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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: MyHomePage());
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  /// Background color for the ExpansionTile
  Color _backgroundColor = randomColor();

  /// Generate random color for the background
  static Color randomColor() => new Color(new Random().nextInt(0xffffffff));

  /// Change event from the ExpansionTile
  void _onExpansion(bool value) {
    /// Change background color. The ExpansionTile doesn't change to the new
    /// _backgroundColor value. The Text element does.
    setState(() {
      _backgroundColor = randomColor();
    });

    print("_onExpansion: value=" + value.toString());
    print("_onExpansion: _backgroundColor=" + _backgroundColor.toString());
  }

  @override
  Widget build(BuildContext context) {
    List<Widget> children = <Widget>[
      /// This view updates to _backgroundColor.
      Text("_backgroundColor=" + _backgroundColor.toString(),
          style: TextStyle(
            color: _backgroundColor,
          )),

      /// The ExpansionTile doesn't change to the new color value.
      ExpansionTile(
        title: Text("_backgroundColor=" + _backgroundColor.toString(),
            style: TextStyle(color: Color(0xff000000))),

        /// backgroundColor: This value will not be updated...
        backgroundColor: _backgroundColor,
        onExpansionChanged: _onExpansion,
        children: <Widget>[
          Text("Text 1"),
          Text("Text 2"),
          Text("Text 3"),
        ],
      )
    ];

    return Scaffold(
        appBar: AppBar(title: Text('Flutter Demo Home Page')),
        body: Center(
            child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          crossAxisAlignment: CrossAxisAlignment.start,
          children: children,
        )));
  }
}