What is build_runner in Flutter?

build_runner is a Dart build system that generates source files based on annotations in your code. Run it with flutter pub run build_runner build for a one-time generation or watch for continuous generation during development. It powers several major packages: json_serializable (JSON models), freezed (immutable classes), injectable (dependency injection), hive_generator (Hive adapters), and riverpod_generator. Generated files end in .g.dart or .freezed.dart and should be committed to version control.