flutter_controller_example.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(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> {
  final MyController _myController = MyController();

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

  @override
  Widget build(BuildContext context) {
    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:',
            ),
            MyWidget(
              controller: _myController,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _myController.onIncrement();
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

class MyController {
  VoidCallback onIncrement;

  void dispose() {
    onIncrement = null;
  }
}

class MyWidget extends StatefulWidget {
  const MyWidget({
    @required this.controller,
  }) : assert(controller != null);

  final MyController controller;

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

class _MyWidgetState extends State<MyWidget> {
  @override
  void initState() {
    super.initState();
    widget.controller.onIncrement = _incrementCounter;
  }

  int _counter = 0;

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

  @override
  Widget build(BuildContext context) {
    return Text(
      '$_counter',
      style: Theme.of(context).textTheme.headline3,
    );
  }
}