dynamic-dropdown-with-get.dart

import 'package:flutter/material.dart';
import 'package:get/get.dart';

class Controller extends GetxController {
  var items = <String>['a'].obs;
  var itemCurrent = 'a'.obs;
  addItem(String item) => items.add(item);
}

void main() => runApp(GetMaterialApp(home: Home()));

class Home extends StatelessWidget {
  final Controller c = Get.put(Controller());

  @override
  Widget build(context) {
    return Scaffold(
        appBar: AppBar(),
        body: MyDropdown(),
        floatingActionButton: FloatingActionButton(
            child: Icon(Icons.add),
            onPressed: () {
              c.addItem((c.items.length + 1).toString());
            }));
  }
}

class MyDropdown extends StatelessWidget {
  @override
  Widget build(context) {
    final Controller c = Get.find();
    return Obx(() => DropdownButton<String>(
        icon: const Icon(Icons.arrow_drop_down),
        value: c.itemCurrent.value,
        items: c.items.map((String item) {
          return DropdownMenuItem<String>(
            value: item,
            child: Text(item),
          );
        }).toList(),
        onChanged: (item) {
          c.itemCurrent.value = item!;
        }));
  }
}