Code generation

Learn code generation with source_gen.

Code generation with source_gen

source_gen is a package that generates Dart code from Dart code. It runs with build_runner; in a pre-compile step it produces .g.dart files.

It replaces reflection: it reads annotations and produces serialization/mapper code for the relevant class. There is no runtime cost; everything is ready before compilation.

Common uses: json_serializable (JSON ↔ class), freezed (immutable data classes), mockito (mock generation).

The Generator class

A source_gen generator implements the Generator abstract class: its generate(LibraryReader, BuildStep) method returns a String (Dart code).

The generator reads source code via the analyzer package (AST), finds annotations and produces methods/fields for the relevant class.

The generated file has the .g.dart extension and is written next to the original. It is included into the source file via the part directive.

Exercise Simulate a simple code generator

Define class FieldGen { final String name; FieldGen(this.name); String build() => 'final String $name;'; }. Inside main create var g = FieldGen('title'); and add print(g.build());.

Show example solution

class FieldGen {
  final String name;
  FieldGen(this.name);
  String build() => 'final String $name;';
}

void main() {
  var g = FieldGen('title');
  print(g.build());
}

Test yourself

Build command

dart run ____ build

Show answer

dart run build_runner build

Match the tools

source_gen

Code generation
build_runner

Build step
analyzer

AST reading
.g.dart

Generated file
When does source_gen run?
- At runtime
- Only on the server
- In a pre-compile build step
- Only in tests
Show answer

In a pre-compile build step — source_gen runs pre-compile via build_runner.

Next lesson: Summary - Metaprogramming

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