Get a Result Back

Part of the Lists and Screens section of Coddy's Flutter journey. Lesson 22 of 33.

Some screens exist to ask a question: pick a color, pick a date. The answer has to travel back to the screen that asked. Navigator.pop can carry it:

Navigator.pop(context, 'Green');

On the other side, Navigator.push gives back a Future: a value that arrives later, when the pushed screen is popped. await waits for it, and the function that waits is marked async:

onPressed: () async {
  final String? color = await Navigator.push(
    context,
    MaterialPageRoute(builder: (context) => const ColorScreen()),
  );
  if (color != null) {
    setState(() {
      chosen = color;
    });
  }
},

The result is a String?: when someone leaves with the back arrow, nothing is sent and color is null. The if keeps the old choice then. A later section tells the whole story of Future and await.

Challenge

Medium

Picking a color closes the color screen, but the home screen never hears about it. Make the choice come back:

on the color screen, pop with the color: Navigator.pop(context, color)
on the home screen, await the push and, when a color came back, show Your color: Green (with the color picked)
leaving with the back arrow keeps the old text

Try it yourself

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: HomeScreen(),
    );
  }
}

class HomeScreen extends StatefulWidget {
  const HomeScreen({super.key});

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  String chosen = 'none';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Paint')),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            Text('Your color: $chosen', style: const TextStyle(fontSize: 24)),
            const SizedBox(height: 16),
            ElevatedButton(
              onPressed: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(builder: (context) => const ColorScreen()),
                );
              },
              child: const Text('Choose a color'),
            ),
          ],
        ),
      ),
    );
  }
}

class ColorScreen extends StatelessWidget {
  const ColorScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Pick a color')),
      body: ListView(
        children: [
          for (String color in ['Red', 'Green', 'Blue'])
            ListTile(
              title: Text(color),
              onTap: () {
                Navigator.pop(context);
              },
            ),
        ],
      ),
    );
  }
}