Integrating Flutter into an existing native app might sound like a complex decision, but in practice, it's a powerful way to modernize your UI, iterate faster, and bring consistency across platforms — all without rewriting your whole codebase.

In this post, we'll walk through how to use Flutter as a module inside native Android and iOS apps using Flutter Add-to-App. We'll also show how to use GoRouter for navigation and Pigeon to bridge native and Dart logic in a type-safe, scalable way.

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Why Add-to-App?

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Flutter is great, but not every team is ready for a full rewrite — and you shouldn’t have to be. With Add-to-App:

• You can incrementally adopt Flutter in high-impact areas
• Developers can experiment without affecting the native app core
• You can build new UI with Flutter’s speed and flexibility
• You get native performance and Flutter UI where it makes sense

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Setting up a Flutter Module

Start by creating a new Flutter module:

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flutter create --template module flutter_module

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This creates a structure optimized for embedding. Inside this module, we recommend:

• Using go_router to manage navigation cleanly
• Defining native-Dart interfaces using pigeon for clean, boilerplate-free platform channels

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Android Integration

Link the module in `settings.gradle`:

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include ':flutter'
setProjectDir(':flutter', file('../flutter_module'))

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Add it to `app/build.gradle`:

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dependencies {
  implementation project(':flutter')
}

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Start a Flutter screen from your activity:

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startActivity(
  FlutterActivity
    .withNewEngine()
    .initialRoute("/qr")
    .build(this)
)

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For production, consider caching a single engine instance to improve launch time.

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iOS Integration

In your iOS project’s `Podfile`:

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flutter_application_path = '../flutter_module'
load File.join(flutter_application_path, '.ios', 'Flutter', 'podhelper.rb')
install_all_flutter_pods(flutter_application_path)

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Then:

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pod install

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Launching Flutter from Swift:

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let flutterEngine = (UIApplication.shared.delegate as! AppDelegate).flutterEngine
let flutterVC = FlutterViewController(engine: flutterEngine, nibName: nil, bundle: nil)
flutterVC.setInitialRoute("/qr")
present(flutterVC, animated: true)

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4. Navigation with GoRouter

Inside your Flutter module:

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final router = GoRouter(
  routes: [
    GoRoute(
      path: '/qr',
      builder: (context, state) => const QrScannerScreen(),
    ),
  ],
);

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Then plug it into your `MaterialApp.router`:

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MaterialApp.router(
  routerConfig: router,
)

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This makes your screens deep-linkable and easy to coordinate from native.

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5. Communication with Pigeon

Pigeon helps you generate type-safe messaging code between Dart and platform code.

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Define your interface:

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@HostApi()
abstract class QrApi {
  String sendResult(String scannedCode);
}

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Generate code for both platforms:

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flutter pub run pigeon \
  --input pigeons/api.dart \
  --dart_out lib/pigeon/api.g.dart \
  --kotlin_out android/app/src/main/kotlin/com/yourcompany/QrApi.kt     
  --swift_out ios/Runner/QrApi.swift

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Implement in native code (e.g. Kotlin):

class QrApiImpl : QrApi {
    override fun sendResult(scannedCode: String): String {
        Log.d("QR", "Received: $scannedCode")
        return "Success"
    }
}

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Then register it:

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QrApi.setUp(flutterEngine.dartExecutor.binaryMessenger, QrApiImpl())

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Final Thoughts

Add-to-App isn’t just for transitional phases. It’s a powerful long-term architecture for hybrid teams and projects that need the best of both worlds.

With tools like Pigeon and GoRouter, integrating Flutter doesn’t just feel native — it is native. And once you're up and running, your team can start shipping faster with a consistent experience across platforms.

Whether you’re shipping a single screen or planning a larger migration, Flutter Add-to-App is ready when you are.

Pro tip: Reuse a single Flutter engine when possible and preload it early to improve performance on first navigation.

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Written by Juan Garcia da Rosa, Senior Engineer at Leenspace.

Happy hybrid coding! 🚀

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