Flutter路由跳转返回后Bloc状态丢失且调用BlocProvider.of()报错的原因

核心问题分析 #

• 路由跳转逻辑错误：HEHE页面的跳转按钮用Navigator.push创建了全新的Home实例，而非通过Navigator.pop返回原首页。这会导致新的Home页面不在初始的BlocProvider上下文树中，无法获取ExampleBloc实例。
• Bloc作用域局限：原代码中BlocProvider仅包裹了初始Home页面，当新增页面实例时，新页面脱离了Bloc的作用范围，自然找不到对应的状态管理器。

修正方案 #

1. 修复HEHE页面的返回逻辑 #

把HEHE页面里的Navigator.push替换为Navigator.pop，回到原来的首页实例：

FloatingActionButton(
  onPressed: () => Navigator.of(context).pop(),
  child: Text('back to home'),
),

2. 提升BlocProvider作用域（推荐） #

如果需要在多页面共享Bloc，将BlocProvider移到MaterialApp外层，让整个应用都能访问Bloc实例，同时用create替代value来自动管理Bloc生命周期：

void main() {
  runApp(
    BlocProvider(
      create: (context) => ExampleBloc(),
      child: MaterialApp(
        home: Home(),
      ),
    ),
  );
}

3. 避免重复创建首页实例 #

永远不要通过push重复创建首页，返回上一页优先用pop；若需重置首页状态，可使用pushReplacement替换当前页面，但常规返回场景用pop即可。

完整修正代码 #

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:equatable/equatable.dart';

void main() {
  runApp(
    BlocProvider(
      create: (context) => ExampleBloc(),
      child: MaterialApp(
        home: Home(),
      ),
    ),
  );
}

class Home extends StatefulWidget {
  Home({Key? key}) : super(key: key);

  @override
  State<Home> createState() => _HomeState();
}

class _HomeState extends State<Home> {
  late TextEditingController teste;
  late String lol;

  @override
  void initState() {
    teste = TextEditingController();
    lol = 'ola';
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Column(
        children: <Widget>[
          TextField(
            controller: teste,
          ),
          Center(child: BlocBuilder<ExampleBloc, ExampleState>(
            builder: (context, state) {
              return Text(state.exampleString ?? 'null');
            },
          ))
        ],
      ),
      floatingActionButton: Row(
        children: [
          FloatingActionButton(
            onPressed: () {
              lol = teste.text;
              BlocProvider.of<ExampleBloc>(context)
                  .add(ExampleUseEvent(exampleString: lol));
            },
            child: Icon(Icons.add),
          ),
          FloatingActionButton(
            onPressed: () => Navigator.push(
              context,
              MaterialPageRoute(builder: (context) => HEHE()),
            ),
            child: Text('next screen'),
          ),
          FloatingActionButton(
            onPressed: () => Navigator.of(context).pop(),
            child: Text('pop'),
          ),
        ],
      ),
    );
  }

  @override
  void dispose() {
    teste.dispose();
    super.dispose();
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: const Center(child: Text('teste')),
      floatingActionButton: Row(
        children: [
          FloatingActionButton(
              onPressed: () => Navigator.of(context).pop(),
              child: Text('back home')),
        ],
      ),
    );
  }
}

class ExampleBloc extends Bloc<ExampleEvent, ExampleState> {
  ExampleBloc() : super(ExampleState()) {
    on<ExampleUseEvent>(exampleFuncion);
  }

  Future<void> exampleFuncion(
    ExampleUseEvent event,
    Emitter<ExampleState> emit,
  ) async {
    emit(state.copyWith(
      exampleString: event.exampleString,
    ));
  }
}

abstract class ExampleEvent {}

class ExampleUseEvent extends ExampleEvent {
  final String exampleString;

  ExampleUseEvent({required this.exampleString});
}

class ExampleState extends Equatable {
  final String? exampleString;

  ExampleState({
    this.exampleString,
  });

  ExampleState copyWith({
    String? exampleString,
  }) {
    return ExampleState(
      exampleString: exampleString ?? this.exampleString,
    );
  }

  @override
  List<Object?> get props => [
        exampleString,
      ];
}

内容的提问来源于stack exchange，提问作者user21632175