 */

import React from 'react';
import { Platform, CameraRoll } from 'react-native';
import RNFS from 'react-native-fs';
import { Toast} from "teaset"


export function _Download(uri) {
  if (!uri) return null;
  return new Promise((resolve, reject) => {
    let dirs = Platform.OS === 'ios' ? RNFS.LibraryDirectoryPath : RNFS.ExternalDirectoryPath; //外部文件，共享目录的绝对路径（仅限android）
    const downloadDest = `${dirs}/${((Math.random() * 10000000) | 0)}.jpg`;
    const formUrl = uri;
    const options = {
      fromUrl: formUrl,
      toFile: downloadDest,
      background: true,
      begin: (res) => {
        console.log('begin', res);
        console.log('contentLength:', res.contentLength / 1024 / 1024, 'M');
      },
    };
    try {
      const ret = RNFS.downloadFile(options);
      ret.promise.then(res => {
        console.log('success', res);
        console.log('file://' + downloadDest)
        var promise = CameraRoll.saveToCameraRoll(downloadDest);
        promise.then(function(result) {
          // alert('保存成功！地址如下：\n' + result);
          Toast.success('保存成功!')
        }).catch(function(error) {
          console.log('error', error);
          // alert('保存失败！\n' + error);
          Toast.info('保存失败!')
        });
        resolve(res);
      }).catch(err => {
        reject(new Error(err))
      });
    } catch (e) {
      reject(new Error(e))
    }

  })

}

```