simple state management

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: BrowserPage(),
    );
  }
}

class BrowserController extends ChangeNotifier {
  int _likeCounter = 0;
  int get likeCounter => _likeCounter;

  void incrementCounter() {
    _likeCounter++;
    notifyListeners();
  }
}

class BrowserPage extends StatefulWidget {
  @override
  _BrowserPageState createState() => _BrowserPageState();
}

class _BrowserPageState extends State<BrowserPage> {
  BrowserController _controller;

  @override
  void initState() {
    _controller = BrowserController();
    super.initState();
  }

  @override
  void setState(fn) {
    assert(false, 'calling setState should not be needed');
    super.setState(fn);
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Column(
          children: <Widget>[
            // Notice that we are not calling setState anywhere
            // AnimatedBuilder will update itself when BrowserController notifies listeners
            AnimatedBuilder(
              animation: _controller,
              builder: (context, _) {
                return Text('${_controller.likeCounter}');
              },
            ),
            FlatButton(
              child: Text('Increment'),
              onPressed: _controller.incrementCounter,
            )
          ],
        ),
      ),
    );
  }
}