click_counter.dart

import 'dart:html';
import 'package:flutter/material.dart';

void main() {
  window.document.onContextMenu.listen((evt) => evt.preventDefault());

  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Click Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter 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;

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

  void _decrementCounter() {
    setState(() {
      _counter--;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'The current count is:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
            SizedBox(height: 48.0),
            GestureDetector(
              onTap: () => _incrementCounter(),
              onLongPress: () => _decrementCounter(),
              onSecondaryTap: () => _decrementCounter(),
              child: Container(
                height: 60,
                width: 200,
                color: Colors.grey.shade300,
                child: Center(
                  child: Text(
                    "Click left to increment\nClick right to decrement",
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}