[cross_file] [web] Implement openRead correctly.

[ditman]

The current implementation of XFile.openRead is not really streaming the contents of the enclosed Blob as a Stream. It just slices a cached blob and returning that slice, in one go.

People are chunking files by computing the chunks from the outside using the start/end variables, and then listening for the (only) event on the Stream. This is very inefficient, and even though it works, it is not what the Stream is supposed to do!

Users need to get a Stream from the slice they care about in the blob, and then read it until its completion. See an example of the dart:io implementation: _FileStream

cross_file needs a _BlobStream class for its web implementation.

[rajrushilmakkar]

I am trying to upload image to google drive on web
Sample code-

uploadFileToGoogleDriveWeb(XFile xFile) async {
    final String serviceAccount =
        await rootBundle.loadString('assets/ServiceAccount.json');
    final accountCredentials =
        ServiceAccountCredentials.fromJson(serviceAccount);
    AuthClient client = await clientViaServiceAccount(
        accountCredentials, ['https://www.googleapis.com/auth/drive']);
    var drive = ga.DriveApi(client);
    ga.File fileToUpload = ga.File();
    fileToUpload.parents = ["parent_id"];
    fileToUpload.name = p.basename(xFile.path);
    var response = await drive.files.create(
      fileToUpload,
      uploadMedia: ga.Media(xFile.openRead(), await xFile.length()),
    );
    return response.id;
  }

I received the xfile using image_picker
I am using googleapis package to upload the file to drive
It’s working fine on android/iOS but for web I get this error-
error: Expected a value of type 'StreamTransformer<Uint8List, String>', but got one of type 'Base64Encoder'
The error is in line

drive.files.create(
      fileToUpload,
      uploadMedia: ga.Media(xFile.openRead(), await xFile.length()),
    ) 

Please let me know if I am doing anything wrong or what's the issue here
Also on mobile devices, I actually used File from 'dart:io' instead of XFile, so file.openRead() didn't have any issue, that's why I assumed that the issue is with xFile.openRead()

[rajrushilmakkar]

Okay, so I actually needed a Stream<List < int > > instead of Stream
So this worked for me

Stream<List<int>> newStream = xFile.openRead().map((event) {
      //event is uint8list
      return List<int>.from(event);
    });

[ditman]

I have a very promising demo here:

https://dit-tests.web.app

The app allows selecting files of any size and stream their contents without impacting the memory usage of the browser greatly.

(It's based off of the PR linked above, and a small tweak to package file_selector.)

[rohintoncollins]

Do we have an ETA for when we think this PR may be merged?

[rohintoncollins]

Please see here for a partial workaround using the File System API: flutter/packages#5158 (comment)

[CosmicPangolin]

Do we have an ETA for when we think this PR may be merged?

Not one to harass on a package but also curious; I feel like I've patiently watched paint dry on this issue for years. I know Stuart is super busy with platform issues - maybe it makes sense to reassign the factories?