添加一些必需的依赖项，我有一个测试演示与android只。请审查file_utils和path_provider，而你在IOS设备实现。

dio: ^3.0.0

  path_provider: ^1.3.0

  simple_permissions: ^0.1.9

  file_utils: ^0.1.3

• • 注意：**simple_permissions已断开连接，请使用此依赖关系permission_handler

您需要在android manifest文件上添加权限。

<uses-permission android:name="android.permission.INTERNET"/>
  <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
  <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

样本代码：

import 'dart:io';
  import 'package:flutter/material.dart';
  import 'package:dio/dio.dart';
  import 'package:path_provider/path_provider.dart';
  import 'dart:async';
  import 'package:simple_permissions/simple_permissions.dart';
  import 'package:file_utils/file_utils.dart';
  import 'dart:math';

  void main() => runApp(Downloader());

  class Downloader extends StatelessWidget {
    @override
    Widget build(BuildContext context) => MaterialApp(
          title: "File Downloader",
          debugShowCheckedModeBanner: false,
          home: FileDownloader(),
          theme: ThemeData(primarySwatch: Colors.blue),
        );
  }

  class FileDownloader extends StatefulWidget {
    @override
    _FileDownloaderState createState() => _FileDownloaderState();
  }

  class _FileDownloaderState extends State<FileDownloader> {

    final imgUrl = "https://images6.alphacoders.com/683/thumb-1920-683023.jpg";
    bool downloading = false;
    var progress = "";
    var path = "No Data";
    var platformVersion = "Unknown";
    Permission permission1 = Permission.WriteExternalStorage;
    var _onPressed;
    static final Random random = Random();
    Directory externalDir;

    @override
    void initState() {
      super.initState();
      downloadFile();
    }

    Future<void> downloadFile() async {
      Dio dio = Dio();
      bool checkPermission1 =
          await SimplePermissions.checkPermission(permission1);
      // print(checkPermission1);
      if (checkPermission1 == false) {
        await SimplePermissions.requestPermission(permission1);
        checkPermission1 = await SimplePermissions.checkPermission(permission1);
      }
      if (checkPermission1 == true) {
        String dirloc = "";
        if (Platform.isAndroid) {
          dirloc = "/sdcard/download/";
        } else {
          dirloc = (await getApplicationDocumentsDirectory()).path;
        }

        var randid = random.nextInt(10000);

        try {
          FileUtils.mkdir([dirloc]);
          await dio.download(imgUrl, dirloc + randid.toString() + ".jpg",
              onReceiveProgress: (receivedBytes, totalBytes) {
            setState(() {
              downloading = true;
              progress =
                  ((receivedBytes / totalBytes) * 100).toStringAsFixed(0) + "%";
            });
          });
        } catch (e) {
          print(e);
        }

        setState(() {
          downloading = false;
          progress = "Download Completed.";
          path = dirloc + randid.toString() + ".jpg";
        });
      } else {
        setState(() {
          progress = "Permission Denied!";
          _onPressed = () {
            downloadFile();
          };
        });
      }
    }

    @override
    Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
          title: Text('File Downloader'),
        ),
        body: Center(
            child: downloading
                ? Container(
                    height: 120.0,
                    width: 200.0,
                    child: Card(
                      color: Colors.black,
                      child: Column(
                        mainAxisAlignment: MainAxisAlignment.center,
                        children: <Widget>[
                          CircularProgressIndicator(),
                          SizedBox(
                            height: 10.0,
                          ),
                          Text(
                            'Downloading File: $progress',
                            style: TextStyle(color: Colors.white),
                          ),
                        ],
                      ),
                    ),
                  )
                : Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      Text(path),
                      MaterialButton(
                        child: Text('Request Permission Again.'),
                        onPressed: _onPressed,
                        disabledColor: Colors.blueGrey,
                        color: Colors.pink,
                        textColor: Colors.white,
                        height: 40.0,
                        minWidth: 100.0,
                      ),
                    ],
                  )));
  }

如果你控制服务器，另一个选择是让用户的系统来处理下载，在服务器上你添加一个Content-Disposition: attachment头到响应中，在nginx中你可以通过sites-enabled配置文件中的add_header指令来完成：

add_header Content-Disposition "attachment";

关于here的更多注解。
然后，在Flutter端，您可以使用url_launcher包启动下载：

await launch('https://www.example.com/my_file.pdf');

这将把下载移交给安装在手机上的浏览器。并且Content-Disposition: attachment头将告诉浏览器它将被下载而不是显示或播放。
这个解决方案非常简单，尽管它不能给予您控制文件下载到哪里。

使用flutter_downloader具有：

• 在android上下载通知完成
• 下载队列
• 下载状态
• 下载过程

在github上查看示例