使用flutter_downloader package时出现异常

Question

I'm trying to use flutter_downloader package to download some files (images/pdf). There is a listView with ListTiles each containing a button to start downloading when clicked but this error occurs when scrolling the list view.

[ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: 'package:flutter_downloader/src/downloader.dart': Failed assertion: line 30 pos 12: ':_initialized'. FlutterDownloader.initialize() must be called only once!

//my code is like this:

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';

class DownloadFile extends StatefulWidget {

  DownloadFile({this.downloadUrl});
  final String downloadUrl;

  @override
  _DownloadFileState createState() => _DownloadFileState();
}

class _DownloadFileState extends State<DownloadFile> {
  String downloadId;
  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) {
      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
  void dispose() {
    super.dispose();
  }
  //---------------------------------------------------------------
  @override
  Widget build(BuildContext context) {
    return FlatButton(
      onPressed: () async {
        if (await _checkPermission()) {
          final taskId = await FlutterDownloader.enqueue(
            url: widget.downloadUrl,
            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;
        }
      },
      child: Text('Downloa File',style: TextStyle(color: Colors.teal),)
    );
  }
}

Answer 1

According to the Usage section in the flutter_downloader package and the error you are getting, you must call the FlutterDownloader.initialize not more than once.

You can do that in the main method of your application, just like so:

WidgetsFlutterBinding.ensureInitialized();
await FlutterDownloader.initialize();