Cannot Find Method 'SetState' within widget

Question

I have a widget that is called in my main scaffolding file later. This widget contains a dropdown menu but, I cannot change the state when selecting another value. The field does not update and I get the error message 'error: Method not found: 'setState'. setState(() {' ^^^^^^^^

I have updated the setState method and removed code from it but, it still says the method not found.

child: DropdownButton(
                  hint: Text('Medical'),
                  value: _selectedCustomerType,
                  onChanged: (newValue) {
                    setState(() {
                      _selectedCustomerType = newValue;
                    });
                  },
                  items: _customerType.map((cusType) {
                    print(cusType);

                    return DropdownMenuItem(
                      child: Text(cusType),
                      value: cusType,
                    );
                  }).toList(),
                ),

I need to be able to update the value and display it when the new value is chosen.

Answer 1

You can't use setState outside of a StatefulWidget so you should wrap your DropdownButton in a StatefulWidget, for example:

class StatefulDropdownButton extends StatefulWidget {
  final List<String> _customerType;

  StatefulDropdownButton(this._customerType);

  @override
  State<StatefulWidget> createState() => DropdownButtonState();
}

class DropdownButtonState extends State<StatefulDropdownButton> {
  String _selectedCustomerType;

  @override
  Widget build(BuildContext context) {
    return DropdownButton(
      hint: Text('Medical'),
      value: _selectedCustomerType,
      onChanged: (newValue) {
        setState(() {
          _selectedCustomerType = newValue;
        });
      },
      items: widget._customerType.map((cusType) {
        print(cusType);

        return DropdownMenuItem(
          child: Text(cusType),
          value: cusType,
        );
      }).toList(),
    );
  }
}