Stateful widget with globalkey

main.dart

import 'dart:math';

import 'package:flutter/material.dart';

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

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

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _currentPosition = 0;
  static const _maxPosition = 2;
  bool _removed = false;

  void _moveDown() {
    setState(() {
      _currentPosition++;
    });
  }

  void _moveUp() {
    setState(() {
      _currentPosition--;
    });
  }

  void _removeBox() {
    setState(() {
      _removed = true;
    });
  }

  void _createBox() {
    setState(() {
      _removed = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: new AppBar(
          title: new Text(widget.title),
        ),
        body: Column(
          mainAxisAlignment: MainAxisAlignment.end,
          children: <Widget>[
            BoxContainer(_currentPosition == 0 && !_removed),
            BoxContainer(_currentPosition == 1 && !_removed),
            BoxContainer(_currentPosition == 2 && !_removed),
            ButtonsContainer(
                _currentPosition < _maxPosition && !_removed ? _moveDown : null,
                _currentPosition > 0 && !_removed ? _moveUp : null,
                !_removed ? _removeBox : null,
                _removed ? _createBox : null),
          ],
        ));
  }
}

class BoxContainer extends StatelessWidget {
  final bool _withChild;

  BoxContainer(this._withChild);

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 100.0,
      child: _withChild
          ? ColoredBox(
              key: boxKey,
            )
          : null,
    );
  }
}

class ButtonsContainer extends StatelessWidget {
  final VoidCallback _downAction;
  final VoidCallback _upAction;
  final VoidCallback _removeAction;
  final VoidCallback _createAction;

  ButtonsContainer(
      this._downAction, this._upAction, this._removeAction, this._createAction);

  @override
  Widget build(BuildContext context) {
    return Container(
      alignment: Alignment.bottomCenter,
      color: Colors.yellow.withAlpha(80),
      child: Row(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          RaisedButton(
            child: Text('Down'),
            onPressed: _downAction,
          ),
          RaisedButton(
            child: Text('Up'),
            onPressed: _upAction,
          ),
          RaisedButton(
            child: Text(_createAction == null ? 'Remove' : 'Create'),
            onPressed: _createAction == null ? _removeAction : _createAction,
          ),
        ],
      ),
    );
  }
}

final Key boxKey = GlobalKey(debugLabel: 'Colored box key');

class ColoredBox extends StatefulWidget {
  ColoredBox({Key key}) : super(key: key);

  @override
  State createState() => _ColoredBoxState();
}

class _ColoredBoxState extends State {
  int _number;
  Color _color;

  @override
  void initState() {
    _number = Random().nextInt(50);
    _color = Color.fromARGB(255, Random().nextInt(255), Random().nextInt(255),
        Random().nextInt(255));
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      color: _color,
      child: Center(
        child: Text(
          _number.toString(),
          style: Theme.of(context).textTheme.display1,
        ),
      ),
    );
  }
}