如何从 Flutter 中的另一个路由/页面增加计数器

Posted 2023-02-19

【中文标题】如何从 Flutter 中的另一个路由/页面增加计数器

【英文标题】：How to increment a counter from another route/page in Flutter

【发布时间】：2022-01-15 15:40:03

【问题描述】：

我正在使用 Bloc/Cubit (flutter_bloc) 和 Auto_Router 包

我在 /counter 路由上有计数器，在 /user_profile 路由上有 FloatingActionButtons 来增加计数器。

如何从不同的页面/路由增加计数器？我应该能够在页面之间来回切换并从任一页面增加/减少，但我收到以下错误消息：

发生了异常。 ProviderNotFoundException（错误：在此 UserProfilePage 小部件上方找不到正确的提供程序

发生这种情况是因为您使用了不包含提供程序的 BuildContext 你的选择。有几种常见的情况：

您在 main.dart 中添加了一个新提供程序并执行了热重载。 要修复，请执行热重启。

您尝试读取的提供程序位于不同的路径中。

提供者是“范围的”。因此，如果您在路线内插入提供者，那么 其他路由将无法访问该提供程序。

您使用了 BuildContext，它是您尝试读取的提供程序的祖先。

确保 UserProfilePage 在您的 MultiProvider/Provider 下。 这通常发生在您创建提供程序并尝试立即读取它时。

例如，而不是：

Widget build(BuildContext context) 
  return Provider<Example>(
    create: (_) => Example(),
    // Will throw a ProviderNotFoundError, because `context` is associated
    // to the widget that is the parent of `Provider<Example>`
    child: Text(context.watch<Example>()),
  ),

考虑像这样使用builder：

Widget build(BuildContext context) 
  return Provider<Example>(
    create: (_) => Example(),
    // we use `builder` to obtain a new `BuildContext` that has access to the provider
    builder: (context) 
      // No longer throws
      return Text(context.watch<Example>()),
    
  ),

如果这些解决方案都不起作用，请考虑在 *** 上寻求帮助： https://***.com/questions/tagged/flutter )

这是我的代码：

home_page.dart

class HomePage extends StatelessWidget 
  const HomePage(Key? key) : super(key: key);

  @override
  Widget build(BuildContext context) 
    return AutoTabsScaffold(
      appBarBuilder: (_, tabsRouter) => AppBar(
        backgroundColor: Colors.indigo,
        title: const Text('FlutterBottomNav'),
        centerTitle: true,
        leading: const AutoBackButton(),
      ),
      backgroundColor: Colors.teal,
      routes: const [
        CounterRouter(),
        PostsRouter(),
        UsersRouter(),
        SettingsRouter(),
      ],
      bottomNavigationBuilder: (_, tabsRouter) 
        return SalomonBottomBar(
          margin: const EdgeInsets.symmetric(
            horizontal: 20,
            vertical: 40,
          ),
          currentIndex: tabsRouter.activeIndex,
          onTap: tabsRouter.setActiveIndex,
          items: [
            SalomonBottomBarItem(
              selectedColor: Colors.amberAccent,
              icon: const Icon(Icons.plus_one, size: 30),
              title: const Text('Counter'),
            ),
            SalomonBottomBarItem(
              selectedColor: Colors.amberAccent,
              icon: const Icon(Icons.post_add, size: 30),
              title: const Text('Posts'),
            ),
            SalomonBottomBarItem(
              selectedColor: Colors.blue[200],
              icon: const Icon(
                Icons.person,
                size: 30,
              ),
              title: const Text('Users'),
            ),
            SalomonBottomBarItem(
              selectedColor: Colors.pinkAccent[100],
              icon: const Icon(
                Icons.settings,
                size: 30,
              ),
              title: const Text('Settings'),
            ),
          ],
        );
      ,
    );
  

counter_cubit.dart

class CounterCubit extends Cubit<int> 
  CounterCubit() : super(0);

  void increment() => emit(state + 1);
  void decrement() => emit(state - 1);

counter_page.dart

class CounterPage extends StatelessWidget 
  const CounterPage(Key? key) : super(key: key);

  @override
  Widget build(BuildContext context) 
    return BlocProvider(
      create: (_) => CounterCubit(),
      child: const CounterView(),
    );
  


class CounterView extends StatelessWidget 
  const CounterView(Key? key) : super(key: key);

  @override
  Widget build(BuildContext context) 
    final l10n = context.l10n;
    return Scaffold(
      appBar: AppBar(title: Text(l10n.counterAppBarTitle)),
      body: const Center(child: CounterText()),
      floatingActionButton: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        crossAxisAlignment: CrossAxisAlignment.end,
        children: [
          FloatingActionButton(
            key: const Key('counterView_star_floatingActionButton'),
            onPressed: () 
              print('star');
            ,
            child: const Icon(Icons.star),
          ),
          const SizedBox(height: 8),
          FloatingActionButton(
            key: const Key('counterView_boat_floatingActionButton'),
            onPressed: () 
              print('boat');
            ,
            child: const Icon(Icons.sailing),
          ),
          FloatingActionButton(
            key: const Key('counterView_increment_floatingActionButton'),
            onPressed: () => context.read<CounterCubit>().increment(),
            child: const Icon(Icons.add),
          ),
          const SizedBox(height: 8),
          FloatingActionButton(
            key: const Key('counterView_decrement_floatingActionButton'),
            onPressed: () => context.read<CounterCubit>().decrement(),
            child: const Icon(Icons.remove),
          ),
        ],
      ),
    );
  


class CounterText extends StatelessWidget 
  const CounterText(Key? key) : super(key: key);

  @override
  Widget build(BuildContext context) 
    final theme = Theme.of(context);
    final count = context.select((CounterCubit cubit) => cubit.state);
    return Text('$count', style: theme.textTheme.headline1);
  

user_profile_page.dart

class UserProfilePage extends StatelessWidget 
  final int userId;
  const UserProfilePage(
    Key? key,
    @PathParam() required this.userId,
  ) : super(key: key);

  @override
  Widget build(BuildContext context) 
    final user = User.users[userId - 1];
    return Scaffold(
      backgroundColor: user.color,
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            UserAvatar(
              avatarColor: Colors.white,
              username: 'user$user.id',
            )
          ],
        ),
      ),
      floatingActionButton: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        crossAxisAlignment: CrossAxisAlignment.end,
        children: [
          FloatingActionButton(
            key: const Key('counterView_star_floatingActionButton'),
            onPressed: () 
              print('star');
            ,
            child: const Icon(Icons.star),
          ),
          const SizedBox(height: 8),
          FloatingActionButton(
            key: const Key('counterView_boat_floatingActionButton'),
            onPressed: () 
              print('boat');
            ,
            child: const Icon(Icons.sailing),
          ),
          FloatingActionButton(
            key: const Key('counterView_increment_floatingActionButton'),
            onPressed: () => context.read<CounterCubit>().increment(),
            child: const Icon(Icons.add),
          ),
          const SizedBox(height: 8),
          FloatingActionButton(
            key: const Key('counterView_decrement_floatingActionButton'),
            onPressed: () => context.read<CounterCubit>().decrement(),
            child: const Icon(Icons.remove),
          ),
        ],
      ),
    );
  

非常感谢任何帮助。谢谢！

【参考方案1】：

请参考以下代码

ValueNotifer 和 ValueListenableBuilder 可用于保存值并通过通知其侦听器并减少重新构建小部件树的次数来更新小部件。

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);
final ValueNotifier<int> counter = ValueNotifier(0);

void main() 
  runApp(MyApp());


class MyApp extends StatelessWidget 
  @override
  Widget build(BuildContext context) 
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: FloatingActionButtonClass(),
    );
  


class FloatingActionButtonClass extends StatelessWidget 
  void _incrementCounter() 
    counter.value++;
  

  @override
  Widget build(BuildContext context) 
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.

      body: Center(
        child: InkWell(
          onTap: () 
            Navigator.push(
              context,
              MaterialPageRoute(builder: (context) => MyHomePage()),
            );
          ,
          child: Text("Floating Action Button"),
        ),
      ),
    );
  


class MyHomePage extends StatefulWidget 
  MyHomePage(
    Key? key,
  ) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();


class _MyHomePageState extends State<MyHomePage> 
  @override
  Widget build(BuildContext context) 
    return Scaffold(
      appBar: AppBar(
        title: Text("Example"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            ValueListenableBuilder(
              valueListenable: counter,
              builder: (context, value, child) 
                return Text(
                  counter.value.toString(),
                  style: Theme.of(context).textTheme.headline4,
                );
              ,
            ),
          ],
        ),
      ),
    );
  

【讨论】：

您可以通过下面提到的链接了解有关 ValueNotifier 和 ValueListenable Builder 的更多信息。 medium.flutterdevs.com/…

非常感谢！看起来你的方式更精简，感谢你，我将阅读 ValueNotifier 和 ValueListenable Builder！

总是乐于提供帮助

【参考方案2】：

我需要创建一个 BlocProvider 并将视图移至子级。

class HomePage extends StatelessWidget 
  const HomePage(Key? key) : super(key: key);

  @override
  Widget build(BuildContext context) 
    return BlocProvider(
      create: (_) => CounterCubit(),
      child: const HomePageView(),
    );
  


class HomePageView extends StatelessWidget 
  const HomePageView(Key? key) : super(key: key);

  @override
  Widget build(BuildContext context) 
    return AutoTabsScaffold(
      appBarBuilder: (_, tabsRouter) => AppBar(
        backgroundColor: Colors.indigo,
        title: const Text('FlutterBottomNav'),
        centerTitle: true,
        leading: const AutoBackButton(),
      ),
      backgroundColor: Colors.teal,
      routes: const [
        CounterRouterB(),
        CounterRouter(),
        PostsRouter(),
        UsersRouter(),
        SettingsRouter(),
      ],
      bottomNavigationBuilder: (_, tabsRouter) 
        return SalomonBottomBar(
          margin: const EdgeInsets.symmetric(
            horizontal: 20,
            vertical: 40,
          ),
          currentIndex: tabsRouter.activeIndex,
          onTap: tabsRouter.setActiveIndex,
          items: [
            SalomonBottomBarItem(
              selectedColor: Colors.redAccent,
              icon: const Icon(Icons.plus_one, size: 30),
              title: const Text('Counter2'),
            ),
            SalomonBottomBarItem(
              selectedColor: Colors.amberAccent,
              icon: const Icon(Icons.plus_one, size: 30),
              title: const Text('Counter'),
            ),
            SalomonBottomBarItem(
              selectedColor: Colors.amberAccent,
              icon: const Icon(Icons.post_add, size: 30),
              title: const Text('Posts'),
            ),
            SalomonBottomBarItem(
              selectedColor: Colors.blue[200],
              icon: const Icon(
                Icons.person,
                size: 30,
              ),
              title: const Text('Users'),
            ),
            SalomonBottomBarItem(
              selectedColor: Colors.pinkAccent[100],
              icon: const Icon(
                Icons.settings,
                size: 30,
              ),
              title: const Text('Settings'),
            ),
          ],
        );
      ,
    );
  

【讨论】：

这是一个不错的解决方案还是笨拙？