when to implements an abstract class in freezed?

Question

I need to understand this code, resoCoder did it on DDD Playlist. why does he implements IEntity inside freezed?

The code is:

@freezed
abstract class TodoItem with _$TodoItem implements IEntity {
  const factory TodoItem({
    @required UniqueId id,
    @required TodoName name,
    @required bool done,
  }) = _TodoItem;

  factory TodoItem.empty() => TodoItem(
        id: UniqueId(),
        name: TodoName(''),
        done: false,
      );
}
}

IEntity code is:

abstract class IEntity {
  UniqueId get id;
}

UniqueId Code is:

class UniqueId extends ValueObject<String> {
  @override
  final Either<ValueFailure<String>, String> value;

  // We cannot let a simple String be passed in. This would allow for possible non-unique IDs.
  factory UniqueId() {
    return UniqueId._(
      right(Uuid().v1()),
    );
  }

  /// Used with strings we trust are unique, such as database IDs.
  factory UniqueId.fromUniqueString(String uniqueIdStr) {
    assert(uniqueIdStr != null);
    return UniqueId._(
      right(uniqueIdStr),
    );
  }

  const UniqueId._(this.value);
}

Answer 1

It ensures consistency; TodoItem must implement everything as per IEntity.

Let's imagine one day you want to add an attribute "createdAt" to IEntity: in this case, you will have to add "createdAt" to every class that implements IEntity across the project, otherwise the compiler will let you know you're missing something :D

---

Some code now.

The result would be

abstract class IEntity {
  UniqueId get id;
  int get createdAt; // let's keep it "int" for the example purpose
}

then you would have to update the freezed class too

@freezed
abstract class TodoItem with _$TodoItem implements IEntity {
  const factory TodoItem({
    @required UniqueId id,
    @required int createdAt,
    @required TodoName name,
    @required bool done,
  }) = _TodoItem;

  factory TodoItem.empty() => TodoItem(
        id: UniqueId(),
        createdAt: 1234,
        name: TodoName(''),
        done: false,
      );
}
}