• flutter_downloader 此库更新极慢，所以问题及多，可以看文档

  dependencies:
  flutter_downloader: ^1.3.1

Java:

// MyApplication.java
package com.example.flutter_demo;  // 设置你的

import io.flutter.app.FlutterApplication;
import io.flutter.plugin.common.PluginRegistry;
import io.flutter.plugins.GeneratedPluginRegistrant;

public class MyApplication extends FlutterApplication implements PluginRegistry.PluginRegistrantCallback {
    @Override
    public void registerWith(PluginRegistry registry) {
        GeneratedPluginRegistrant.registerWith(registry);
    }
}

Or Kotlin:

// MyApplication.kt
package com.example.flutter_demo // 设置你的

import io.flutter.app.FlutterApplication
import io.flutter.plugin.common.PluginRegistry;
import io.flutter.plugins.GeneratedPluginRegistrant

internal class MyApplication : FlutterApplication(), PluginRegistry.PluginRegistrantCallback {
    override fun registerWith(registry: PluginRegistry) {
        GeneratedPluginRegistrant.registerWith(registry)
    }
}


<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.flutter_demo">

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

    <!-- io.flutter.app.FlutterApplication is an android.app.Application that
         calls FlutterMain.startInitialization(this); in its onCreate method.
         In most cases you can leave this as-is, but you if you want to provide
         additional functionality it is fine to subclass or reimplement
         FlutterApplication and put your custom class here. -->

         <!-- 更新  android:name -->
    <application
        android:name=".MyApplication"
        android:label="flutter_demo"
        android:icon="@mipmap/ic_launcher">
        <activity
            android:name=".MainActivity"
            android:launchMode="singleTop"
            android:theme="@style/LaunchTheme"
            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
            android:hardwareAccelerated="true"
            android:windowSoftInputMode="adjustResize">
            <!-- This keeps the window background of the activity showing
                 until Flutter renders its first frame. It can be removed if
                 there is no splash screen (such as the default splash screen
                 defined in @style/LaunchTheme). -->
            <meta-data
                android:name="io.flutter.app.android.SplashScreenUntilFirstFrame"
                android:value="true" />
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>

        <!--   flutter_downloader     new-->
        <provider
            android:name="vn.hunghd.flutterdownloader.DownloadedFileProvider"
            android:authorities="${applicationId}.flutter_downloader.provider"
            android:exported="false"
            android:grantUriPermissions="true">
            <meta-data
                android:name="android.support.FILE_PROVIDER_PATHS"
                android:resource="@xml/provider_paths"/>
        </provider>

        <provider
            android:name="androidx.work.impl.WorkManagerInitializer"
            android:authorities="${applicationId}.workmanager-init"
            android:enabled="false"
            android:exported="false" />

        <provider
            android:name="vn.hunghd.flutterdownloader.FlutterDownloaderInitializer"
            android:authorities="${applicationId}.flutter-downloader-init"
            android:exported="false">
            <!-- changes this number to configure the maximum number of concurrent tasks -->
            <meta-data
                android:name="vn.hunghd.flutterdownloader.MAX_CONCURRENT_TASKS"
                android:value="5" />
        </provider>
        <!--        new -->
    </application>
</manifest>


// main.dart
import 'dart:io';
import 'dart:isolate';
import 'dart:ui';

import 'package:flutter/material.dart';
import 'package:flutter_downloader/flutter_downloader.dart';
import 'package:path_provider/path_provider.dart';
import 'package:permission_handler/permission_handler.dart';

void main() async {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  String downloadId;
  String _link = "https://i.loli.net/2019/10/18/gE3PTycd9SF5VqY.jpg";
  String _localPath;

  ReceivePort _port = ReceivePort();

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

  Future<void> _init() async {
    await FlutterDownloader.initialize();
    IsolateNameServer.registerPortWithName(
        _port.sendPort, 'downloader_send_port');
    _port.listen((dynamic data) {
      print('UI Isolate Callback: $data');
      String id = data[0];
      DownloadTaskStatus status = data[1];
      int progress = data[2];

      print("status: $status");
      print("progress: $progress");
      print("id == downloadId: ${id == downloadId}");
    });
    FlutterDownloader.registerCallback(downloadCallback);

    _localPath = (await _findLocalPath()) + '/Download';
    final savedDir = Directory(_localPath);
    bool hasExisted = await savedDir.exists();
    if (!hasExisted) {
      savedDir.create();
    }
  }

  static void downloadCallback(
      String id, DownloadTaskStatus status, int progress) {
    print(
        'Background Isolate Callback: task ($id) is in status ($status) and process ($progress)');
    final SendPort send =
        IsolateNameServer.lookupPortByName('downloader_send_port');
    send.send([id, status, progress]);
  }

  Future<String> _findLocalPath() async {
    final directory = await getExternalStorageDirectory();
    return directory.path;
  }

  Future<bool> _checkPermission() async {
    if (Theme.of(context).platform == TargetPlatform.android) {
      PermissionStatus permission = await PermissionHandler()
          .checkPermissionStatus(PermissionGroup.storage);
      if (permission != PermissionStatus.granted) {
        Map<PermissionGroup, PermissionStatus> permissions =
            await PermissionHandler()
                .requestPermissions([PermissionGroup.storage]);
        if (permissions[PermissionGroup.storage] == PermissionStatus.granted) {
          return true;
        }
      } else {
        return true;
      }
    } else {
      return true;
    }
    return false;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home Page'),
      ),
      body: Center(
        child: Text('home page'),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.arrow_downward),
        onPressed: () async {
          if (await _checkPermission()) {
            final taskId = await FlutterDownloader.enqueue(
              url: _link,
              savedDir: _localPath,
              showNotification:
                  true, // show download progress in status bar (for Android)
              openFileFromNotification:
                  true, // click on notification to open downloaded file (for Android)
            );
            downloadId = taskId;
          }
        },
      ),
    );
  }
}



<resources>
<string name="flutter_downloader_notification_started">下载开始</string>
<string name="flutter_downloader_notification_in_progress">正在下载</string>
<string name="flutter_downloader_notification_canceled">下载取消</string>
<string name="flutter_downloader_notification_failed">下载错误</string>
<string name="flutter_downloader_notification_complete">下载完成</string>
<string name="flutter_downloader_notification_paused">下载暂停</string>
</resources>