ScaffoldMessenger Example

scaffold_messenger_app.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,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {

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

class _MyHomePageState extends State<MyHomePage> {

  bool _isLoading = false;

  void _loadData(BuildContext context) {
    setState(() => _isLoading = !_isLoading);

    Future.delayed(Duration(seconds: 5), () {
      ScaffoldMessenger.of(context).showSnackBar(SnackBar(
        content: Text('Problemas de conexão a internet.'),
      ));
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(_isLoading? 'Loading data...' : 'Page 1'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            Builder(
              builder: (context) {
                return ElevatedButton(child: Text('Navigate to Chat page'),
                  onPressed: () {
                    Navigator.push(context, MaterialPageRoute(
                      builder: (_) => ChatPage()
                    ));
                  },
                );
              }
            ),
            SizedBox(
              height: 50,
              width: 110,
              child: Builder(
                builder: (context) {
                  return ElevatedButton(child: _isLoading ? Padding(
                    padding: const EdgeInsets.symmetric(vertical: 8),
                    child: CircularProgressIndicator(
                      valueColor: AlwaysStoppedAnimation(Colors.white),
                    ),
                  ) : Text('Load Data'),
                    onPressed: () => _loadData(context),
                  );
                }
              ),
            ),
            ElevatedButton(child: Text('Navigate to Users page'),
              onPressed: () {
                Navigator.push(context, MaterialPageRoute(
                    builder: (_) => UsersPage()
                ));
              },
            ),
          ],
        )
      ),
    );
  }
}


class ChatPage extends StatefulWidget {

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

class _ChatPageState extends State<ChatPage> {

  bool _isLoading = false;

  void _loadData(BuildContext context) {
    setState(() => _isLoading = !_isLoading);

    Future.delayed(Duration(seconds: 5), () {
      setState(() => _isLoading = !_isLoading);

      ScaffoldMessenger.of(context).showSnackBar(SnackBar(
        content: Text('Problemas de conexão a internet.'),
      ));
    });
  }

  @override
  Widget build(BuildContext context) {
    return ScaffoldMessenger(
      child: Scaffold(
        appBar: AppBar(
          title: Text('Chat'),
        ),
        body: Center(
          child: Builder(
            builder: (context) {
              if(_isLoading) {
                return CircularProgressIndicator();
              }
              return ElevatedButton(
                child: Text('Load data'),
                onPressed: () => _loadData(context),
              );
            }
          ),
        ),
      ),
    );
  }
}


class UsersPage extends StatelessWidget {

  final style = TextStyle(fontWeight: FontWeight.bold);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Users'),
      ),
      body: ListView.builder(
        itemCount: 30,
        itemBuilder: (context, index) {
          return ListTile(
            leading: CircleAvatar(child: Text('$index', style: style),),
            title: Text('User $index', style: style),
            subtitle: Text('[email protected]'),
          );
        },
      ),
    );
  }
}