Optimizing Build Sizes: Tree Shaking and Deferred Components in Flutter

Tree Shaking in Flutter

Flutter’s tree shaking runs automatically during a release build. It removes unreachable Dart code and unused font glyphs. To trigger tree shaking, compile with flutter build --release (or --obfuscate --split-debug-info). The compiler performs a global analysis, discarding methods, classes, and assets not referenced by your entry points or deferred imports.

Key points:

• Unused Dart classes and functions vanish in release mode.

• Dead CSS and JS assets (for Flutter web) are also pruned.

• Fonts strip out unused glyphs when you specify font families in pubspec.yaml.

Example pubspec.yaml snippet for fonts:

fonts:
  - family: Roboto
    fonts:
      - asset

Include only the variants you need to reduce asset bloat.

Deferred Components for Dynamic Loads

Deferred components (Android App Bundles) let you split your app into install-time and on-demand modules. You define deferred libraries in Dart and map them to modules in Android Gradle.

In Dart, mark a library as deferred:

import 'package:my_app/heavy_feature.dart' deferred as heavy;

Future<void> loadHeavyFeature() async {
  await heavy.loadLibrary();
  heavy.openHeavyFeature();
}

In android/app/src/main/AndroidManifest.xml, declare modules:

<dist:module dist:instant="false" dist:title="HeavyFeature">
  <dist:fusing dist:include="true" />
</dist:module>

When the user invokes loadHeavyFeature(), the Flutter engine downloads and installs that module, keeping the base APK smaller.

Profiling and Measuring Build Size

Before and after optimizations, measure build size with flutter build --analyze-size. This generates a JSON analysis you can explore via DevTools:

1. Run: flutter build apk --release --analyze-size

2. Locate flutter_app_size_*.json in build/flutter_analysis.

3. Run: flutter pub global activate devtools && flutter pub global run devtools

4. Open the JSON in DevTools’ “App Size” tab.

Key metrics:

• Code size per package or library

• Asset contribution

• Deferred module breakdown

Compare builds to quantify savings from tree shaking and deferred loading.

Combining Optimization Techniques

Tree shaking and deferred components are complementary. Use tree shaking as a baseline for all release builds. Identify heavy screens, large asset sets, or seldom-used features and convert them into deferred modules. For Flutter web, use deferred imports for optional widgets or admin dashboards. On desktop, consider code splitting via Dart zones or plugin separation.

Workflow:

1. Profile current build size.

2. Remove or refactor dead code.

3. Identify heavy features; convert to deferred modules.

4. Re-profile and iterate.

Best Practices for Minimizing Build Size

• Depend only on necessary packages. Avoid monolithic libraries.

• Use const constructors and final fields to help the compiler optimize.

• Limit asset folders to required images and fonts. Compress or vectorize where possible.

• For web builds, enable --dart2js-optimization with --minify.

• Obfuscate and split debug info to reduce APK size and safeguard your code: flutter build apk --obfuscate --split-debug-info=/.

• Automate size checks in CI: fail builds if size exceeds thresholds.

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Conclusion

Optimizing build sizes in Flutter hinges on effective tree shaking and strategic use of deferred components. Combine these with rigorous profiling and best practices around dependencies and assets. The result is a faster download, lower memory footprint, and improved user experience across devices.