Flutter: provider 使用小部件的小部件构建的依赖注入系统

• 文档

dependencies:
  provider:

import 'package:dart_printf/dart_printf.dart';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class Counter with ChangeNotifier {
  Counter() {
    printf('init');
  }
  int _n = 0;
  int get n => _n;
  set n(v) {
    _n = v;
    notifyListeners();
  }
}

void main() {
  var _app = MultiProvider(
    providers: [
      ChangeNotifierProvider(create: (_) => Counter()),

      // var counter = Counter();
      // ChangeNotifierProvider.value(value: counter),
    ],
    child: MyApp(),
  );
  runApp(_app);
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home Page'),
      ),
      body: Column(
        children: [
          Center(
            child: Text('home page ${context.watch<Counter>().n}'),
          ),
          W1(),
          W2(),
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          context.read<Counter>().n++;
        },
      ),
    );
  }
}

class W1 extends StatefulWidget {
  @override
  _W1State createState() => _W1State();
}

class _W1State extends State<W1> {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 100,
      height: 100,
      child: Text(context.watch<Counter>().n.toString()),
    );
  }
}

class W2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 100,
      height: 100,
      child: Text(context.watch<Counter>().n.toString()),
    );
  }
}