カウンターアプリ

import 'package:flutter/material.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:riverpod_annotation/riverpod_annotation.dart';

part 'main.g.dart';

// A Counter example implemented with riverpod

void main() {
  runApp(
    // Adding ProviderScope enables Riverpod for the entire project
    const ProviderScope(child: MyApp()),
  );
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: Home());
  }
}

/// Annotating a class by `@riverpod` defines a new shared state for your application,
/// accessible using the generated [counterProvider].
/// This class is both responsible for initializing the state (through the [build] method)
/// and exposing ways to modify it (cf [increment]).
@riverpod
class Counter extends _$Counter {
  /// Classes annotated by `@riverpod` **must** define a [build] function.
  /// This function is expected to return the initial state of your shared state.
  /// It is totally acceptable for this function to return a [Future] or [Stream] if you need to.
  /// You can also freely define parameters on this method.
  @override
  int build() => 0;

  void increment() => state++;
  void decrement() => state--;
}

class Home extends ConsumerWidget {
  @override
  Widget build(BuildContext context, WidgetRef ref) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter example')),
      body: Center(
          child: Column(children: [
        Text('${ref.watch(counterProvider)}'),
        OutlinedButton(
            onPressed: () {
              ref.read(counterProvider.notifier).decrement();
            },
            child: Text("-"))
      ])),
      floatingActionButton: FloatingActionButton(
        // The read method is a utility to read a provider without listening to it
        onPressed: () => ref.read(counterProvider.notifier).increment(),
        child: const Icon(Icons.add),
      ),
    );
  }
}