当未显式设置bloc时，为什么从不调用BlocBuilder

// 引入所需的包
import 'package:flutter_bloc/flutter_bloc.dart';

// 事件类
abstract class CounterEvent {}

class IncrementEvent extends CounterEvent {}

class DecrementEvent extends CounterEvent {}

// 状态类
abstract class CounterState {}

class InitialState extends CounterState {}

class UpdatedState extends CounterState {
  final int count;

  UpdatedState(this.count);
}

// Bloc类
class CounterBloc extends Bloc<CounterEvent, CounterState> {
  CounterBloc() : super(InitialState());

  @override
  Stream<CounterState> mapEventToState(CounterEvent event) async* {
    if (event is IncrementEvent) {
      yield UpdatedState(state.count + 1);
    } else if (event is DecrementEvent) {
      yield UpdatedState(state.count - 1);
    }
  }
}

// 应用程序入口
void main() {
  runApp(MyApp());
}

// UI组件
class MyApp extends StatelessWidget {
  final CounterBloc counterBloc = CounterBloc();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Counter App'),
        ),
        body: BlocBuilder<CounterBloc, CounterState>(
          bloc: counterBloc,
          builder: (context, state) {
            if (state is UpdatedState) {
              return Center(
                child: Text(
                  'Count: ${state.count}',
                  style: TextStyle(fontSize: 24),
                ),
              );
            }
            return Container();
          },
        ),
        floatingActionButton: Column(
          mainAxisAlignment: MainAxisAlignment.end,
          children: [
            FloatingActionButton(
              onPressed: () => counterBloc.add(IncrementEvent()),
              child: Icon(Icons.add),
            ),
            SizedBox(height: 8),
            FloatingActionButton(
              onPressed: () => counterBloc.add(DecrementEvent()),
              child: Icon(Icons.remove),
            ),
          ],
        ),
      ),
    );
  }
}