internals_basic.dart

import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      home: ApplicationSample(),
    ),
  );
}

class ApplicationSample extends StatefulWidget {
  @override
  _ApplicationSampleState createState() => _ApplicationSampleState();
}

class _ApplicationSampleState extends State<ApplicationSample> {
  final Key _keyGesture = Key("MyFakeButton");
  bool _isOn = false;
  
  @override
  Widget build(BuildContext context) {
    return Material(
      child: Center(
        child: Stack(
          alignment: Alignment.center,
          children: <Widget>[
            _isOn ? ChildOn() : ChildOff(),
            GestureDetector(
              key: _keyGesture,
              onTap: (){
                setState((){
                  _isOn = !_isOn;
                });
              },
              child: const Icon(Icons.swap_horizontal_circle),
            ),
          ],
        ),
      ),
    );
  }
}

class ChildOn extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 150.0,
      height: 150.0,
      color: Colors.green,
    );
  }
}

class ChildOff extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 100.0,
      height: 100.0,
      color: Colors.red,
    );
  }
}