Designing On-Demand Feature Modules in Flutter

Introduction

Building large-scale mobile applications often demands breaking functionality into independent, on-demand modules. Flutter’s single-binary model may seem at odds with a modular architecture, but by leveraging deferred imports and platform-specific feature delivery, you can design on-demand feature modules that load only when needed. This tutorial covers structuring your codebase, integrating Android Dynamic Feature Modules, managing dependencies, and testing for seamless, performant mobile development.

Defining Modular Boundaries

The first step in designing on-demand feature modules is clearly defining boundaries. Each feature module should encapsulate its UI, business logic, and any native bridges it needs. In Flutter, you can isolate a feature into a separate Dart package within the same repository. For example:

• Create a packages/feature_x directory with its own pubspec.yaml.

• Expose only the public API (widgets, services) needed by the host app.

• Avoid direct imports from other features; communicate via contracts or events.

This approach enforces clear separation, lets multiple teams work in parallel, and keeps compile times manageable during mobile development iterations.

Implementing Deferred Components

Flutter supports deferred loading at the Dart level with deferred as imports, and on Android you can integrate with Play Feature Delivery. Start by tagging libraries for deferred import:

import 'package:feature_x/feature_x.dart' deferred as feature_x;

Future<void> loadFeatureX(BuildContext context) async {
  await feature_x.loadLibrary();
  Navigator.push(
    context,
    MaterialPageRoute(builder: (_) => feature_x.FeatureXScreen()),
  );
}

On the Android side, add a dynamic feature module in settings.gradle and configure build.gradle for Play Feature Delivery: specify onDemand true, list asset packs, and ensure your feature’s Dart entry is compiled into a separate ABI split. The host APK remains lean, while feature_x is fetched when loadFeatureX is invoked.

Managing Dependencies and Communication

Within each feature, manage dependencies locally and avoid global singletons leaking across modules. Use a service locator or dependency injection to register feature-specific blocs or services:

class FeatureXModule {
  static void register() {
    GetIt.I.registerFactory<FeatureXBloc>(() => FeatureXBloc());
  }
}

// In host app initialization:
FeatureXModule.register();

For inter-module communication, define interfaces or use an event bus. The host app can pass parameters or callbacks when pushing a route. For example, wrap results in a typed callback:

typedef FeatureResult = String;

Navigator.push<FeatureResult>(
  context,
  MaterialPageRoute(builder: (_) => feature_x.FeatureXScreen()),
).then((result) {
  if (result != null) handleFeatureResult(result);
});

This ensures loose coupling and testable interactions across on-demand modules in your mobile development pipeline.

Testing and Deployment

Testing on-demand modules requires simulating dynamic delivery. For unit tests, import feature packages directly and mock platform channels. For integration tests on Android, use the gradlew bundlePlayRelease task with the Play Core plugin to install on-demand modules locally:

• Run adb shell pm install-create and adb shell pm install-write commands to push the base and dynamic modules.
• Automate installation in CI to validate feature loading and performance.

On iOS, since the App Store lacks true on-demand installation, bundle feature modules and guard them behind runtime flags. This keeps code paths consistent but does not reduce binary size.

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Conclusion

Designing on-demand feature modules in Flutter blends Dart’s deferred loading with platform-specific delivery. By defining modular boundaries, implementing deferred imports, managing dependencies cleanly, and setting up realistic tests, you can ship lean host binaries and load features only when required. This approach scales large mobile development teams, improves startup performance, and adapts to evolving user needs without monolithic rebuilds.