some flutter code for imperative vs declarative article

main.dart

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


void main() => runApp(HomePage());

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  final String _title = 'simple ui';
  final String _greetings = 'change my looks!';
  Color _currentColor;
  final Color _initialColor = Colors.deepOrange;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: Scaffold(
        body: Container(
            alignment: Alignment.center,
            decoration: BoxDecoration(color: _currentColor),
            child: ButtonBar(
              children: <Widget>[
                FlatButton(
                  child: Text(_greetings),
                  onPressed: () {
                    var rand = Random();
                    var c = rand.nextInt(3);

                    setState(() {
                      if (c == 0) _currentColor = Colors.green;
                      if (c == 1) _currentColor = Colors.blue;
                      if (c == 2) _currentColor = Colors.red;
                    });
                  },
                )
              ],
            )),
      ),
    );
  }

  @override
  void initState() {
    _currentColor = _initialColor;
    super.initState();
  }
}