How to declare factory constructor in abstract classes?

Question

I want to declare, but not define a factory constructor in an abstract class.

In my case, I want to create a method that accepts any class that implements a St

Answer 1

As suggested in the accepted answer, I ended up creating a Serializer type that got implemented by a serializer for each class:

Turns out, this has several benefits over just having toJson/fromJson on the classes directly:

• It decouples the serialization logic from the actual classes. That means better code readability because classes only contain methods that relate directly to the class — serializers can even be put into their own files.
• Currently, extensions can't create constructors. So having serializers separately makes it possible to write serializers for existing classes, like String or Flutter's Color, where you can't just add a fromColor constructor.
• Both these points combined mean it also works well with code generation — the classes are hand-written and the serializer can be generated in a separate file.

---

Code example:

class Fruit {
  Fruit(this.name, this.color);

  final String name;
  final String color;
}

// in another file

class FruitSerializer extends Serializer {
  Map toJson(Fruit fruit) {
    return ...;
  }

  Fruit fromJson(Map data) {
    return Fruit(...);
  }
}

An then also pass the serializer to the code that needs it:

someMethod(Serializer serializer, T value) {
  ...
}

someMethod(FruitSerializer(), someFruit);
final fruit = recreateFruit(FruitSerializer());

Obviously, you can't pass an object that can't be serialized to the code, because the method expects a Serializer.