An example of a page that does expensive work when recreated every time the keyboard pops on and off

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: MyHomePage());
  }
}

class MyHomePage extends StatelessWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Test Expensive Work'),
      ),
      body: Center(
        child: RaisedButton(
          child: Text('Open Page'),
          onPressed: () => _openExpensivePage(context),
        ),
      ),
    );
  }

  _openExpensivePage(BuildContext context) {
    Navigator.push(
        context,
        MaterialPageRoute(
            builder: (BuildContext context) =>
                ExpensivePage(bloc: ExpensivePageBloc())));
  }
}

class ExpensivePage extends StatefulWidget {
  ExpensivePage({@required this.bloc}) : assert(bloc != null);

  final ExpensivePageBloc bloc;

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

class _ExpensivePageState extends State<ExpensivePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Padding(
        padding: const EdgeInsets.all(32.0),
        child: Center(
          child: TextField(
            decoration: InputDecoration(hintText: 'Enter some text'),
          ),
        ),
      ),
    );
  }
}

class ExpensivePageBloc {
  ExpensivePageBloc() {
    print('!!! doing something expensive');
  }
}

When clicking the TextField and then hitting Back a couple of times the Bloc is created every time:

13:57:32.370 4 info flutter.tools I/flutter (17940): !!! doing something expensive
13:57:34.124 5 info flutter.tools I/flutter (17940): !!! doing something expensive
13:57:35.629 6 info flutter.tools I/flutter (17940): !!! doing something expensive
13:57:36.997 7 info flutter.tools I/flutter (17940): !!! doing something expensive
13:57:38.834 8 info flutter.tools I/flutter (17940): !!! doing something expensive
13:57:40.135 9 info flutter.tools I/flutter (17940): !!! doing something expensive