myhomie_app/lib/main.dart

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:mqtt_client/mqtt_client.dart';
import 'package:mqtt_client/mqtt_server_client.dart';

import 'constants.dart';

void main() {
  String initialRoute;

  initialRoute = '/home';

  final MyApp myApp = MyApp(
    initialRoute: initialRoute,
    //context: localContext,
  );

  runApp(myApp);
}

class MyApp extends StatefulWidget {
  final String initialRoute;
  //final Context context;
  MyApp({this.initialRoute});

  // This widget is the root of your application.
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        debugShowCheckedModeBanner: false,
        title: 'MyHomie App Demo',
        initialRoute: widget.initialRoute,
        /*supportedLocales: [
          const Locale('en', 'US'),
          //const Locale('fr', 'FR'),
        ],*/
        theme: ThemeData(
          primarySwatch: Colors.blue,
          scaffoldBackgroundColor: kBackgroundColor,
          //fontFamily: "Vollkorn",
          textTheme: TextTheme(bodyText1: TextStyle(color: kBodyTextColor)),
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        routes: {
          '/home': (context) => MyHomePage(title: 'MyHomie App Demo Home Page')
        }
    );
  }
}

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;
  final MqttServerClient client = MqttServerClient.withPort('192.168.31.140', 'flutter_client00', 1883); //192.168.31.140

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

      const pubTopic = 'topic/test';
      final builder = MqttClientPayloadBuilder();
      builder.addString('Hello MQTT');
      client.publishMessage(pubTopic, MqttQos.atLeastOnce, builder.payload);
    });
  }

  // connection succeeded
  void onConnected() {
    print('Connected !!!!!!!!!!!! ----------------------------------');

    client.updates.listen((List<MqttReceivedMessage<MqttMessage>> c) {
      final MqttPublishMessage message = c[0].payload;
      final payload = MqttPublishPayload.bytesToStringAsString(message.payload.message);
      print('Received message:$payload from topic: ${c[0].topic}>');
    });
  }

// unconnected
  void onDisconnected() {
    print('Disconnected');
  }

// subscribe to topic succeeded
  void onSubscribed(String topic) {
    print('Subscribed topic: $topic');
  }

// subscribe to topic failed
  void onSubscribeFail(String topic) {
    print('Failed to subscribe $topic');
  }

// unsubscribe succeeded
  void onUnsubscribed(String topic) {
    print('Unsubscribed topic: $topic');
  }

  Future<MqttServerClient> connect() async {
    client.logging(on: false);
    client.keepAlivePeriod = 20;
    client.onDisconnected = onDisconnected;
    client.onConnected = onConnected;
    client.onSubscribed = onSubscribed;

    /// Security context
    /*SecurityContext context = new SecurityContext()
      ..useCertificateChain('path/to/my_cert.pem')
      ..usePrivateKey('path/to/my_key.pem', password: 'key_password')
      ..setClientAuthorities('path/to/client.crt', password: 'password');*/
    //client.setProtocolV31();
    //client.secure = true;
    //client.securityContext = context;

    final connMessage = MqttConnectMessage()
    /*.authenticateAs('thomas', 'MyCore,1')
     .keepAliveFor(60)
    .withWillTopic('willtopic')
        .withWillMessage('Will message')*/
    .withClientIdentifier("TESSST")
        .startClean();
        //.withWillQos(MqttQos.atLeastOnce);
    //client.secure = true;
    client.connectionMessage = connMessage;
    try {
      await client.connect();
    } catch (e) {
      print('Exception: $e');
      client.disconnect();
    }

    client.subscribe("#", MqttQos.atLeastOnce);

    return client;
  }

  @override
  Widget build(BuildContext context) {
    connect();
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      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.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}