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

void main() {
  runApp(
    ChangeNotifierProvider(
      create: (_) => Counter(),
      child: const MyApp(),
    ),
  );
}

class Counter with ChangeNotifier {
  int _count = 0;

  int get count => _count;

  void increment() {
    _count++;
    notifyListeners();
  }
}

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

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('轻松上手provider'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          mainAxisAlignment: MainAxisAlignment.center,
          children: const <Widget>[
            Text('点击按钮看看效果。'),

            /// 性能优化：这里将会频繁触发build的操作封装到单独的widget中来避免对整个页面频繁的build
            Count(),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        key: const Key('点我'),

        ///性能优化： 在这里调用`context.read`而不是`context.watch`，主要是为了避免当Counter变化时而带来的rebuild
        onPressed: () => context.read<Counter>().increment(),
        tooltip: '+1',
        child: const Icon(Icons.add),
      ),
    );
  }
}

class Count extends StatelessWidget {
  const Count({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Text(

        ///使用 `context.watch` 可以在`Counter`改变之后重新构建Count
        '${context.watch<Counter>().count}',
        key: const Key('counterState'),
        style: Theme.of(context).textTheme.headline4);
  }
}