main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: PageView.builder(
          itemBuilder: (context, position) {
            Key key = ValueKey(position);
            return Level(key);
          },
          itemCount: 3,
        ),
      ),
    );
  }
}

class Level extends StatefulWidget {
  Level(Key key) : super(key: key);

  @override
  _LevelState createState() => _LevelState();
}

class _LevelState extends State<Level> {
  static const MaterialColor COLOR_GAME = Colors.deepPurple;
  static const MaterialColor COLOR_SUCCESS = Colors.green;

  static const STATE_PLAYING = 'playing';
  static const STATE_WON = 'won';

  String gameState;

  _LevelState() {
    gameState = STATE_PLAYING;
  }

  @override
  void dispose() {
    print("Disposing of level, state = " + gameState);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    MaterialColor headerColor = COLOR_GAME;
    String textToDisplay = "Click the toggle to win";
    if (gameState == STATE_WON) {
      headerColor = COLOR_SUCCESS;
      textToDisplay = "You won!";
    }

    return Column(children: <Widget>[
      Material(
        color: headerColor[300],
        child: InkWell(
          child: Padding(
            padding: const EdgeInsets.all(8.0),
            child: Text(textToDisplay),
          ),
        ),
      ),
      Expanded(
        child: SwitchListTile(
          title: Text("Make me win"),
          value: gameState != STATE_PLAYING,
          onChanged: gameState != STATE_PLAYING
              ? null
              : (bool value) {
            setState(() {
              gameState = STATE_WON;
            });
          },
        ),
      )
    ]);
  }
}