counter_cubit.dart

import 'package:flutter_bloc/flutter_bloc.dart';

class CounterCubit extends Cubit<int> {
  CounterCubit() : super(0);
  void increment() => { emit(state + 1) };
  void decrement() => { emit(state - 1) };
  void reset() => { emit(0) };
}

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:kitchensink/counter_cubit.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      // 加筆修正
      home: BlocProvider<CounterCubit>(
        create: (_) => CounterCubit(),
        child: 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++;
    });
  }

  // カウントダウン
  void _decrementCounter() {
    setState(() {
      _counter--;
    });
  }

  // リセット
  void _resetCounter() {
    setState(() {
      _counter = 0;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: BlocBuilder<CounterCubit, int>(builder: (_, count) {
        return Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text(
                'You have pushed the button this many times:',
              ),
              Text(
                '$count',
                style: Theme.of(context).textTheme.headline4,
              ),
            ],
          ),
        );
      }),
      floatingActionButton: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        crossAxisAlignment: CrossAxisAlignment.end,
        children: [
          FloatingActionButton(
            // 修正
            onPressed: () => context.read<CounterCubit>().increment(),
            tooltip: 'Increment',
            child: Icon(Icons.add),
          ),
          SizedBox(height: 5.0),
          FloatingActionButton(
            onPressed: () => context.read<CounterCubit>().decrement(),
            tooltip: 'Decrement',
            child: Icon(Icons.remove),
          ),
          SizedBox(height: 5.0),
          FloatingActionButton(
            onPressed: () => context.read<CounterCubit>().reset(),
            tooltip: 'Reset',
            child: Icon(Icons.refresh),
          )
        ],
      ),
    );
  }
}