Testing async await future after widget tree is diposed.

future_widget_dispose.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

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

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

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

class _FirstRouteState extends State<FirstRoute> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('First Route'),
      ),
      body: Center(
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            RaisedButton(
              child: Text('Open route'),
              onPressed: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(
                      builder: (context) =>
                          SecondRoute(initialValue: 'Home Page')),
                );
              },
            ),
          ],
        ),
      ),
    );
  }
}

class SecondRoute extends StatefulWidget {
  SecondRoute({Key key, this.initialValue}) : super(key: key);
  String initialValue;

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

class _SecondRouteState extends State<SecondRoute> {
  Future<String> createOrderMessage() async {
    var order = await fetchUserOrder();
    return 'Your order is: $order';
  }

  Future<String> fetchUserOrder() {
    return Future.delayed(
      Duration(seconds: 5),
      () => 'Large Latte',
    );
  }

  @override
  void initState() {
    print("initState()");
    super.initState();
  }

  @override
  void dispose() {
    print("dispose()");
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.initialValue),
      ),
      body: Center(
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            RaisedButton(
              child: Text('Go back'),
              onPressed: () {
                Navigator.pop(context);
              },
            ),
            RaisedButton(
              child: Text('Call await future'),
              onPressed: () async {
                final _someValue = await createOrderMessage();
                print("value is: $_someValue");
                if (this.mounted) {
                  setState(() {
                    widget.initialValue = _someValue;
                  });
                }
              },
            ),
          ],
        ),
      ),
    );
  }
}