Part 6: Flutter part

FRB Template App: Applying the modified Workflow.

Flutter instructions

Create helper file

Let's make a new file in the lib/ directory called ffi.dart. This file will help us load the library into our code. Here's what you should put inside the file:

// This file initializes the dynamic library and connects it with the stub
// generated by flutter_rust_bridge_codegen.

import 'dart:ffi';

import 'bridge_generated.dart';
import 'bridge_definitions.dart';
export 'bridge_definitions.dart';

// Re-export the bridge so it is only necessary to import this file.
export 'bridge_generated.dart';
import 'dart:io' as io;

const _base = 'rust';

// On MacOS, the dynamic library is not bundled with the binary,
// but rather directly **linked** against the binary.
final _dylib = io.Platform.isWindows ? '$_base.dll' : 'lib$';

final Rust api = RustImpl(io.Platform.isIOS || io.Platform.isMacOS
    ? DynamicLibrary.executable()

Create your main.dart file

This is where your Flutter app starts its journey! Usually, you'd have to write all the code yourself, but in this case, simply replace the entire content of the file with:

import 'package:flutter/material.dart';
import 'ffi.dart';

void main() {
  runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
      home: const MyHomePage(title: 'Flutter Demo Home Page'),

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  State<MyHomePage> createState() => _MyHomePageState();

class _MyHomePageState extends State<MyHomePage> {
  // These futures belong to the state and are only initialized once,
  // in the initState method.
  late Future<Platform> platform;
  late Future<bool> isRelease;

  void initState() {
    platform = api.platform();
    isRelease = api.rustReleaseMode();

  Widget build(BuildContext context) {
    // This method is rerun every time setState is called.
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the method, and use it to set our appbar title.
        title: Text(widget.title),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          children: <Widget>[
            const Text("You're running on"),
            // To render the results of a Future, a FutureBuilder is used which
            // turns a Future into an AsyncSnapshot, which can be used to
            // extract the error state, the loading state and the data if
            // available.
            // Here, the generic type that the FutureBuilder manages is
            // explicitly named, because if omitted the snapshot will have the
            // type of AsyncSnapshot<Object?>.
              // We await two unrelated futures here, so the type has to be
              // List<dynamic>.
              future: Future.wait([platform, isRelease]),
              builder: (context, snap) {
                final style = Theme.of(context).textTheme.headline4;
                if (snap.error != null) {
                  // An error has been encountered, so give an appropriate response and
                  // pass the error details to an unobstructive tooltip.
                  return Tooltip(
                    message: snap.error.toString(),
                    child: Text('Unknown OS', style: style),

                // Guard return here, the data is not ready yet.
                final data =;
                if (data == null) return const CircularProgressIndicator();

                // Finally, retrieve the data expected in the same order provided
                // to the FutureBuilder.future.
                final Platform platform = data[0];
                final release = data[1] ? 'Release' : 'Debug';
                final text = const {
                      Platform.Android: 'Android',
                      Platform.Ios: 'iOS',
                      Platform.MacApple: 'MacOS with Apple Silicon',
                      Platform.MacIntel: 'MacOS',
                      Platform.Windows: 'Windows',
                      Platform.Unix: 'Unix',
                      Platform.Wasm: 'the Web',
                    }[platform] ??
                    'Unknown OS';
                return Text('$text ($release)', style: style);