[Feature Request] Support for transferable objects in web workers (postMessage transfer list)

[kartikey321]

Problem

When using isolate_manager on web platforms, large binary data (e.g., Uint8List, ArrayBuffer) is copied during postMessage, causing significant performance issues for data-intensive applications like file uploads/downloads.

Performance Impact:

• 5MB chunk transfer: ~40-50ms (with copy) vs <2ms (with transfer)
• Memory overhead: 2x data size during active transfers
• Affects concurrent operations with multiple workers

Proposed Solution

Add support for JavaScript's transferable objects by accepting an optional transfer parameter in web worker communication, enabling zero-copy transfers.

Example API

Current behavior (copies data):

final data = Uint8List(5 * 1024 * 1024); // 5MB
isolateManager.compute(myFunction, data); // Data is copied

Proposed behavior (zero-copy transfer):

final data = Uint8List(5 * 1024 * 1024); // 5MB
isolateManager.compute(
  myFunction, 
  data,
  transfer: [data.buffer], // Data ownership transferred
);
// data.buffer is now detached (neutered)

Implementation Suggestion

Modify IsolateContactorControllerImplWorker.sendIsolate() in isolate_contactor_controller_web_worker.dart:

@override
void sendIsolate(P message, {List<JSAny>? transfer}) {
  final jsMessage = message is ImType 
    ? message.unwrap.jsify() 
    : message.jsify();
    
  if (transfer != null && transfer.isNotEmpty) {
    _delegate.postMessage(jsMessage, transfer.toJS);
  } else {
    _delegate.postMessage(jsMessage);
  }
}

Benefits

• 50-95% faster for large binary data transfers
• 50% memory reduction for concurrent operations (data exists in one context only)
• Semantic alignment with native TransferableTypedData on Dart VM
• Standard web API - follows JavaScript Web Workers spec

Use Cases

1. File processing: Multipart uploads/downloads with 5MB+ chunks
2. Image processing: Transferring decoded image data between workers
3. Cryptographic operations: Zero-copy for hashing/encryption of large buffers
4. Video/audio streaming: Real-time processing pipelines

Current Workaround

We can use direct package:web Worker API with manual transfer lists, but native isolate_manager integration would benefit the entire ecosystem and maintain platform abstraction.

Additional Context

• Related: MDN - Transferable Objects
• Transferable types: ArrayBuffer, MessagePort, ImageBitmap, OffscreenCanvas
• After transfer, original object becomes detached (cannot be used)

Would be happy to contribute a PR if this aligns with the project's roadmap!

[lamnhan066]

The idea looks really promising. A PR is welcome and please also add tests that show the performance improvement.