Builder

Learn code generators with Builder.

Builder

Builder is the code generator interface that build_runner runs. The build(BuildStep) method reads source files and writes generated files.

A Builder declares: buildExtensions (input → output map, e.g. '.dart' → '.g.dart') and the build method itself.

The BuildStep context provides file reading/writing, dependency resolution and logging. Builders are stateless; they are recreated on each build.

The buildExtensions map

buildExtensions maps input extension to output extension: {'.dart': '.g.dart'}. build_runner uses this map to know which files to generate.

A Builder can produce multiple outputs: {'.dart': ['.g.dart', '.freezed.dart']}. Each output is a separate file.

When writing a Builder, you first define buildExtensions, then the build method. Together they form the generation contract.

Exercise Define a Builder skeleton

Define a Builder class with String get outputExtension => '.g.dart'; and String build(String input) => '// generated from $input'; methods. In main create final builder = Builder();; print builder.outputExtension and builder.build('user.dart').

Show example solution

class Builder {
  String get outputExtension => '.g.dart';
  String build(String input) => '// generated from $input';
}

void main() {
  final builder = Builder();
  print(builder.outputExtension);
  print(builder.build('user.dart'));
}

Test yourself

The build method

Future<void> ____(BuildStep step) async {
  // read source, write output
}

Show answer

Future<void> build(BuildStep step) async {
  // read source, write output
}

Which method does a Builder implement?
- compile(Library)
- generate(String)
- build(BuildStep)
- run(Builder)
Show answer

build(BuildStep) — a Builder implements the build(BuildStep) method.
Match the Builder parts

build(BuildStep)

Build method
BuildStep

Input context
buildExtensions

Extension map
Future<void>

Return type

Next lesson: source_gen

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