错误：在此 BlocBuilder<InternetBloc> Widget<InternetBloc, InternetState> 上方找不到正确的提供程序

问题描述 投票：0回答：2

我在尝试将 flutter_bloc 集成到 flutter 项目中时遇到错误。

错误：在此 BlocBuilder Widget 上方找不到正确的提供者

这是我正在使用的代码示例。

internet_state.dart

abstract class InternetState{}
class InitialInternetState extends InternetState{}
class LostConnectionInternetState extends InternetState{}
class GainConnectionInternetState extends InternetState{}


internet_event.dart

abstract class InternetEvent{}
class LostConnectionEvent extends InternetEvent{}
class GainConnectionEvent extends InternetEvent{}

internet_bloc.dart

class InternetBloc extends Bloc<InternetEvent, InternetState>{
  final Connectivity _connectivity = Connectivity();
  StreamSubscription? streamSubscription;
  InternetBloc() : super(InitialInternetState()){
    on<GainConnectionEvent>((event, emit) => emit(GainConnectionInternetState()));
    on<LostConnectionEvent>((event, emit) => emit(LostConnectionInternetState()));
    _connectivity.onConnectivityChanged.listen((event) {
      if(event == ConnectivityResult.mobile || event == ConnectivityResult.wifi){
        add(GainConnectionEvent());
      } else {
        add(LostConnectionEvent());
      }
    });
  }
  @override
  Future<void> close() {
    streamSubscription?.cancel();
    return super.close();
  }
}

main.dart

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(

        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  Widget build(BuildContext context) {
    return  Scaffold(
      appBar: AppBar(title: Text(title),),
      body: SafeArea(
        child: Center(child: BlocBuilder<InternetBloc, InternetState>(
          builder: (context, state) {
            return const Text("hello");
          },
        )),
      ),);
  }
}

我明白了。

======== 小部件库捕获异常====================================== =================== 构建 MediaQuery(MediaQueryData(size: Size(392.7, 783.3), devicePixelRatio: 2.8, textScaler: noscaling, platformBrightness: Brightness.light, padding: EdgeInsets.zero, viewPadding: EdgeInsets.zero, viewInsets: EdgeInsets 时抛出以下 ProviderNotFoundException。零，systemGestureInsets：EdgeInsets（29.8，24.0，29.8，8.0），alwaysUse24HourFormat：假，accessibleNavigation：假，highContrast：假，onOffSwitchLabels：假，disableAnimations：假，invertColors：假，boldText：假，navigationMode：传统，gestureSettings：DeviceGestureSettings （touchSlop：8.0），显示功能：[]））： 错误：无法在此 BlocBuilder Widget

之上找到正确的提供程序

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。有以下几种常见场景：

  • 您在

    main.dart
    中添加了一个新的提供程序并执行了热重载。 要修复此问题，请执行热重启。

  • 您尝试阅读的提供商处于不同的路线。

    提供商具有“范围”。因此，如果您在路线中插入提供者，那么 其他路线将无法访问该提供商。

  • 您使用了 

    BuildContext
    ，它是您尝试读取的提供程序的祖先。

    确保 BlocBuilder 位于您的 MultiProvider/Provider 下。 当您创建提供程序并尝试立即读取它时，通常会发生这种情况。

    例如，代替：

    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>().toString()),
  );
}

    考虑使用

    builder
    ，如下所示：

    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, child) {
      // No longer throws
      return Text(context.watch<Example>().toString());
    }
  );
}

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

The relevant error-causing widget was: 
  SafeArea SafeArea:file:///Users/../Projects/flutterTest/bloc_test/lib/main.dart:37:13
When the exception was thrown, this was the stack: 
#0      Provider._inheritedElementOf (package:provider/src/provider.dart:343:7)
#1      Provider.of (package:provider/src/provider.dart:293:30)
#2      ReadContext.read (package:provider/src/provider.dart:649:21)
#3      _BlocBuilderBaseState.initState (package:flutter_bloc/src/bloc_builder.dart:130:36)
#4      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:5602:55)
#5      ComponentElement.mount (package:flutter/src/widgets/framework.dart:5447:5)
...     Normal element mounting (4 frames)
#9      Element.inflateWidget (package:flutter/src/widgets/framework.dart:4326:16)
#10     Element.updateChild (package:flutter/src/widgets/framework.dart:3831:20)
#11     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#12     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#13     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#14     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#15     SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6743:14)
#16     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#17     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#18     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#19     StatelessElement.update (package:flutter/src/widgets/framework.dart:5547:5)
#20     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#21     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#22     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#23     StatelessElement.update (package:flutter/src/widgets/framework.dart:5547:5)
#24     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#25     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#26     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#27     StatelessElement.update (package:flutter/src/widgets/framework.dart:5547:5)
#28     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#29     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#30     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#31     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#32     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#33     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#34     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#35     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#36     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#37     Element.updateChildren (package:flutter/src/widgets/framework.dart:3964:32)
#38     MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6896:17)
#39     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#40     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#41     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#42     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#43     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#44     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#45     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5634:11)
#46     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#47     StatefulElement.update (package:flutter/src/widgets/framework.dart:5657:5)
#48     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#49     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#50     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5634:11)
#51     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#52     StatefulElement.update (package:flutter/src/widgets/framework.dart:5657:5)
#53     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#54     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#55     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#56     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#57     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#58     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#59     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5634:11)
#60     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#61     StatefulElement.update (package:flutter/src/widgets/framework.dart:5657:5)
#62     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#63     SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6743:14)
#64     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#65     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#66     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#67     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#68     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#69     SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6743:14)
#70     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#71     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#72     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5634:11)
#73     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#74     StatefulElement.update (package:flutter/src/widgets/framework.dart:5657:5)
#75     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#76     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#77     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5634:11)
#78     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#79     StatefulElement.update (package:flutter/src/widgets/framework.dart:5657:5)
#80     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#81     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#82     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#83     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#84     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#85     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#86     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#87     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#88     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#89     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#90     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#91     ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#92     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#93     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#94     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5634:11)
#95     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#96     StatefulElement.update (package:flutter/src/widgets/framework.dart:5657:5)
#97     Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#98     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#99     Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#100    ProxyElement.update (package:flutter/src/widgets/framework.dart:5800:5)
#101    Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#102    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#103    StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5634:11)
#104    Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#105    StatefulElement.update (package:flutter/src/widgets/framework.dart:5657:5)
#106    Element.updateChild (package:flutter/src/widgets/framework.dart:3815:15)
#107    ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5496:16)
#108    Element.rebuild (package:flutter/src/widgets/framework.dart:5187:7)
#109    BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2895:19)
#110    WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:984:21)
#111    RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:457:5)
#112    SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1325:15)
#113    SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1255:9)
#114    SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:978:7)
#118    _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)
(elided 3 frames from class _Timer and dart:async-patch)
====================================================================================================

我不想使用像 MultiProvider 这样的全局提供程序。

flutter flutter-bloc
2个回答
0
投票

您应该在某处创建 InternetBloc 并提供它。 MultiProvider 本身并不是全局的，它只是多个。 更多信息在这里：https://bloclibrary.dev/#/flutterbloccoreconcepts?id=blocprovider

所以，如果您不想使用全局提供程序，它将看起来像这样：

class MyHomePage extends StatelessWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: SafeArea(
        child: Center(
          child: BlocProvider<InternetBloc>(
            create: ( context) => InternetBloc(),
            child: BlocBuilder<InternetBloc, InternetState>(
              builder: (context, state) {
                return const Text("hello");
              },
            ),
          ),
        ),
      ),
    );
  }
}
0
投票

要使用 BlocBuilder，您需要在 Widget 树中放置一个 BlocProvider。最佳实践是将其放置得尽可能深。在您的情况下，只需将 BlocProvider 放在 BlocBuilder 之上。 此外 MultiProvider / MultiBlocProvider 不是全局 Provider。它只是一个提供多个 Provider/BlocProvider 的便捷小部件。

class MyHomePage extends StatelessWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: SafeArea(
        child: Center(
          // Now everything below that can access the InternetBloc
          child: BlocProvider<InternetBloc>( 
            create: (_) => InternetBloc(),
            child: BlocBuilder<InternetBloc, InternetState>(
              builder: (context, state) {
                return const Text("hello");
              },
            ),
          ),
        ),
      ),
    );
  }
}
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