BlocProvider.of() 使用不包含 CounterBloc 类型的 Cubit 的上下文调用

Question

The following is a example code from :

https://bloclibrary.dev/#/flutterbloccoreconcepts

I am getting the following exception,

How can I fix the following Exception and why is this happening:

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════

The following assertion was thrown building CounterPage(dirty):

BlocProvider.of() called with a context that does not contain a Cubit of type CounterBloc.

No ancestor could be found starting from the context that was passed to BlocProvider.of().

This can happen if the context you used comes from a widget above the BlocProvider.

The context used was: CounterPage(dirty)

main.dart

import 'package:flutter/material.dart';

import 'counter_page.dart';

void main() {
  runApp(MyApp());
} 
class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Bloc Counter Example',
      home: CounterPage(),
    );
  }
}

counter_page.dart

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import'package:flutterbloc_counter_app/counter_bloc.dart';

class CounterPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final CounterBloc counterBloc = BlocProvider.of<CounterBloc>(context);

    return Scaffold(
      appBar: AppBar(title: Text('Counter')),
      body: BlocBuilder<CounterBloc, int>(
        builder: (context, count) {
          return Center(
            child: Text(
              '$count',
              style: TextStyle(fontSize: 24.0),
            ),
          );
        },
      ),
      floatingActionButton: Column(
        crossAxisAlignment: CrossAxisAlignment.end,
        mainAxisAlignment: MainAxisAlignment.end,
        children: <Widget>[
          Padding(
            padding: EdgeInsets.symmetric(vertical: 5.0),
            child: FloatingActionButton(
              child: Icon(Icons.add),
              onPressed: () {
                counterBloc.add(CounterEvent.increment);
              },
            ),
          ),
          Padding(
            padding: EdgeInsets.symmetric(vertical: 5.0),
            child: FloatingActionButton(
              child: Icon(Icons.remove),
              onPressed: () {
                counterBloc.add(CounterEvent.decrement);
              },
            ),
          ),
        ],
      ),
    );
  }
}

counter_bloc.dart

import 'package:flutter_bloc/flutter_bloc.dart';
    
enum CounterEvent { increment, decrement }

class CounterBloc extends Bloc<CounterEvent, int> {
  CounterBloc() : super(0);

  @override
  Stream<int> mapEventToState(CounterEvent event) async* {
    switch (event) {
      case CounterEvent.decrement:
        yield state - 1;
        break;
      case CounterEvent.increment:
        yield state + 1;
        break;
    }
  }
}

Answer 1

You can not use 'BlocProvider.of' without BlocProvider at the parent.
You just create a 'CounterBloc' and pass the bloc to BlocBuilder.

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import'package:flutterbloc_counter_app/counter_bloc.dart';

class CounterPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final CounterBloc counterBloc = CounterBloc();

    return Scaffold(
      appBar: AppBar(title: Text('Counter')),
      body: BlocBuilder<CounterBloc, int>(
        bloc: counterBloc,
        builder: (context, count) {
          return Center(
            child: Text(
              '$count',
              style: TextStyle(fontSize: 24.0),
            ),
          );
        },
      ),
      floatingActionButton: Column(
        crossAxisAlignment: CrossAxisAlignment.end,
        mainAxisAlignment: MainAxisAlignment.end,
        children: <Widget>[
          Padding(
            padding: EdgeInsets.symmetric(vertical: 5.0),
            child: FloatingActionButton(
              child: Icon(Icons.add),
              onPressed: () {
                counterBloc.add(CounterEvent.increment);
              },
            ),
          ),
          Padding(
            padding: EdgeInsets.symmetric(vertical: 5.0),
            child: FloatingActionButton(
              child: Icon(Icons.remove),
              onPressed: () {
                counterBloc.add(CounterEvent.decrement);
              },
            ),
          ),
        ],
      ),
    );
  }
}

Answer 2

It looks like I forgot to provide an instance of the bloc to the CounterPage. Wrapping the CounterPage widget in a BlocProvider and create the instance of the bloc like this will do the trick:

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Bloc Counter Example',
      home: BlocProvider(
        create: (context) => CounterBloc(),
        child: CounterPage(),
      ),
    );
  }
}