Freezed class with generic callback - flutter

Question

I would like to define a freezed class [https://pub.dev/packages/freezed] with a generic callback.
Freezed class:
import 'package:freezed_annotation/freezed_annotation.dart';
part 'foo.freezed.dart';
#freezed
abstract class Foo<T> with _$Foo {
factory Foo({
// String Function() callBackOne,
String Function(T) callBackTwo,
}) = _Foo;
}
Widget using the Freezed class:
class MyHomePage extends StatelessWidget {
// final fooOne = Foo<int>(callBackOne: () => 'Result: 42');
final fooTwo = Foo<int>(callBackTwo: (value) => 'Result: ${value * 3}');
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Text(fooTwo.callBackTwo(14)),
),
);
}
}
Error:
lib/foo.freezed.dart:128:26: Error: The return type of the method '_Foo.callBackTwo' is 'String Function(T)', which does not match the return type, 'String Function(dynamic)', of the overridden method, '_$Foo.callBackTwo'.
Change to a subtype of 'String Function(dynamic)'.
String Function(T) get callBackTwo;
^
lib/foo.freezed.dart:31:26: Context: This is the overridden method ('callBackTwo').
String Function(T) get callBackTwo;
^
Do you know what is wrong with my code? Is it a limitation of Freezed? Do you know a workaround?
Thank you!!!

Answer 1

It looks like a flaw in Dart type system. I've encouraged something like that either. I don't know a clean workaround. You can specify not a direct function but a function wrapped into a class with a "strong" method signature. Something like that should work:
#freezed
abstract class Foo<T> with _$Foo {
factory Foo({
Func<T> callBackTwo,
}) = _Foo;
}
class Func<T> {
final String Function(T) _apply;
Func(this._apply) : assert(_apply != null);
String call(T value) {
return _apply(value);
}
}
class MyHomePage extends StatelessWidget {
final fooTwo = Foo<int>(Func<int>((value) => 'Result: ${value * 3}'));
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Text(fooTwo.callBackTwo(14)),
),
);
}
}
It's not so good, because you are to type more. But we can minimize typing a bit:
#freezed
abstract class Foo<T> with _$Foo {
factory Foo({
Func<T> callBackTwo,
}) = _Foo;
factory Foo.from(String Function(T) arg) {
return Foo<T>(callBackTwo: Func<T>(arg));
}
}
class MyHomePage extends StatelessWidget {
final fooTwo = Foo<int>.from((value) => 'Result: ${value * 3}');
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Text(fooTwo.callBackTwo(14)),
),
);
}
}