Inheritance in Null Safety - Flutter - flutter

Question

I'm working on Flutter Clean Architecture. Initially I developed my project without sound null-safety. I recently migrated it to null-safety.
Migrating to null-safety was pretty smooth but unfortunately there is an issue with inheriting classes that I'm unable to resolve.
Here is a sample of my code I'm working with:
ENTITIES:
UserEntity:
class UserEntity extends Equatable {
final int userID;
final String email;
const UserEntity({required this.userID, required this.email});
#override
List<Object> get props => [userID, email];
}
CourseEntity:
class CourseEntity extends Equatable {
final int courseID;
final String courseName;
const CourseEntity({required this.courseID, required this.courseName});
#override
List<Object> get props => [courseID, courseName];
}
DataEntity:
class DataEntity extends Equatable {
final UserEntity? user;
final CourseEntity? course;
const DataEntity({required this.user, required this.course});
#override
List<Object> get props => [user, course];
}
MODELS:
UserModel:
class UserModel extends UserEntity {
const UserModel({
required int userID,
required String email,
}) : super(userID: userID, email: email);
factory UserModel.fromJson(Map<dynamic, dynamic> json) {
return UserModel(
userID: json["userID"],
email: json["email"],
);
}
Map<String, dynamic> toJson() {
return {
"userID": userID,
"email": email,
};
}
}
CourseModel:
class CourseModel extends CourseEntity {
const CourseModel({
required int courseID,
required String courseName,
}) : super(courseID: courseID, courseName: courseName);
factory CourseModel.fromJson(Map<dynamic, dynamic> json) {
return CourseModel(
courseID: json["courseID"],
courseName: json["courseName"],
);
}
Map<String, dynamic> toJson() {
return {
"courseID": courseID,
"courseName": courseName,
};
}
}
DataModel:
class DataModel extends DataEntity {
const DataModel({
required UserModel? user,
required CourseModel? course,
}) : super(user: user, course: course);
factory DataModel.fromJson(Map<dynamic, dynamic> json) {
return DataModel(
user: UserModel.fromJson(json["user"]),
course: CourseModel.fromJson(json["course"]),
);
}
}
LOCAL DATASOURCE:
Saving Data Locally:
abstract class LocalDataSource {
Future<void> saveData(DataModel model);
Future<void> saveUser(UserModel user);
Future<void> saveCourse(CourseModel course);
}
class LocalDataSourceImpl implements LocalDataSource {
#override
Future<void> saveData(DataModel model) async {
await saveUser(model.user);
await saveCourse(model.course);
}
#override
Future<void> saveUser(UserModel model) async {
final box = await Hive.openBox("USER");
return box.put("user", model.toJson());
}
#override
Future<void> saveCourse(CourseModel model) async {
final box = await Hive.openBox("COURSE");
return box.put("course", model.toJson());
}
}
Explanation:
I hope all the sample classes are easy to understand.
There are 2 entities User and Course. The Data entity is a combination of these 2 entities. The models are extending these entities which means that the entities are the parent classes for their respective models.
The class LocalDataSource is used to store data locally on the device. I'm using Hive to cache the data. There are 2 main futures namely saveUser and saveCourse. The saveData future is the combination of these 2 futures just like the entities.
Problem:
The problem exists in the future saveData of class LocalDataSourceImpl. The future requires 1 argument i.e. DataModel. The other futures also require 1 argument i.e. UserModel and CourseModel.
In saveData(model) future, when I provide the values for respective arguments to saveUser and saveCourse as saveUser(model.user) and saveCourse(model.course), this is the error that I receive on arguments passed:
The argument type 'UserEntity' can't be assigned to the parameter type 'UserModel'.dart(argument_type_not_assignable)
Workarounds:
First workaround could be using getters:
class DataModel extends DataEntity {
final UserModel? _user;
final CourseModel? _course;
const DataModel({
required UserModel user,
required CourseModel course,
}) : _user = user,
_course = course, super(
user: user,
course: course,
);
UserModel? get user => _user;
CourseModel? get course => _course;
...
}
Second workaround could be overriding fields:
class DataModel extends DataEntity {
final UserModel? user;
final CourseModel? course;
const DataModel({
required this.user,
required this.course,
}) : super(
user: user,
course: course,
);
...
}
I'm confused if any of these 2 is a good practice, because there must be no assignment error as Models are actually the subclasses of Entities.
Thanks in advance!

Answer 1

Simplest and Easiest way is to add type cast, and it works great!
class LocalDataSourceImpl implements LocalDataSource {
#override
Future<void> saveData(DataModel model) async {
await saveUser(model.user as UserModel);
await saveCourse(model.course as CourseModel);
}
...