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flutterdartoopmodifiersealed

sealed class in Flutter / Dart. Meaning of "library" in Flutter / Dart

I'm looking through of sealed usage in my projects, and I can't figure it out.

The "sealed" modifier states "The sealed modifier prevents a class from being extended or implemented outside its own library. Sealed classes are implicitly abstract." on dart.dev page.

// sign_result_entity.dart

sealed class SignResult extends Equatable {
  @override
  List<Object?> get props => [];
}

// sign_result_test.dart

import 'sign_result_entity.dart';

class UniqueSignResult extends SignResult {}

The problem is - I'm trying to implement sealed class outside of file, and I get the next message:

enter image description here

The file located in a same directory, same folder.

enter image description here

I believe the "library" means a different thing than we would expect. Extending it in a same file works fine.

Solution

  • Every Dart file (plus its parts) is a library, even if it doesn't use a library directive

    Since every Dart file is a library, you cannot extend SignResult from another file. The only way to do that is to use part/part of.

    part 'sign_result_test.dart';

sealed class SignResult extends Equatable {
  @override
  List<Object?> get props => [];
}

    part of 'sign_result_entity.dart';

class UniqueSignResult extends SignResult {}