simple push and pop demo with return

push_pop.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(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Push / Pop demo'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _navigateToSecond() async {
    int ret = await Navigator.of(context)
        .push(MaterialPageRoute<int>(builder: (BuildContext context) {
      return SecondPage(counter: _counter);
    }));
    setState(() {
      // null が返された場合は、元の値を表示
      _counter = ret == null ? _counter : ret;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button on Second Page:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _navigateToSecond,
        tooltip: 'To Second Page',
        child: Icon(Icons.forward),
      ),
    );
  }
}

class SecondPage extends StatefulWidget {
  SecondPage({Key key, this.counter}) : super(key: key);

  final int counter;

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

class _SecondPageState extends State<SecondPage> {
  int _counter;

  @override
  void initState() {
    super.initState();
    _counter = widget.counter;
  }

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Second Page'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
            FloatingActionButton(
              onPressed: _incrementCounter,
              tooltip: 'Increment',
              child: Icon(Icons.add),
            ),
            Row(
              children: <Widget>[
                Expanded(
                  flex: 1,
                  child: RaisedButton(
                    // counter をリセット
                    onPressed: () => Navigator.of(context).pop(0),
                    child: Icon(Icons.cancel),
                    color: Colors.red,
                  ),
                ),
                Expanded(
                  flex: 1,
                  child: RaisedButton(
                    onPressed: () => Navigator.of(context).pop(_counter),
                    child: Icon(Icons.check_circle),
                    color: Colors.green,
                  ),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}