wasm_tools 0.1.0-preview.0

Tools to create WebAssembly components from Dart.

wasm_tools

Tools to compile Dart to standalone WebAssembly targets, including the WebAssembly component model.

Caution

This functionality is highly experimental. It requires a Dart 3.13 SDK with this patch applied.

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. This will generate:

• lib/src/component.g.dart, a file to bridge between Dart types and the component model.
• hook/wasm_abi.json, containing a copy of the WIT file with 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/component.g.dart';

void main(List<String> arguments) {
  defineInstanceExport(unnamedExport0: const _Greeting());
}

final class _Greeting implements Greeting {
  const _Greeting();

  @override
  String generateGreeting() {
    return 'Hello from Dart!';
  }
}

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

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

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

void main(List<String> args) => build(args, (input, output) async {
  if (input.config.buildWasmComponent) {
    final abi = input.packageRoot.resolve('hook/wasm_abi.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 -W all-proposals=y --invoke 'generate-greeting()' bin/greeting.wasm

Note

Enabling Garbage Collection, Exceptions and Typed Function References is required to run Dart in wasmtime. These flags will be enabled by default in wasmtime version 46.0.