webtransport

WebTransport (HTTP/3 over QUIC) for Flutter, with one portable client API that runs either on the browser's native WebTransport or on a Rust implementation of the protocol (wtransport) compiled into the app.

import 'package:webtransport/webtransport.dart';

final transport = createWebTransport(
  'https://example.com:4433/echo',
  WebTransportOptions(
    serverCertificateHashes: [WebTransportHash(value: digest)],
  ),
);
await transport.ready;

transport.datagrams.write(utf8.encode('ping'));       // unreliable, unordered
transport.datagrams.readable.listen(print);

final stream = await transport.createBidirectionalStream();
await stream.writable.write(utf8.encode('hello'));    // reliable, ordered
await stream.writable.close();
await for (final chunk in stream.readable.bytes) {
  print(chunk);
}

await transport.close(const WebTransportCloseInfo(closeCode: 0));

The type and method names mirror the W3C WebTransport API, so the same code runs unchanged everywhere. Nothing extra has to be imported or configured: createWebTransport selects the implementation at compile time.

configuration	backend
Flutter desktop / mobile	the Rust code asset, driven from a worker isolate
Flutter web (JS / Wasm)	the browser's own WebTransport
Dart VM (tests, servers)	the Rust code asset

Architecture

The binding is layered, and every layer has a single responsibility:

layer	file(s)	responsibility
L0 raw ABI	src/webtransport.h (generated), lib/webtransport_bindings_generated.dart (generated)	the wt_* C functions and wt_*_t opaque handles
L1 memory safety	lib/src/native/memory.dart	status → exception, string / header marshalling, wt_buffer_t ownership, copying an event out of its batch before it is freed. No other layer does pointer arithmetic or malloc.
L2 async plumbing	lib/src/native/runtime.dart	starts the tokio runtime, registers the single NativeCallable.listener, correlates completions with their callers, drains each event batch exactly once
L3 objects	lib/src/native/{transport,session,server}.dart, lib/src/web/transport.dart, lib/src/native/isolate_backend.dart	clients, sessions, streams, datagrams — one implementation per backend
L4 contract	lib/src/api.dart	the platform neutral WebTransport surface every backend implements
L5 facade	lib/webtransport.dart	the portable API, wired to a backend by a conditional export

Native-only functionality lives in a separate library, lib/webtransport_ffi.dart: the server side, the in-process backend, the worker isolate, NativeBuffer and the raw wt_* bindings. It reaches into dart:ffi, so it is not part of the web build.

Why it never blocks the UI isolate

The Rust side owns a multi-threaded tokio runtime: no exported function waits on the network, every asynchronous operation reports its outcome through one callback, and completions are coalesced into batches so a burst of traffic costs one isolate message instead of one per event.

On top of that, the portable API runs the whole native stack inside a worker isolate:

• the calling isolate never performs a native call, an allocation or a decode — it only exchanges messages;
• reads are pull based, so a slow consumer stops asking and QUIC flow control slows the peer down;
• stream chunks cross the isolate boundary without a copy: Dart isolates share the address space, so the worker sends the address of the native buffer it read into and the receiving isolate wraps that memory in a Uint8List that frees the handle when it is collected. QUIC → application, zero copies.

If the calling isolate already owns the runtime (for example because it started a WebTransportServer), createWebTransport transparently creates the session in-process instead: the native library exposes a single event sink, so it cannot be shared with a worker isolate. The interface is identical either way.

Layout

path	contents
rust/	the Rust crate implementing the C ABI (staticlib + cdylib + rlib)
rust/cbindgen.toml	cbindgen configuration — the header is generated from Rust
rust/rust-toolchain.toml	pinned toolchain and target list
src/webtransport.h	generated C header — the ABI contract
hook/build.dart	the Flutter build hook
lib/	the Dart layers described above
tool/generate_bindings.dart	regenerates the header and the Dart bindings
example/	Flutter app that runs a server and a client, on native and on web

Requirements

• Flutter / Dart SDK with native assets support (developed against Dart 3.13).
• rustup. rust/rust-toolchain.toml pins 1.99.0 and the supported targets; rustup fetches whatever is missing on first use.
• clang / libclang — only to regenerate the Dart bindings.

Building

cd example
flutter build windows      # or: flutter run -d windows
flutter build web          # the browser backend, no Rust involved

On native the hook compiles rust/ in release mode (fat LTO, codegen-units = 1) and Flutter bundles the result, so the product contains:

example/build/windows/x64/runner/Release/
├── webtransport_example.exe
├── flutter_windows.dll
└── webtransport.dll          <-- the code asset built by hook/build.dart

Regenerating the bindings

The Rust sources are the single source of truth; the C header is generated from them. Never edit src/webtransport.h or lib/webtransport_bindings_generated.dart by hand.

dart run tool/generate_bindings.dart

Server side (native only)

import 'package:webtransport/webtransport_ffi.dart';

final config = WebTransportServerConfig()
  ..identity = const WebTransportSelfSignedIdentity(['localhost', '127.0.0.1'])
  ..bindAddress = '127.0.0.1';
final server = await WebTransportServer.bind(config);
print(server.port);

await for (final request in server.requests) {
  final session = await request.accept();          // or request.reject(404)
  session.incomingBidirectionalStreams.listen((stream) async {
    await stream.writable.write(await stream.readable.toBytes());
    await stream.writable.close();
  });
}

server.certificateHashes is the digest a client passes as WebTransportOptions.serverCertificateHashes — the portable way to trust a self-signed server, exactly as a browser does.

Notes

• CARGO_HOME / RUSTUP_HOME. Flutter build hooks run with a filtered environment: hooks_runner forwards only a white-list of variables, which does not include the Rust toolchain homes. Left alone, rustup falls back to %USERPROFILE%\.rustup and downloads a second copy of the pinned toolchain into a location the developer never chose. hook/build.dart therefore resolves both variables — from the inherited environment, from hooks: user_defines: or from the Windows user profile — and re-executes itself once with them set. An explicit override always wins:

  hooks:
    user_defines:
      webtransport:
        cargo_home: D:\Devel\Rust\Cargo
        rustup_home: D:\Devel\Rust\Rustup
  

• TMP / TEMP. While the Rust build runs, the hook points TMP/TEMP at a directory inside its own output directory: the MSVC archiver used by ring creates a scratch file in %TEMP%, and on hosts where the inherited temp directory is not writable that surfaces as LNK1104: cannot open file '...\lnk{...}.tmp'.

• Crypto backend. The crate uses wtransport's ring backend, which needs neither CMake nor NASM, so the hook stays self-contained on every host.

• Platform differences. Dart has neither WritableStream nor ReadableStream: a send stream is an object with write/close/abort returning Futures, a receive stream is consumed through its bytes Stream or read(), and datagrams.readable is a Stream. maxDatagramSize on the native backend reports the QUIC limit, and the native WebTransportStats only fills in the round-trip time — the rest stays zero rather than being guessed.