final List<String> items = [

  'A_Item1',

  'A_Item2',

  'A_Item3',

  'A_Item4',

  'B_Item1',

  'B_Item2',

  'B_Item3',

  'B_Item4',

];

String? selectedValue;

final TextEditingController textEditingController = TextEditingController();

@override

void dispose() {

  textEditingController.dispose();

  super.dispose();

}

String getTurkish(String text) {
 text = text.toLowerCase().replaceAll("i̇", "i"); // buradaki iki i farklıdır

  text = text.replaceAll("ç", "c");
  text = text.replaceAll("ğ", "g");
  text = text.replaceAll("ı", "i");
  text = text.replaceAll("ö", "o");
  text = text.replaceAll("ş", "s");
  text = text.replaceAll("ü", "u");
  return text;
}

@override

Widget build(BuildContext context) {

  return Scaffold(

    body: Center(

      child: DropdownButtonHideUnderline(

        child: DropdownButton2(

          isExpanded: true,

          hint: Text(

            'Select Item',

            style: TextStyle(

              fontSize: 14,

              color: Theme.of(context).hintColor,

            ),

          ),

          items: items

                  .map((item) => DropdownMenuItem<String>(

            value: item,

            child: Text(

              item,

              style: const TextStyle(

                fontSize: 14,

              ),

            ),

          ))

                  .toList(),

          value: selectedValue,

          onChanged: (value) {

            setState(() {

              selectedValue = value as String;

            });

          },

          buttonHeight: 40,

          buttonWidth: 200,

          itemHeight: 40,

          dropdownMaxHeight: 200,

          searchController: textEditingController,

          searchInnerWidget: Padding(

            padding: const EdgeInsets.only(

              top: 8,

              bottom: 4,

              right: 8,

              left: 8,

            ),

            child: TextFormField(

              controller: textEditingController,

              decoration: InputDecoration(

                isDense: true,

                contentPadding: const EdgeInsets.symmetric(

                  horizontal: 10,

                  vertical: 8,

                ),

                hintText: 'Search for an item...',

                hintStyle: const TextStyle(fontSize: 12),

                border: OutlineInputBorder(

                  borderRadius: BorderRadius.circular(8),

                ),

              ),

            ),

          ),

        searchMatchFn: (item, searchValue) {
       String aranacak = getTurkish(searchValue);
       String listItem = getTurkish(item.value.toString());
        return (listItem.contains(aranacak));
        },

          //This to clear the search value when you close the menu

          onMenuStateChange: (isOpen) {

            if (!isOpen) {

              textEditingController.clear();

            }

          },

        ),

      ),

    ),

  );

}

 1. dropdownr.dart

import 'package:dropdown_button2/dropdown_button2.dart';
import 'package:flutter/material.dart';

class DropDownR extends StatefulWidget {
  List<String> items = [];
  String secili = "";
  String title = "Seçiniz...";
  Function secilenClick;

  DropDownR(
      {required this.items,
      required this.secili,
      required this.title,
      required this.secilenClick});

  @override
  State<DropDownR> createState() => _DropDownRState();
}

class _DropDownRState extends State<DropDownR> {
  List<String> items = [];
  String? secili = null;
  String title = "";
  late Function secilenClick;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    this.items = widget.items;
    this.secili = widget.secili;
    this.title = widget.title;
    this.secilenClick = widget.secilenClick;
  }

  final TextEditingController textEditingController = TextEditingController();

  @override
  void dispose() {
    textEditingController.dispose();
    super.dispose();
  }

  String getTurkish(String text) {
    text = text.toLowerCase().replaceAll("i̇", "i"); // buradaki iki i farklıdır
    text = text.replaceAll("ç", "c");
    text = text.replaceAll("ğ", "g");
    text = text.replaceAll("ı", "i");
    text = text.replaceAll("ö", "o");
    text = text.replaceAll("ş", "s");
    text = text.replaceAll("ü", "u");
    return text;
  }

  @override
  Widget build(BuildContext context) {
    // DropdownButtonHideUnderline
    return Center(
      child: Container(
        decoration: BoxDecoration(
          borderRadius: BorderRadius.circular(5.0),
          border: Border.all(
              color: Colors.grey, style: BorderStyle.solid, width: 0.80),
        ),
        child: DropdownButtonHideUnderline(
          child: DropdownButton2(
            isExpanded: true,

            hint: TextFormField(
              style: TextStyle(
                fontSize: 14,
                color: Theme.of(context).hintColor,
              ),
              decoration: InputDecoration(
                icon: Icon(Icons.file_present),
                labelText: title,
                border: InputBorder.none,
                labelStyle: TextStyle(fontSize: 14),
              ),
            ),
            items: items
                .map((item) => DropdownMenuItem<String>(
                      value: item,
                      child: Container(
                        padding: EdgeInsets.only(left: 10),
                        child: Text(
                          item,
                          style: const TextStyle(
                            fontSize: 14,
                          ),
                        ),
                      ),
                    ))
                .toList(),
            value: secili == "" ? null : secili,
            onChanged: (value) {
              setState(() {
                secili = value as String;
                secilenClick(secili!);
              });
            },
            buttonHeight: 40,
            buttonWidth: 200,
            itemHeight: 30,
            dropdownMaxHeight: 600,
            searchController: textEditingController,
            searchInnerWidget: Padding(
              padding: const EdgeInsets.only(
                top: 8,
                bottom: 4,
                right: 8,
                left: 8,
              ),
              child: TextFormField(
                controller: textEditingController,
                decoration: InputDecoration(
                  isDense: true,
                  contentPadding: const EdgeInsets.symmetric(
                    horizontal: 10,
                    vertical: 8,
                  ),
                  hintText: "Ara...",
                  hintStyle: const TextStyle(fontSize: 12),
                  border: OutlineInputBorder(
                    borderRadius: BorderRadius.circular(8),
                  ),
                ),
              ),
            ),
            searchMatchFn: (item, searchValue) {
              String aranacak = getTurkish(searchValue);
              String listItem = getTurkish(item.value.toString());
              return (listItem.contains(aranacak));
            },
            //This to clear the search value when you close the menu
            onMenuStateChange: (isOpen) {
              if (!isOpen) {
                textEditingController.clear();
              }
            },
          ),
        ),
      ),
    );
  }
}

 2. kullanımı

final List<String> items = [

  'A_Item1',

  'A_Item2',

  'A_Item3',

  'A_Item4',

  'B_Item1',

  'B_Item2',

  'B_Item3',

  'B_Item4',

];

String secilenBelgeTip="";

Widget getBelgeContent() {
  var dropDown= DropDownR(items: items ,secili: secilenBelgeTip,title: "Belge tipini seçiniz...", secilenClick: (String secilenBelgeTip){
    print(secilenBelgeTip);
    this.secilenBelgeTip=secilenBelgeTip;
  },);
  return dropDown;
}