Extending from FormField in a Stateful Widget

I have a Stateful widget with properties and functions. I would like to extend the widget from FormField, because I need to validate some user input on the widget.

It seems that it is not possible to extend from multiple classes (StatefulWidget and FormField).

If I try to create a normal class that extends from FormField:

class CustomInput extends FormField<bool> {
  final Widget label;
  final void Function(bool?) onChanged;

  userClicked() {
    print('User clicked');
  }

  CustomInput({
    required this.label,
    required this.onChanged,
    FormFieldValidator<bool>? validator,
  }) : super(
          validator: validator,
          builder: (field) {           
            return Column(
              children: [
                TextButton(
                  onPressed: () => userClicked(), //<-- The instance member 'userClicked' can't be accessed in an initializer.
                  child: Text('Call internal function'),
                ),

... then I cannot access internal methods because of the 'The instance member 'userClicked' can't be accessed in an initializer.' error.

So how do you create a widget that extends from FormField to use the validator, onChanged, onSaved etc., but at the same time have internal logic that can be accessed normally from the widget tree?

Solution

Instead of extending FormField, the solution was to return a FormField from the StatefulWidget. In this example the validation content is a string array, but it can of course be whatever:

class CustomInput extends StatefulWidget {
  final String? Function(List<String>)? validator;
  final Function(List<String>)? onChanged;

  const CustomInput({this.onChanged, this.validator, Key? key}) : super(key: key);

  @override
  State<CustomInput> createState() => _CustomInputState();
}

class _CustomInputState extends State<CustomInput> {

  @override
  void initState() {
    super.initState();
  }
  
 userClicked() {
    print('User clicked');
  }

  @override
  Widget build(BuildContext context) {
    return FormField(validator: ((value) {
      if (widget.validator != null) {
        return widget.validator!(widget.imagePaths);
      }
      return null;
    }), builder: (FormFieldState<List<String>> state) {
      return Column(
              children: [
                TextButton(
                  onPressed: () => userClicked(), //<-- The instance member 'userClicked' can't be accessed in an initializer.
                  child: Text('Call internal function'),
                ),
                ]
          );
        
    });
  }
}