Flutter Web 多部分表单数据文件上传进度条

如何解决Flutter Web 多部分表单数据文件上传进度条

我使用 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，无论是缓冲区还是流。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);

我的尝试： 当访问 onData 的 response.stream 时，它仅在服务器发送完成的请求时响应（即使方法声明它应该获得一些进度指示）。

Dio 封装代码

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：

似乎该包能够获得一些进度指示，但是用于 Flutter web 的 Dio 包有一个尚未修复的错误：请求会阻止 ui，并且应用程序在上传完成之前会冻结。

解决方法

您好，可以使用universal_html/html.dart包来做进度条，步骤如下：

1. 导入通用包

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

2. 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;
    });
 }

3. 将upload_worker.js创建到web文件夹中，我的例子是上传到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,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',true);
    xhr.send(formData);
}

4. 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，希望能帮到你。