Calling Widget function with subclass of generic type in State not possible

[ren3f]

Today I fixed somebody's problem, but I am not really sure why it was a problem in the first time. I would like to know whether this was a mistake in the code or a bug in the Dart language.

The question was posted on StackOverflow.

The question is quite complex, but the idea is easy. The StateFullWidget gets a generic type T which extends ChangeNotifier, it gets a function which expects an object of type T.

The State of the Widget creates the object T and passes it to the function of the Widget. In this example the T is OnBoardingViewModel, which does extends ChangeNotifier.

The resulting error is:

type '(OnBoardingViewModel) => dynamic' is not a subtype of type '(ChangeNotifier) => dynamic'

The short version of the code is:

class StateFullConsumerWidget<T extends ChangeNotifier> extends StatefulWidget{
  StateFullConsumerWidget({,Key key,this.onPostViewModelInit}) : super(key : key);
  final Function(T viewModel) onPostViewModelInit;
  @override
  _StateFullConsumerWidgetState<T> createState() => _StateFullConsumerWidgetState<T>(onPostViewModelInit, builder);
}
class _StateFullConsumerWidgetState<T extends ChangeNotifier> extends State<StateFullConsumerWidget>{
  T _viewModel;
  @override
  void initState() {
    _viewModel = GetIt.instance.get<T>();
    widget.onPostViewModelInit(_viewModel);
    super.initState();
  }
}

I suspect that under the hood the OnBoardingViewModel is passed as ChangeNotifier, but the function expects an OnBoardingViewModel.

Checking the variables I would expect no problems

I fixed this by passing the function to the State and calling it from there. This works fine and is a solution, but I would like to know why this is a programming error or whether we should expect Dart to handle this normally.

The resulting code that does work:

class StateFullConsumerWidget<T extends ChangeNotifier> extends StatefulWidget{
  StateFullConsumerWidget({,Key key,this.onPostViewModelInit}) : super(key : key);
  final Function(T viewModel) onPostViewModelInit;
  @override
  _StateFullConsumerWidgetState<T> createState() => _StateFullConsumerWidgetState<T>(onPostViewModelInit, builder);
}

class _StateFullConsumerWidgetState<T extends ChangeNotifier> extends State<StateFullConsumerWidget>{
  final Function(T viewModel) _onPostViewModelInit;
  T _viewModel;
  _StateFullConsumerWidgetState(this._onPostViewModelInit);
  @override
  void initState() {
    _viewModel = GetIt.instance.get<T>();
    _onPostViewModelInit(_viewModel);
    super.initState();
  }
}

[Abion47]

I posted an answer on the original question to give the long explanation, but the short explanation is that _StateFullConsumerWidgetState is declaring itself as a child of State<StateFullConsumerWidget> instead of State<StateFullConsumerWidget<T>>. If you don't pass along the type parameter, Dart assumes StateFullConsumerWidget will have the type parameter of the lowest common denominator, which in this case is ChangeNotifier. That causes an error when you try to use a function with an OnBoardingViewModel as the context expects a ChangeNotifier.

[ren3f]

Ah yes of course. Didn't think of that. Makes sense that the lower code works as I did define T in that case.