Flutter：我应该如何使用 InheritedWidget？

Question

简介：我认为这个问题原则上可能是共享的，但我觉得这个具体的应用程序不是。我谦虚地提交它以供考虑，所以请温柔。我的代码在这里偶然发现了一个明显的陷阱，我根本不知道如何修复它。考虑了几个小时后，我什至不知道该怎么做。

问题：InheritedWidget 没有更新其上方小部件的状态。

期望：我希望当我在 HomePage 的 PageView 上滑动时，Text BottomAppBar 将更新。但是，这不会发生。

描述：我正在尝试使用一个 InheritedWidget (InheritedPageIndex)，它是在根小部件 (MaterialApp) via 一个 Provider 方法（请参阅下面的代码）。其中，有一个值会向下传递到小部件树中它下方的小部件 (MenuPage)。我希望这个值在状态改变时更新；我尝试通过在小部件树中它下方的小部件中滑动 PageView (HomePage) 来进行修改。

代码：我已经重写并简化了我的代码，以便我们可以更轻松地诊断和解决问题，它看起来像这样：

page_index_model.dart

import 'package:flutter/material.dart';

    class PageIndex {
      PageIndex({@required this.index});
      double index = 0;
    }

page_index_provider.dart

import 'package:flutter/cupertino.dart';
import 'package:testinginheritedwidget/models/page_index_model.dart';

class InheritedPageIndex extends InheritedWidget {
  InheritedPageIndex({Key key, @required this.indexData, Widget child})
      : super(key: key, child: child);

  final PageIndex indexData;

  @override
  bool updateShouldNotify(InheritedWidget oldWidget) => true;

  static PageIndex of(BuildContext context) => context
      .dependOnInheritedWidgetOfExactType<InheritedPageIndex>()
      .indexData;
}

ma​​in.dart

import 'package:flutter/material.dart';
import 'package:testinginheritedwidget/models/page_index_model.dart';
import 'package:testinginheritedwidget/pages/menu_page.dart';
import 'package:testinginheritedwidget/providers/page_index_provider.dart';

    void main() => runApp(App());

    class App extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return InheritedPageIndex(
          indexData: PageIndex(index: 0),
          child: MaterialApp(
            debugShowCheckedModeBanner: false,
            title: 'Testing Inherited Widgets',
            home: MenuPage(),
          ),
        );
      }
    }

menu_page.dart

    import 'package:flutter/material.dart';
    import 'package:testinginheritedwidget/models/page_index_model.dart';
    import 'package:testinginheritedwidget/pages/home_page.dart';
    import 'package:testinginheritedwidget/providers/page_index_provider.dart';

    class MenuPage extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        PageIndex _pageIndex = InheritedPageIndex.of(context);

        return DefaultTabController(
          length: 3,
          child: Scaffold(
            //TODO: dot indicator
            resizeToAvoidBottomInset: false,
            appBar: AppBar(
              backgroundColor: Colors.black,
              title: Center(
                child: Text('Testing Inherited Widgets',
                    style: TextStyle(fontSize: 20.0, fontWeight: FontWeight.bold)),
              ),
              bottom: TabBar(
                onTap: (tabIndex) {},
                labelColor: Colors.white,
                indicatorColor: Colors.white,
                tabs: const <Widget>[
                  Tab(icon: Icon(Icons.flight), child: Text('Flight')),
                  Tab(icon: Icon(Icons.map), child: Text('Map')),
                  Tab(icon: Icon(Icons.print), child: Text('Print')),
                ],
              ),
            ),
            body: SafeArea(
              child: TabBarView(
                children: <Widget>[
                  HomePage(),
                  HomePage(),
                  HomePage(),
                ],
              ),
            ),
            bottomNavigationBar: BottomAppBar(
              child: Row(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
// *** PROBLEM 1 IS HERE ***
                  Text('${_pageIndex.index}'),
                ],
              ),
            ),
          ),
        );
      }
    }

home_page.dart

    import 'package:flutter/material.dart';
    import 'package:testinginheritedwidget/models/page_index_model.dart';
    import 'package:testinginheritedwidget/providers/page_index_provider.dart';

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

    class _HomePageState extends State<HomePage> {
      List<Widget> cards = [];

      @override
      Widget build(BuildContext context) {
        PageIndex _pageIndex = InheritedPageIndex.of(context);

        for (var index = 0; index < 7; index++) {
          cards.add(
            Center(
              child: Text(
                '$index',
                style: TextStyle(fontSize: 25, fontWeight: FontWeight.bold),
              ),
            ),
          );
          print(index);
        }
        return PageView(
          onPageChanged: (index) {
            setState(() {
//*** PROBLEM 2 IS HERE ***
              _pageIndex.index = index.toDouble();
              print('page index: ${_pageIndex.index}');
            });
          },
          children: cards,
        );
      }
    }

假设：经过大量研究和反思。我相信我无法更新 Provider 的价值；因为它在第一个实例化的 InheritedWidget 和改变状态的方法之间。也许另一种说法是，我想改变的状态就在它的正上方；并且，它取而代之的是第二个提供者的状态，而不是使用第一个？

问题：

1)为了我的利益，我的假设是正确的，还是这里发生了其他事情？

2) 如何解决我的问题？我不认为实现 BLoC 方法可以解决这个问题，因为它也使用了提供程序？

在这种情况下，如果没有 InheritedWidget，我不知道如何将状态向上传递到小部件树。

Answer 1

你根本不需要一个包。一个有点hacky的方法是在inheritedWidget周围创建一个Wrapper，因为继承的小部件不会自行重建，您需要在有状态的小部件中使用它。

class MyWrapper extends StatefulWidget {
  const MyWrapper({this.child});
  final Widget child;
  @override
  _MyWrapperState createState() => _MyWrapperState();
}

class _MyWrapperState extends State<MyWrapper> {
  int index = 0;

  changePageIndex(int newIndex) {
    setState(() {
      index = newIndex;
    });
  }

  @override
  Widget build(BuildContext context) {
    return InheritedPageIndex(indexData: index , child: widget.child, state: this);
  }
}

class InheritedPageIndex extends InheritedWidget {
  InheritedPageIndex(
      {Key key, @required this.indexData, @required this.state, Widget child})
      : super(key: key, child: child);
  final _MyWrapperState state;
  final int indexData;

  @override
  bool updateShouldNotify(InheritedPageIndex oldWidget) =>
      indexData != oldWidget.indexData;

  static InheritedPageIndex of(BuildContext context) =>
      context.dependOnInheritedWidgetOfExactType<InheritedPageIndex>();
}

那就这样用吧

MyWrapper(
  child: MaterialApp(
    debugShowCheckedModeBanner: false,
    title: 'Testing Inherited Widgets',
    home: MenuPage(),
  ),
);

最后像这样访问/更改它

int indexData = InheritedPageIndex.of(context).indexData; //we access the variable

InheritedPageIndex.of(context).state.changePageIndex(1); //We change it, and all the widgets that accessed the data with InheritedPageIndex.of(context).indexData rebuild themselves

Answer 2

包解决方案

简介：为了彻底起见，我已将代码作为使用 Provider 包的最终答案包含在内，如上面的 cmets 中所建议的那样。如您所料，此代码比上面不使用包的答案更简单。

代码：

ma​​in.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:stackoverflowquestionthree/counter_model.dart';
import 'package:stackoverflowquestionthree/page_menu.dart';

void main() => runApp(App());

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Testing Inherited Widgets',
      home: ChangeNotifierProvider(
        create: (BuildContext context) => Counter(0),
        child: MenuPage(),
      ),
    );
  }
}

counter_model.dart

import 'package:flutter/material.dart';

class Counter with ChangeNotifier {
  int _counter;

  Counter(this._counter);

  getCounter() => _counter;

  setCounter(int counter) => _counter = counter;

  void increment(int counter) {
    _counter = counter;
    notifyListeners();
  }
}

page_menu.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:stackoverflowquestionthree/counter_model.dart';
import 'package:stackoverflowquestionthree/home_page.dart';

class MenuPage extends StatefulWidget {
  @override
  _MenuPageState createState() => _MenuPageState();
}

class _MenuPageState extends State<MenuPage> {
  var _bloc;

  @override
  Widget build(BuildContext context) {
    final counter = Provider.of<Counter>(context);

    return DefaultTabController(
      length: 3,
      child: Scaffold(
        //TODO: dot indicator
        resizeToAvoidBottomInset: false,
        appBar: AppBar(
          backgroundColor: Colors.black,
          title: Center(
            child: Text('Testing Inherited Widgets',
                style: TextStyle(fontSize: 20.0, fontWeight: FontWeight.bold)),
          ),
          bottom: TabBar(
            onTap: (tabIndex) {
              counter.increment(0);
            },
            labelColor: Colors.white,
            indicatorColor: Colors.white,
            tabs: const <Widget>[
              Tab(icon: Icon(Icons.flight), child: Text('Flight')),
              Tab(icon: Icon(Icons.map), child: Text('Map')),
              Tab(icon: Icon(Icons.print), child: Text('Print')),
            ],
          ),
        ),
        body: SafeArea(
          child: TabBarView(
            children: <Widget>[
              HomePage(),
              HomePage(),
              HomePage(),
            ],
          ),
        ),
        bottomNavigationBar: BottomAppBar(
          child: Row(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              //TODO: insert value below.
              Text(counter.getCounter().toString()),
            ],
          ),
        ),
      ),
    );
  }
}

home_page.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:stackoverflowquestionthree/counter_model.dart';

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

class _HomePageState extends State<HomePage> {
  List<Widget> cards = [];

  @override
  Widget build(BuildContext context) {
    final counter = Provider.of<Counter>(context);

    for (var index = 0; index < 7; index++) {
      cards.add(
        Center(
          child: Text(
            '$index',
            style: TextStyle(fontSize: 25, fontWeight: FontWeight.bold),
          ),
        ),
      );
      print(index);
    }
    return PageView(
      onPageChanged: (index) {
        setState(() {
          counter.increment(index);
        });
      },
      children: cards,
    );
  }
}

Answer 3

您应该检查这个包：https://pub.dev/packages/provider，它是 InheritedWidget 的包装器。并检查ChangeNotiferProvider，它公开了ChangeNotifier 类，并允许在更新时通知侦听器。如果您需要更多解释，我随时待命