Flutter Web 多部分formdata文件上传进度条

Posted 2023-02-23

【中文标题】Flutter Web 多部分formdata文件上传进度条

【英文标题】：Flutter Web multipart formdata file upload progress bar

【发布时间】：2021-05-18 12:46:12

【问题描述】：

我正在使用 Flutter web 和strapi headless cms 作为后端。我能够成功发送文件，但想要它的进度指示。后端限制：文件上传必须是多部分表单数据，可以是缓冲区或流。前端限制：Flutter web 无权访问系统文件目录；文件必须加载到内存中并使用其字节发送。

我可以使用flutter的http包或Dio包上传文件，但是在尝试以某种方式访问​​上传进度时遇到以下问题：

Http 示例代码：

http.StreamedResponse response;

final uri = Uri.parse(url);
final request = MultipartRequest(
  'POST',
  uri,
);
request.headers['authorization'] = 'Bearer $_token';
request.files.add(http.MultipartFile.fromBytes(
  'files',
  _fileToUpload.bytes,
  filename: _fileToUpload.name,
));

response = await request.send();

var resStream = await response.stream.bytesToString();
var resData = json.decode(resStream);

我尝试了什么： 当访问 onData 的 response.stream 时，它只在服务器发送完成的请求时做出响应（即使方法声明它应该获得一些进度指示）。

Dio包代码

Response response = await dio.post(url,
    data: formData,
    options: Options(
      headers: 
        'authorization': 'Bearer $_token',
      ,
    ), onSendProgress: (int sent, int total) 
  setState(() 
    pm.progress = (sent / total) * 100;
  );

问题：

该包似乎能够获得一些进度指示，但 Flutter web 的 Dio 包有一个尚未修复的错误：请求阻塞 ui 并且应用程序冻结直到上传完成。

【问题讨论】：

dio 问题仍未解决：github.com/flutterchina/dio/issues/961github.com/flutterchina/dio/issues/925

【参考方案1】：

你好，可以使用universal_html/html.dart包做进度条，步骤如下：

导入通用包

import 'package:universal_html/html.dart' as html;

从 html 输入元素中选择文件，而不是使用文件选择器包

 _selectFile() 
    html.FileUploadInputElement uploadInput = html.FileUploadInputElement();
    uploadInput.multiple = false;
    uploadInput.accept = '.png,.jpg,.glb';
    uploadInput.click();

    uploadInput.onChange.listen((e) 
      _file = uploadInput.files.first;
    );
 

在 web 文件夹中创建 upload_worker.js，我的示例是上传到 S3 post presigned url

self.addEventListener('message', async (event) => 
    var file = event.data.file;
    var url = event.data.url;
    var postData = event.data.postData;
    uploadFile(file, url, postData);
);

function uploadFile(file, url, presignedPostData) 

    var xhr = new XMLHttpRequest();
    var formData = new FormData();

    Object.keys(presignedPostData).forEach((key) => 
        formData.append(key, presignedPostData[key]);
    );
    formData.append('Content-Type', file.type);
    // var uploadPercent;
    formData.append('file', file);

    xhr.upload.addEventListener("progress", function (e) 
        if (e.lengthComputable) 
            console.log(e.loaded + "/" + e.total);
            // pass progress bar status to flutter widget
            postMessage(e.loaded/e.total);
        
    );

    xhr.onreadystatechange = function () 
        if (xhr.readyState == XMLHttpRequest.DONE) 
            // postMessage("done");
        
    
    xhr.onerror = function () 
        console.log('Request failed');
        // only triggers if the request couldn't be made at all
        // postMessage("Request failed");
    ;

    xhr.open('POST', url, true);
    xhr.send(formData);

Flutter web call upload worker 上传和监听进度条状态

class Upload extends StatefulWidget 
  @override
  _UploadState createState() => _UploadState();


class _UploadState extends State<Upload> 
  html.Worker myWorker;
  html.File file;

  _uploadFile() async 
    String _uri = "/upload";
    final postData = ;

    myWorker.postMessage("file": file, "uri": _uri, postData: postData);
  

  _selectFile() 
    html.InputElement uploadInput = html.FileUploadInputElement();
    uploadInput.multiple = false;
    uploadInput.click();

    uploadInput.onChange.listen((e) 
      file = uploadInput.files.first;
    );
  

  @override
  void initState() 
    myWorker = new html.Worker('upload_worker.js');
    myWorker.onMessage.listen((e) 
      setState(() 
        //progressbar,...
      );
    );

    super.initState();
  

  @override
  Widget build(BuildContext context) 
    return Column(
      children: [
        RaisedButton(
          onPressed: _selectFile,
          child: Text("Select File"),
        ),
        RaisedButton(
          onPressed: _uploadFile,
          child: Text("Upload"),
        ),
      ],
    );
  

就是这样，希望对你有帮助。