Creating a custom Flutter DropDown button as a separate widget

custom_dropdown.dart

import 'package:flutter/material.dart';

class TestPage extends StatefulWidget {
  @override
  _TestPageState createState() => _TestPageState();
}

class _TestPageState extends State<TestPage> {
  List<Activities> _activities = [
    Activities(id: 1, icon: Icons.place, title: 'Travel'),
    Activities(id: 2, icon: Icons.food_bank, title: 'Eat'),
  ];
  List<DropdownMenuItem<Activities>> activityListDropDownItems = [];
  Activities selectedActivity;
  int selectedActivityId = 0;
  String selectedActivityTitle = '';
  IconData selectedActivityIcon = Icons.addchart;

  List<DropdownMenuItem<Activities>> buildActivityList(List activities) {
    List<DropdownMenuItem<Activities>> items = [];
    for (Activities activity in activities) {
      items.add(
        DropdownMenuItem(
          value: activity,
          child: Row(
            children: [
              Text(
                '${activity.title}',
                style: TextStyle(
                  color: Colors.red,
                ),
              ),
            ],
          ),
        ),
      );
    }
    return items;
  }

  // VOID
  void onChangeActivityListDropDownItem(Activities selected) {
    setState(() {
      selectedActivity = selected;
      selectedActivityId = selected.id;
      selectedActivityTitle = selected.title;
      selectedActivityIcon = selected.icon;
    });
  }

  @override
  void initState() {
    activityListDropDownItems = buildActivityList(_activities);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Custom DropDown'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('$selectedActivityTitle'),
            SizedBox(height: 20.0),
            // pass the class
            MyDropDown<Activities>(
              hintText: 'Activites',
              icon: selectedActivityIcon,
              value: selectedActivity,
              items: activityListDropDownItems,
              onChanged: onChangeActivityListDropDownItem,
            ),
          ],
        ),
      ),
    );
  }
}

// class that can accept type T
class MyDropDown<T> extends StatelessWidget {
  final List<DropdownMenuItem> items;
  final IconData icon;
  final T value;
  final String hintText;
  // ValueChanged with the passed type
  final ValueChanged<T> onChanged;

  const MyDropDown(
      {Key key,
      @required this.items,
      this.icon,
      this.value,
      this.hintText,
      this.onChanged})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.symmetric(horizontal: 10.0),
      decoration: BoxDecoration(
        color: Colors.white,
        border: Border.all(
          color: Colors.grey,
          width: 0.3,
        ),
        borderRadius: BorderRadius.all(
          Radius.circular(
            30.0,
          ),
        ),
      ),
      child: Row(
        children: [
          SizedBox(width: 10.0),
          Icon(
            icon,
            color: Colors.red.withOpacity(0.7),
          ),
          SizedBox(width: 10.0),
          Expanded(
            // T
            child: DropdownButton<T>(
              hint: Text(
                hintText,
                style: TextStyle(),
              ),
              isExpanded: true,
              value: value,
              items: items,
              onChanged: onChanged,
              underline: Container(),
            ),
          )
        ],
      ),
    );
  }
}

class Activities {
  final int id;
  final IconData icon;
  final String title;

  Activities({@required this.id, @required this.icon, @required this.title});
}