How to write a mixin for a generic class State<T extends StatefulWidget>

Question

I'm in need to write an extension for Flutter's State<T extends StatefulWidget> so I can use a function in all of my States, let's say showSnackBar("Hello world", 5). I tried writing a mixin

mixin BaseState on State<ProfileScreen> {
  final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>();

  void showSnackBar(String text) {
    setState(() {
      scaffoldKey.currentState.showSnackBar(new SnackBar(
          content: new Row(
            children: <Widget>[
              new CircularProgressIndicator(),
              new Text(text == null ? "  Logging in" : "      $text")
            ],
          )));
    });
  }

  void hideSnackBar() {
    setState(() {
      scaffoldKey.currentState.hideCurrentSnackBar();
    });
  }
}

As you can see, it is now mixed on State<ProfileScreen>. It's a problem because I only can use this mixin in class ProfileScreenState extends State<ProfileScreen>. Without the type notation I end up with an error:

error: The class 'ProfileScreenState' cannot implement both 'State<ProfileScreen>' and 'State<StatefulWidget>' because the type arguments are different. (conflicting_generic_interfaces at [mobile] lib/Screens/profile.dart:17)
error: Type parameters could not be inferred for the mixin 'BaseState' because no type parameter substitution could be found matching the mixin's supertype constraints (mixin_inference_no_possible_substitution at [mobile] lib/Screens/profile.dart:17)

I tried to Google a lot, seen questions like these but without a success.

And yes I know composition is preferred over inheritance in Flutter, but I think this is a thing I don't know I would make work with composition and I feel it will be OK with inheritance.

Answer 1

I think you are looking for something like https://github.com/flutter/flutter/blob/3fbd140e77601686acb336cd2af2838d03ba0572/packages/flutter/lib/src/widgets/ticker_provider.dart#L155

mixin BaseState<T extends StatefulWidget> on State<T> {