theme_extensions_gen

A modular, annotation-based code generation system for scalable theming in Flutter using ThemeExtension.

Features

• Declarative @ThemeExtensionTemplate() interface for theme structure

• @ThemeExtensionImpl() for centralized theme value lists

• Auto-generated:

  • copyWith, lerp, equality, and mixins
  • BuildContext extensions
  • Merged ThemeExtension lists for ThemeData
  • debugFillProperties() if Diagnosticable is used

• Group-based output (light, dark, etc.)

• Fully configurable via build.yaml

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Before / After

Hand-written ThemeExtension	With @ThemeExtensionTemplate

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Install

To use theme_extensions_gen, you will need your typical build_runner code generator setup.

First, install the annotations and the generator by adding them to your project:

dart pub add theme_extensions_gen_annotations
dart pub add dev:theme_extensions_gen
dart pub add dev:build_runner

This adds:

• theme_extensions_gen_annotations, a package containing annotations for theme_extensions_gen
• theme_extensions_gen, the code generator
• build_runner, the tool to run code generators

Run the generator

To run the code generator, execute the following command:

dart run build_runner watch

Like most code generators, theme_extensions_gen requires importing the theme_extensions_gen_annotations package and including a part directive at the top of your file.

Thus, any file using theme_extensions_gen should begin like this:

import 'package:theme_extensions_gen_annotations/theme_extensions_gen_annotations.dart';

part 'file_name.g.dart';

Annotations

@ThemeExtensionTemplate()

Marks an abstract interface as a ThemeExtension template. The generator creates:

• A concrete class with a factory constructor
• copyWith, lerp, ==, hashCode
• debugFillProperties() if Diagnosticable is used in the mixins
• BuildContext extensions with accessors like context.themedButtonTheme

Example:

@ThemeExtensionTemplate()
abstract interface class ThemedButtonTheme extends ThemeExtension<ThemedButtonTheme>
    with _$ThemedButtonThemeMixin, Diagnosticable {
  const factory ThemedButtonTheme({
    required Decoration decoration,
    required TextStyle textStyle,
    required EdgeInsets padding,
  }) = _$ThemedButtonTheme;
}

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@ThemeExtensionImpl({ String? group })

Marks a List<ThemeExtension> as a named implementation group to be collected into a generated file.

• Lists without group are merged into a default file.
• Lists with group are merged into a separate file (e.g. for dark, light, etc.).

@ThemeExtensionImpl()
List<ThemeExtension> get someFeatureThemeExtensions =>
    const [
      ThemedCardTheme(/* ... */),
      ThemedButtonTheme(/* ... */),
    ];

@ThemeExtensionImpl(group: 'dark')
List<ThemeExtension> get someFeatureThemeExtensions =>
    const [
      ThemedCardTheme(/* ... */),
      ThemedButtonTheme(/* ... */),
    ];

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Configuration (build.yaml)

targets:
  $default:
    builders:
      themeExtensionsImplCombiner:
        options:
          default_output:
            path: "lib/generated/theme_extensions/theme_extensions.dart"
            list_name: "themeExtensions"
          groups:
            dark:
              path: "lib/generated/theme_extensions/theme_extensions_dark.dart"
              list_name: "themeExtensionsDark"

      contextExtensionsGenerator:
        options:
          output_path: "lib/generated/theme_extensions/context_extensions.dart"

To disable contextExtensionsGenerator, use:

builders:
  contextExtensionsGenerator:
    enabled: false

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Example Usage

final theme = Theme.of(context).extension<ThemedCardTheme>();
// or
final theme = context.themedCardTheme;

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Example Project

See the example/ for:

• Shared ThemeExtension templates
• Light and dark theme implementations
• Runtime theme switching
• Organized resource files (app_colors.dart, app_text_styles.dart, etc.)

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Acknowledgements

Special thanks to
Vladyslav Ulianytskyi
for inspiration, technical discussions, and early feedback.

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Happy theming!