bloc_maybe_statechange

main.dart

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

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Builder(builder: (context) {
        return BlocProvider(
          create: (context) {
            return DataBloc(() {
              ScaffoldMessenger.of(context).showSnackBar(
                const SnackBar(content: Text('Bloc has no new data :(')),
              );
            });
          },
          child: const FirstScreen(),
        );
      }),
    );
  }
}

class FirstScreen extends StatefulWidget {
  const FirstScreen({
    Key? key,
  }) : super(key: key);

  @override
  State<FirstScreen> createState() => _FirstScreenState();
}

class _FirstScreenState extends State<FirstScreen> {
  bool _loading = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: ElevatedButton(
          onPressed: _loading
              ? null
              : () async {
                  setState(() => _loading = true);
                  final bloc = context.read<DataBloc>();
                  final stateUpdate = bloc.stream.first;
                  bloc.add(
                      const DataEvent()); // this will only cause a state change every second time
                  final newState = await stateUpdate;
                  setState(() => _loading = false);
                  Navigator.of(context).push(
                    MaterialPageRoute(
                      builder: (_) => SecondScreen(data: newState.data),
                    ),
                  );
                },
          child: const Text('Go to second screen'),
        ),
      ),
    );
  }
}

class SecondScreen extends StatelessWidget {
  final List<int> data;

  const SecondScreen({Key? key, required this.data}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Text('New data is: $data. Now go back and try again.'),
      ),
    );
  }
}

class DataBloc extends Bloc<DataEvent, DataState> {
  VoidCallback onNoNewData;
  bool _hasNewData = true;
  int newData = 1;

  DataBloc(this.onNoNewData) : super(const DataInitial()) {
    on<DataEvent>((event, emit) async {
      if (_hasNewData) {
        await Future.delayed(
          const Duration(seconds: 2),
          () => emit(DataState([newData++])),
        );
      } else {
        onNoNewData();
      }
      _hasNewData = !_hasNewData;
    });
  }
}

class DataEvent {
  const DataEvent();
}

class DataState /*extends Equatable*/ {
  final List<int> data;

  const DataState(this.data);

  @override
  bool operator ==(Object other) {
    return other is DataState && other.data.every(data.contains);
  }
}

class DataInitial extends DataState {
  const DataInitial() : super(const []);
}