bloc_provider_example.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: BlocProvider(
        bloc: CounterBloc(),
        child: MyHomePage(),
      ),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    CounterBloc _counterBloc = BlocProvider.of<CounterBloc>(context);
    return Scaffold(
      appBar: AppBar(
        title: Text("Bloc Provider"),
        actions: <Widget>[
          CartCount(),
        ],
      ),
      body: Center(
        child: StreamBuilder<int>(
          initialData: 0,
          stream: _counterBloc.outCounter,
          builder: (context, snapshot) {
            return Text(snapshot.data.toString(),
                style: Theme.of(context).textTheme.display1);
          },
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _counterBloc.incrementCount();
        },
        child: Icon(Icons.add),
      ),
    );
  }
}

class CounterBloc implements BlocBase {
  int _counter = 0;

  StreamController<int> _counterController = StreamController<int>.broadcast();

  Stream<int> get outCounter => _counterController.stream;

  @override
  void dispose() {
    _counterController.close();
  }

  void incrementCount() {
    _counter++;
    _counterController.sink.add(_counter);
  }
}

abstract class BlocBase {
  void dispose();
}

// Generic BLoC provider
class BlocProvider<T extends BlocBase> extends StatefulWidget {
  BlocProvider({
    Key key,
    @required this.child,
    @required this.bloc,
  }) : super(key: key);

  final T bloc;
  final Widget child;

  @override
  _BlocProviderState<T> createState() => _BlocProviderState<T>();

  static T of<T extends BlocBase>(BuildContext context) {
    final type = _typeOf<BlocProvider<T>>();
    BlocProvider<T> provider = context.ancestorWidgetOfExactType(type);
    return provider.bloc;
  }

  static Type _typeOf<T>() => T;
}

class _BlocProviderState<T> extends State<BlocProvider<BlocBase>> {
  @override
  void dispose() {
    widget.bloc.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return widget.child;
  }
}

class CartCount extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    CounterBloc _counterBloc = BlocProvider.of<CounterBloc>(context);
    print(_counterBloc);
    return Center(
        child: Stack(
      fit: StackFit.loose,
      children: <Widget>[
        Align(
            alignment: AlignmentDirectional.center,
            child: Padding(
              padding: const EdgeInsets.only(right: 16.0),
              child: Icon(Icons.shopping_cart),
            )),
        Align(
          alignment: AlignmentDirectional.topEnd,
          child: Padding(
            padding: const EdgeInsets.only(left: 16.0),
            child: CircleAvatar(
              backgroundColor: Colors.red,
              radius: 10.0,
              child: StreamBuilder<int>(
                initialData: 0,
                stream: _counterBloc.outCounter,
                builder: (context, snapshot) {
                  return Text(snapshot.data.toString());
                },
              ),
            ),
          ),
        ),
      ],
    ));
  }
}