Flutter/package:provider

package:providerとは

テストしやすいように状態とロジックを分離したもの。 (provider と ChangeNotifier)

公式

https://pub.dev/packages/provider

インストール

pubspec.yaml

dependencies:
  provider: ^4.0.3

package:provider対応前

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> {
  int _counter = 0;
  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }
  @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:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

package:provider対応後

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
class CounterStore with ChangeNotifier {
  var counter = 0;
  void incrementCounter() {
    counter++;
    notifyListeners();
  }
}
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: ChangeNotifierProvider(
        create: (context) => CounterStore(),
        child: MyHomePage(title: 'Flutter Demo Home Page'),
      ),
    );
  }
}
class MyHomePage extends StatelessWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;
  @override
  Widget build(BuildContext context) {
    return Consumer<CounterStore>(
      builder: (context, counterStore, _) {
        return Scaffold(
          appBar: AppBar(
            title: Text(title),
          ),
          body: Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Text(
                  'You have pushed the button this many times:',
                ),
                Text(
                  '${counterStore.counter}',
                  style: Theme.of(context).textTheme.display1,
                ),
              ],
            ),
          ),
          floatingActionButton: FloatingActionButton(
            onPressed: counterStore.incrementCounter,
            tooltip: 'Increment',
            child: Icon(Icons.add),
          ),
        );
      },
    );
  }
}

変更箇所

MyHomePage が StatelessWidget になってる。 counterがcounterStoreのcounterへ

参考

https://ja.unflf.com/flutter/provider/