DropdownButton - Set value: with value NOT contained by The list of items the user can select - Flutter

Question

I have this DropdownButton

with two items in it's List

When i Select Value 2 i'd like it to show a value not contained in the list

Like this :

Is it even possible ?

It would look something like this :

DropdownButton<String>(
        value: dropdownValue,
        items: <String>[
          'Value 1',
          'Value 2',
        ].map<DropdownMenuItem<String>>((String value) {
          return DropdownMenuItem<String>(
            value: value,
            child: Text(value),
          );
        }).toList(),
        onChanged: (String? newValue) {
          setState(() {
            dropdownValue = newValue!;

            if (dropdownValue == 'Value 2') {
              dropdownValue = 'Value Not in  list of items';
            }
          });
        },
      ),

---

As you can see I of course get an error if i set dropdownValue = 'Value Not in list of items'; as it's not in the List - but i'd like it to work :)

This works on the other hand, as expected:

if (dropdownValue == 'Value 2') {
  dropdownValue = 'Value 1';
}

as 'Value 1' is in the List - but i'd like to display a value not in the list when i click on Value 2

I hope someone can help ! : )

---

[for SEO sake here is the error]:

Assertion failed:
../…/material/dropdown.dart:880
items == null ||
              items.isEmpty ||
              value == null ||
              items.where((DropdownMenuItem<T> item) {
                    return item.value == value;
                  }).length ==
                  1
"There should be exactly one item with [DropdownButton]'s value: Value Not in  list of items. \nEither zero or 2 or more [DropdownMenuItem]s were detected with the same value"

Answer 1

If you just want to display some text when Value 2 is chosen, you can try hint widget.

  String hintValue = 'Select Value'; // ---- hint
  var dropdownValue;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Column(
          children: [
            ElevatedButton(child: Text('Press here'), onPressed: null), // ---- ignore
            DropdownButton<String>(
              value: dropdownValue,
              hint: Text(hintValue), // ---- hint
              isExpanded: true,
              items: <String>[
                'Value 1',
                'Value 2',
              ].map<DropdownMenuItem<String>>((String value) {
                return DropdownMenuItem<String>(
                  value: value,
                  child: Text(value),
                );
              }).toList(),
              onChanged: (newValue) {
                setState(() {
                  // ----- if Value 2 is selected
                  if (newValue == 'Value 2') {
                    hintValue = 'Value Not in list of items';
                  } else
                    hintValue = '$newValue';  
                });
              },
            ),
          ],
        ));
  }

Output

Note : Documentation for using hint widget