在flutter中使用provider时如何避免不必要的重建？

Question

我在我的应用程序中使用提供程序，但是我面临不必要的构建。

例

class AllWidget extends StatelessWidget{

  @override
  Widget build(BuildContext context){
    print('state build called');
    return ChangeNotifierProvider(
            builder: (_) => MyCounter(),
            child: Column(children: <Widget>[
                  MyCounterText(),
                  MyIncreaseButton(),
                  MyDecreaseButton(),
            ],
          ),
    );
  }
}

class MyCounterText extends StatelessWidget{

  @override
  Widget build(BuildContext context) {
    final myCounter = Provider.of<MyCounter>(context, listen: false);
    print('MyCounterText');
    return Text(myCounter.num.toString());

  }
}

class MyIncreaseButton extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
    final myCounter = Provider.of<MyCounter>(context, listen: false);
    print('MyIncreaseButton');
    return RaisedButton(
      child: Text('Increase +'),
      onPressed: ()=> myCounter.increament(),
    );

  }
}


class MyDecreaseButton extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
    final myCounter = Provider.of<MyCounter>(context, listen: false);
    print('MyDecreaseButton');
    return RaisedButton(
      child: Text('Decrease -'),
      onPressed: ()=> myCounter.decreament(),
    );

  }
}

现在，如果我单击MyIncreaseButton小部件以增加值MyDecreaseButton，即使我没有单击它，小部件也将构建。

反之亦然，如果我单击MyDecreaseButton小部件以减小值，那么MyIncreaseButton即使我没有单击它，小部件也将构建。

我的期望是：

单击MyIncreaseButton窗口小部件时，MyDecreaseButton不应构建窗口小部件。

Answer 1

有多种解决方案：

• 使用context.read或传递listen: false给Provider.of：

RaisedButton(
  onPressed: () {
    context.read<MyModel>().increment();
    // alternatively, do:
    Provider.of<MyModel>(context, listen: false).increment();
  },
  child: Child(),
);

• 使用context.select：

Widget build(context) {
  final increment = context.select((MyModel m) => m.increment);

  return RaisedButton(
    onPressed: increment,
    child: Child(),
  ); 
}

• 使用Selector：

Widget build(context) {
  return Selector<MyModel, VoidCallback>(
    selector: (_, model) => model.increment,
    builder: (_, increment) {
      return RaisedButton(
        onPressed: increment,
        child: Child(),
      );
    },
  );
}

Answer 2

选择器正是您所需要的。使用选择器，您可以过滤更新。例如，仅在名称更改时更新，您可以执行以下操作

Selector<AppStore, String>(
  selector: (_, store) => store.name,
  builder: (_, name, __) {
    return Text(name);
  },
);

Answer 3

我只是通过编辑代码来避免不必要的渲染，如下所示：

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

class MyCounter with ChangeNotifier {
  int _num = 0;

  int get num => _num;

  set num(int n) {
    _num = n;
    notifyListeners();
  }

  void increaent() {
    _num = _num + 1;
    notifyListeners();
  }

  void decrement() {
    _num = _num - 1;
    notifyListeners();
  }
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('root build called');
    return ChangeNotifierProvider(
        builder: (context) => MyCounter(),
        child: MaterialApp(
          title: 'MyAppJan',
          home: Scaffold(
            appBar: AppBar(title: Text('Home')),
            body: AllWidget(),
          ),
          theme: ThemeData(primarySwatch: Colors.orange),
        ));
  }
}

class AllWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('state build called');
    return Center(
      child: Column(
        children: <Widget>[
          MyCounterText(),
          SizedBox(height: 10),
          MyIncreaseButton(),
          SizedBox(height: 10),
          MyDecreaseButton(),
        ],
        mainAxisAlignment: MainAxisAlignment.center,
        mainAxisSize: MainAxisSize.min,
      ),
    );
  }
}

class MyCounterText extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('MyCounterText');
    return Consumer<MyCounter>(
      builder: (context, myCounter, _) {
        return Text(myCounter.num.toString());
      },
    );
  }
}

class MyIncreaseButton extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final _items = Provider.of<MyCounter>(context,listen: false);
    print('MyIncreaseButton');
    return RaisedButton(
      child: Text('Increase ++'),
      onPressed: () => _items.increment(),
    );
  }
}

class MyDecreaseButton extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final _items = Provider.of<MyCounter>(context,listen: false);
    print('MyDecreaseButton');
    return RaisedButton(
      child: Text('Decrease --'),
      onPressed: () => _items.decrement(),
    );
  }
}