Proposal: const *Route.withArgs() generated constructor

[mernen]

Suppose we had the following routes:

@RoutePage()
class FirstPage extends StatelessWidget {
  const FirstPage({super.key});

  @override
  Widget build(BuildContext context) => const Placeholder();
}

@RoutePage()
class SecondPage extends StatelessWidget {
  const SecondPage({super.key, this.param});

  final String? param;

  @override
  Widget build(BuildContext context) => const Placeholder();
}

The generated routes would look something like:

/// generated route for
/// [_i1.FirstPage]
class FirstRoute extends _i2.PageRouteInfo<void> {
  const FirstRoute({List<_i2.PageRouteInfo>? children})
      : super(
          FirstRoute.name,
          initialChildren: children,
        );

  static const String name = 'FirstRoute';

  static _i2.PageInfo page = _i2.PageInfo(
    name,
    builder: (data) {
      return const _i1.FirstPage();
    },
  );
}

/// generated route for
/// [_i1.SecondPage]
class SecondRoute extends _i2.PageRouteInfo<SecondRouteArgs> {
  SecondRoute({
    _i3.Key? key,
    String? param,
    List<_i2.PageRouteInfo>? children,
  }) : super(
          SecondRoute.name,
          args: SecondRouteArgs(
            key: key,
            param: param,
          ),
          initialChildren: children,
        );

  static const String name = 'SecondRoute';

  static _i2.PageInfo page = _i2.PageInfo(
    name,
    builder: (data) {
      final args = data.argsAs<SecondRouteArgs>(
          orElse: () => const SecondRouteArgs());
      return _i1.SecondPage(
        key: args.key,
        param: args.param,
      );
    },
  );
}

class SecondRouteArgs {
  const SecondRouteArgs({
    this.key,
    this.param,
  });

  final _i3.Key? key;

  final String? param;

  @override
  String toString() {
    return 'SecondRouteArgs{key: $key, param: $param}';
  }
}

We can see that due to the presence of a parameter in SecondPage (key is ignored if it's the only parameter), the generated constructor is no longer const, and a SecondRouteArgs class was created.

I'd like to propose in this situation to produce a second constructor:

class SecondRoute extends _i2.PageRouteInfo<SecondRouteArgs> {
  // ...all existing constructors and members unchanged

  const SecondRoute.withArgs(
    SecondRouteArgs args, {
    List<_i2.PageRouteInfo>? children,
  }) : super(
          SecondRoute.name,
          args: args,
          initialChildren: children,
        );
}

The purpose here being solely to offer a const constructor, as it can be useful in some situations like when declaring default values or enum constructor arguments.

Because the *RouteArgs classes are already present and public, no changes at all are necessary to what is already being generated, and adding constructors is in no way a breaking change.