mix_generator

mix_generator is a build_runner  package that generates mixins and helpers for Mix’s core types. You annotate Spec, Styler, and property (“Mix”) classes in mix_annotations; the builder emits part files that implement copyWith, lerp, merge, resolve, setters, and related plumbing.

What gets generated

The package registers three builders (see build.yaml in the mix repo):

source_gen combines these parts into your hand-authored part '…g.dart'; library.

Installation

Add runtime and dev dependencies:

dart pub add mix mix_annotations
dart pub add dev:build_runner dev:mix_generator

Minimal pubspec.yaml shape:

dependencies:
  mix: ^2.0.0
  mix_annotations: ^2.0.0
 
dev_dependencies:
  build_runner: ^2.0.0
  mix_generator: ^2.0.0

Versions should match what your Mix release expects; check the changelog  when upgrading.

Running code generation

From your package root:

dart run build_runner build --delete-conflicting-outputs

During development, watch mode regenerates on save:

dart run build_runner watch --delete-conflicting-outputs

Contributors working inside the Mix repository often use melos run gen:build to run the repo’s codegen scripts across packages.

Annotations (from mix_annotations)

@MixableSpec

Use on immutable Spec classes that extend Spec<T>. Controls which methods and components are generated via integer flags (see GeneratedSpecMethods and GeneratedSpecComponents in mix_annotations).

import 'package:flutter/foundation.dart';
import 'package:mix/mix.dart';
import 'package:mix_annotations/mix_annotations.dart';
 
part 'my_spec.g.dart';
 
@MixableSpec()
@immutable
final class MySpec extends Spec<MySpec> with _$MySpecMethods {
  final String? name;
  final int? age;
 
  const MySpec({this.name, this.age});
}

Flags let you skip generation you do not need, for example:

• GeneratedSpecMethods.skipLerp — omit lerp
• GeneratedSpecMethods.skipCopyWith — omit copyWith

@MixableStyler

Use on Styler classes. In Mix 2, concrete box-like Stylers typically extend MixStyler<YourStyler, YourSpec> and combine the generated mixin with shared style mixins (padding, borders, and so on). The generated piece adds setters, merge, resolve, debugFillProperties, props, and optionally widget call. Configure with GeneratedStylerMethods flags if you need to omit pieces (for example GeneratedStylerMethods.skipCall).

See BoxStyler in the Mix repository for a full reference implementation (imports, hand-written mixins, const .create, and part wiring).

@MixableField

Annotate individual Prop<…> fields on a Styler when you need to:

• ignoreSetter: true — no fluent setter generated for that field.
• setterType: — override the setter parameter type when it must differ from Prop<T>’s type argument.

@Mixable

Use on property / constraint classes (types extending Mix’s property bases) so the builder emits merge, resolve, equality-related props, and diagnostics. Optional resolveToType supplies the target type name when it cannot be inferred from the superclass.

GeneratedMixMethods flags control whether merge, resolve, props, or debugFillProperties are emitted.

Authoring part files

Point your library at the generated part Dart will pull in:

part 'box_spec.g.dart';

After a successful build, box_spec.g.dart contains the part of directive and the combined output from the relevant builders. If generation fails, fix analyzer errors in the annotated file first—generators depend on resolved types.

Troubleshooting

• No output / stale errors: Run build_runner with --delete-conflicting-outputs if a previous run left incompatible *.g.dart files.
• Builder not running: Ensure mix_generator is in dev_dependencies and that your package (or app) imports files that carry the annotations; builders only visit included sources.
• Monorepo build.yaml: The Mix framework repo narrows generate_for globs for its own layout. Your app typically relies on the builders’ auto_apply: dependents behavior unless you add a custom build.yaml.

Debugging the generator

To step through generator code in this repository, use the VS Code Debug build_runner launch configuration under packages/mix_generator, or run build_runner with the VM service enabled and attach a debugger (see the package README in packages/mix_generator).

Related docs

• mix_lint — Mix-specific analyzer rules, including Styler structure
• Styling guide — how Spec, Styler, and widgets fit together
• API composition guidelines  — fluent chaining and merge patterns (repository guide)