BLOC pattern flutter

counter_bloc.dart

import 'dart:async';

import 'package:bloc_vanilla_tut/counter_event.dart';

class CounterBloc {
  int _counter = 0;

  final _counterStateController = StreamController<int>();
  StreamSink<int> get _inCounter => _counterStateController.sink;
  // For state, exposing only a stream which outputs data
  Stream<int> get counter => _counterStateController.stream;

  final _counterEventController = StreamController<CounterEvent>();
  // For events, exposing only a sink which is an input
  Sink<CounterEvent> get counterEventSink => _counterEventController.sink;

  CounterBloc() {
    // Whenever there is a new event, we want to map it to a new state
    _counterEventController.stream.listen(_mapEventToState);
  }

  void _mapEventToState(CounterEvent event) {
    if (event is IncrementEvent)
      _counter++;
    else
      _counter--;

    _inCounter.add(_counter);
  }

  void dispose() {
    _counterStateController.close();
    _counterEventController.close();
  }
}

counter_event.dart

 
abstract class CounterEvent {}

class IncrementEvent extends CounterEvent {}

class DecrementEvent extends CounterEvent {}

main.dart

import 'package:bloc_vanilla_tut/counter_bloc.dart';
import 'package:bloc_vanilla_tut/counter_event.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 _bloc = CounterBloc();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: StreamBuilder(
          stream: _bloc.counter,
          initialData: 0,
          builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
            return Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Text(
                  'You have pushed the button this many times:',
                ),
                Text(
                  '${snapshot.data}',
                  style: Theme.of(context).textTheme.display1,
                ),
              ],
            );
          },
        ),
      ),
      floatingActionButton: Row(
        mainAxisAlignment: MainAxisAlignment.end,
        children: <Widget>[
          FloatingActionButton(
            onPressed: () => _bloc.counterEventSink.add(IncrementEvent()),
            tooltip: 'Increment',
            child: Icon(Icons.add),
          ),
          SizedBox(width: 10),
          FloatingActionButton(
            onPressed: () => _bloc.counterEventSink.add(DecrementEvent()),
            tooltip: 'Decrement',
            child: Icon(Icons.remove),
          ),
        ],
      ),
    );
  }

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