Is it possible to define an abstract named constructor in Dart? - firebase

Question

I'm trying to create an abstract class Firestorable which will ensure that subclasses override a named constructor fromMap(Map<String, dynamic> map)
The code looks like this ...
abstract class Firestorable {
/// Concrete implementations will convert their state into a
/// Firestore safe [Map<String, dynamic>] representation.
Map<String, dynamic> toMap();
/// Concrete implementations will initialize its state
/// from a [Map] provided by Firestore.
Firestorable.fromMap(Map<String, dynamic> map);
}
class WeaponRange implements Firestorable {
int effectiveRange;
int maximumRange;
WeaponRange({this.effectiveRange, this.maximumRange});
#override
WeaponRange.fromMap(Map<String, dynamic> map) {
effectiveRange = map['effectiveRange'] ?? 5;
maximumRange = map['maximumRange'] ?? effectiveRange;
}
#override
Map<String, int> toMap() {
return {
'effectiveRange': effectiveRange,
'maximumRange': maximumRange ?? effectiveRange,
};
}
}
I don't get any errors when I do this, however I also don't get a compile error when I leave out the concrete implementation of the fromMap(..) constructor.
For example the following code will compile without any errors:
abstract class Firestorable {
/// Concrete implementations will conver thier state into a
/// Firestore safe [Map<String, dynamic>] representation.
Map<String, dynamic> convertToMap();
/// Concrete implementations will initialize its state
/// from a [Map] provided by Firestore.
Firestorable.fromMap(Map<String, dynamic> map);
}
class WeaponRange implements Firestorable {
int effectiveRange;
int maximumRange;
WeaponRange({this.effectiveRange, this.maximumRange});
// #override
// WeaponRange.fromMap(Map<String, dynamic> map) {
// effectiveRange = map['effectiveRange'] ?? 5;
// maximumRange = map['maximumRange'] ?? effectiveRange;
// }
#override
Map<String, int> convertToMap() {
return {
'effectiveRange': effectiveRange,
'maximumRange': maximumRange ?? effectiveRange,
};
}
}
Am I not able to define an abstract named constructor and have it be a require implementation in a concrete class? If not, what would be the correct way to do this?

Answer 1

As stated in the official guide for the Dart Language constructors aren’t inherited so you can't enforce a factory constructor to subclasses. To ensure an implementation it should be part of the class' interface which constructors are not. You can check these related stackoverflow questions for more information:
How to declare factory constructor in abstract classes
How do i guarentee a certain named constructor in dart