wasm_tools

Tools to compile Dart to the WebAssembly component model.

Warning

These tools should be considered a proof of concept. They perform very poorly due to a lack of GC support in the Component Model. A much better way to embed dart2wasm into existing applications is to use a custom embedder.

Installation

This package should be installed as a dev-dependency: dart pub add --dev wasm_tools

Setup

To compile a Dart app as a WebAssembly component, we first need a world.wit file defining imports and exports for the component. Let's use this as a simple example:

// test.wit
package demo:component;

world root {
  export greeting;
}

interface greeting {
  generate-greeting: func() -> string;
}

First, run dart run wasm_tools witgen -i test.wit. In lib/src/components (the directory can be changed with -o), this generates:

  • demo_component.dart, a Dart interface describing greeting.
  • demo_component_root.dart, bindings to bridge between the Dart interface and the low-level component ABI.
  • demo_component_root.json, additional metadata for the compiler.

Compiling a component

With the bridge and ABI file generated, the WIT world can be implemented in Dart. Create a bin/greeting.dart with these contents:

// bin/greeting.dart
import 'package:greeting/src/components/demo_component.dart';
import 'package:greeting/src/components/demo_component_root.dart';

void main() {
  rootComponent((_) => _Greeting());
}

final class const _Greeting() implements Greeting {
  @override
  String generateGreeting() {
    return 'Hello from Dart!';
  }
}

To inform the compiler about the ABI file, also create a hook/link.dart file containing:

// hook/link.dart
import 'dart:convert';
import 'dart:io';

import 'package:hooks/hooks.dart';
import 'package:wasm_tools/hooks.dart';

void main(List<String> args) => link(args, (input, output) async {
  if (input.config.buildWasmComponent) {
    final abi = input.packageRoot.resolve(
      'lib/src/components/demo_component_root.json',
    );

    output.dependencies.add(abi);
    output.assets.webAssemblyComponents.add(
      WasmComponentAsset(
        encoded: json.decode(
          File(abi.toFilePath()).readAsStringSync(),
        ) as Map<String, Object?>,
      ),
    );
  }
});

With everything in place, it's time to compile Dart into a WebAssembly component:

dart run wasm_tools compile bin/greeting.dart

This generates a bin/greeting.wasm, a WebAssembly component, which can be run with wasmtime:

wasmtime --invoke 'generate-greeting()' bin/greeting.wasm

Libraries

hooks