Builder
- Dart course
- Expert
- Lesson 26/47
- 5 min
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.
Practice 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').
Starter code
void main() {
// define a Builder class (outputExtension getter, build method)
// create a Builder; print outputExtension and build('user.dart')
}Sample 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'));
}Expected output
.g.dart // generated from user.dart
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 The build method
Future<void> ____(BuildStep step) async {
// read source, write output
}Future<void> build(BuildStep step) async {
// read source, write output
}Explanation: the build(BuildStep) method does the generation.
2 Which method does a Builder implement?
- compile(Library)
- generate(String)
- build(BuildStep) Correct answer
- run(Builder)
Explanation: a Builder implements the build(BuildStep) method.
3 Match the Builder parts
| Term | Meaning |
|---|---|
build(BuildStep) |
Build method |
BuildStep |
Input context |
buildExtensions |
Extension map |
Future<void> |
Return type |