

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.only(top: 10.0, left: 20.0, right: 20.0),
      child: SingleChildScrollView(
        physics: BouncingScrollPhysics(),
        scrollDirection: Axis.vertical,
        child: Column(
          children: <Widget>[
            SizedBox(
              height: 20,
            ),
            Column(
              children: [
                Align(
                  alignment: Alignment.centerLeft,
                  child: Text(
                    "Select visibility:",
                    style: TextStyle(
                      color: Colors.indigoAccent,
                      fontWeight: FontWeight.bold,
                      fontSize: 16,
                    ),
                  ),
                ),
              ],
            ),
            DropdownButton(
              icon: Icon(
                Icons.visibility,
                color: Colors.black45,
              ),
              isExpanded: true,
              value: _selectedVisibility,
              items: _dropdownMenuItems,
              onChanged: (ParentCategoryComboBox selectedVisibility) {
                setState(() {
                  _selectedVisibility = selectedVisibility;
                });
              },
            ),
            SizedBox(
              height: 35,
            ),
            Container(
              padding: new EdgeInsets.symmetric(horizontal: 5.0),
              decoration: BoxDecoration(
                  border: Border.all(
                    width: 1,
                    color: Colors.grey,
                  ),
                  borderRadius: BorderRadius.circular((10))),
              child: Column(
                children: <Widget>[
                  SizedBox(
                    height: 10,
                  ),
                  Align(
                    alignment: Alignment.centerLeft,
                    child: Text(
                      "Select platform reach:",
                      style: TextStyle(
                        color: Colors.indigoAccent,
                        fontWeight: FontWeight.bold,
                        fontSize: 16,
                      ),
                    ),
                  ),
                  SizedBox(
                    height: 20,
                  ),
                  Row(
                    children: <Widget>[
                      Expanded(
                        child: SearchableDropdown(
                          isExpanded: true,
                          value: _selectedPlatformReach,
                          items: _dropdownPlatformReach,
                          isCaseSensitiveSearch: false,
                          onChanged:
                              (PlatformReachComboBox selectedPlatformReach) {
                            setState(() {
                              _selectedPlatformReach = selectedPlatformReach;
                            });
                          },
                        ),
                        flex: 2,
                      ),
                      Expanded(
                        child: Container(
                          height: 50.0,
                          width: 80,
                          child: Material(
                            borderRadius: BorderRadius.circular(20.0),
                            shadowColor: Colors.blue,
                            color: Colors.deepPurpleAccent,
                            elevation: 0.0,
                            child: GestureDetector(
                              onTap: () {
                                setState(() {
                                  mylist.add(_selectedPlatformReach.toString());
                                });
                              },
                              child: Center(
                                child: Text(
                                  'ADD',
                                  style: TextStyle(
                                    color: Colors.white,
                                    fontWeight: FontWeight.bold,
                                  ),
                                ),
                              ),
                            ),
                          ),
                        ),
                        flex: 0,
                      ),
                    ],
                  ),
                  SizedBox(
                    height: 10,
                  ),
                  Align(
                    alignment: Alignment.centerLeft,
                    child: Text(
                      "Added:",
                      style: TextStyle(
                        color: Colors.indigoAccent,
                        fontWeight: FontWeight.bold,
                        fontSize: 16,
                      ),
                    ),
                  ),
                  SizedBox(
                    height: 10,
                  ),
                  ListView.builder(
                    padding: EdgeInsets.only(left: 10),
                    scrollDirection: Axis.vertical,
                    shrinkWrap: true,
                    itemCount: mylist.length,
                    itemBuilder: (BuildContext ctxt, int Index) {
                      return Text(
                          mylist[Index]
                      );

                    },
                  ),
//                  ListView.builder(
//                    padding: EdgeInsets.only(left: 10),
//                    scrollDirection: Axis.vertical,
//                    shrinkWrap: true,
//                    itemCount: _platformReach.length,
//                    itemBuilder: (BuildContext ctxt, int Index) {
//                      return Text(
//                        "Galati"
//                      );
////                      return DropdownButton(
////                        value: _selectedPlatformReach.toString(),
////                        items: _dropdownPlatformReach,
////                        onChanged: (value) {
////                          _selectedPlatformReach = value;
////                          setState(() {});
////                        },
////                        hint: Text('Select drowdown'),
////                      );
////                                return new Text(_dropdownPlatformReach[Index].value);
//                    },
//                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}


Mock data class:
import 'package:flutter/material.dart';

class PlatformReachComboBox {
  String name;
  String hint;

  PlatformReachComboBox(this.name, this.hint);

  static List<PlatformReachComboBox> getPlatformReach() {
    return <PlatformReachComboBox>[
      PlatformReachComboBox('Jud Galati', '(RO, [Galati] County)'),
      PlatformReachComboBox('Jud Braila', '(RO, [Braila] County)'),
      PlatformReachComboBox('Jud Prahova', '(RO, [Ploiesti] County)'),
      PlatformReachComboBox('Jud Maramures', '(RO, [Baia Mare] County)'),
    ];
  }
}

List<DropdownMenuItem<PlatformReachComboBox>>
    buildDropdownMenuItemsPlatformReach(List platforms) {
  List<DropdownMenuItem<PlatformReachComboBox>> items = List();
  for (PlatformReachComboBox platform in platforms) {
    items.add(
      DropdownMenuItem(
        value: platform,
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            Text(
              platform.name,
              style: TextStyle(fontWeight: FontWeight.bold),
              textAlign: TextAlign.start,
            ),
            Text(
              platform.hint,
              style:
                  TextStyle(fontWeight: FontWeight.normal, color: Colors.grey),
              textAlign: TextAlign.end,
            )
          ],
        ),
      ),
    );
  }
  return items;
}

And this is where I encounter the problem, I don't know what to return in ListView for me pressing the button, and then the item to be added in there:
   ListView.builder(
                    padding: EdgeInsets.only(left: 10),
                    scrollDirection: Axis.vertical,
                    shrinkWrap: true,
                    itemCount: _platformReach.length,
                    itemBuilder: (BuildContext ctxt, int Index) {
                      return Text(
                        "Galati"
                      );

                    },
                  ),


A: please try this way... define a list
List<String> mylist = List();

and use it below
   ListView.builder(
                padding: EdgeInsets.only(left: 10),
                scrollDirection: Axis.vertical,
                shrinkWrap: true,
                itemCount: mylist.length,
                itemBuilder: (BuildContext ctxt, int Index) {
                  return Text(
                    mylist[Index]
                  );

                },
              ),

and After button Clicked
setState(() {
   mylist.add(_selectedVisibility.name);
});

Located on 7, Ferrandière Street in Lyon, The Whisky Lodge is a real institution for spirit amateurs from the surroundings of Lyon. Besides its specialization in whisky and rhum, which distinguishes it from the usual whine store, the shop can boast about being a part of France’s malt history.
It’s mostly the story of the Tissandier family. The father, Gabriel, œnologist, started a long career in whines and spirit. He began by realizing blends in the fields of Cognac and Champagne. Then, in 1968, he founds the ‘Caves Tissandier’ (Tissandier Cellars in French