How to <T extend BaseClass> where BaseClass has a factory function? - dart

Question

Following is a simple class that provides a few helper functions for reading and writing data.
class BaseDAO<T> {
final String _modelName;
static late final StoreRef<int, Map<String, Object?>> _store;
BaseDAO(this._modelName) {
_store = intMapStoreFactory.store(_modelName);
}
Future<Database> get _db async => await AppDatabase().database;
Future<void> create(T object) async {
await _store.add(await _db, object.toJSON()); //The method 'toJSON' can't be unconditionally invoked because the receiver can be 'null'.
}
}
Now the issue with this is that the generic type T doesn't have toJSON function. I tried fixing this by writing an abstract class.
abstract class BaseModel {
Map<String, dynamic> toJSON();
factory BaseModel.fromJson(Map<String, dynamic> json);
}
and extending T with BaseModel. This presents all kinds of issues one of them being that I am unable to write an abstract class.
Any solution will be greatly appreciated.

Answer 1

In your case T object is empty. You need to extend it to some object which provides a method toMap().
Example:
class BaseDAO<T extends BaseModel> {
final String _modelName;
static late final StoreRef<int, Map<String, Object?>> _store;
BaseDAO(this._modelName) {
_store = intMapStoreFactory.store(_modelName);
}
Future<Database> get _db async => await AppDatabase().database;
Future<void> create(T object) async {
await _store.add(await _db, object.toMap()); // <- Dart see that this object extends to BaseModel and has a method `toMap()`,
}
}
abstract class BaseModel {
Map<String, dynamic> toMap();
}
class User extends BaseModel {
User({this.name});
final String? name;
#override
Map<String, dynamic> toMap() => {'name': name};
}
Future<void> create() async {
final user = User(name: 'Superman');
final base = BaseDAO('ModelName');
await base.create(user); // <- The user object will be added as `Map`.
}