bug in updating state without delay for refresh view

[DJafari]

i have a problem in this library :

when i use this code :

import 'package:bloc/bloc.dart';

enum TestState {
  INIT, LOADING, LOCAL_SUCCESS, REMOTE_SUCCESS, FINISH
}

enum TestEvent {
  Get
}

class TestBloc extends Bloc<TestEvent, TestState> {
  @override
  TestState get initialState => TestState.INIT;

  @override
  Stream<TestState> mapEventToState(TestEvent event) async* {
    if(event == TestEvent.Get) {
      yield TestState.LOADING;
      yield TestState.LOCAL_SUCCESS;
      yield TestState.REMOTE_SUCCESS;
      yield TestState.FINISH;
    }
  }
}

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

import 'test_bloc.dart';

class TestPage extends StatefulWidget {
  @override
  _TestPageState createState() => _TestPageState();
}

class _TestPageState extends State<TestPage> {
  final bloc = TestBloc();

  @override
  void initState() {
    bloc.dispatch(TestEvent.Get);
    super.initState();
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: BlocBuilder(
        bloc: bloc,
        builder: (context, state) {
          print(state);
          return Container();
        },
      ),
    );
  }
}

when i run app, my output is :

I/flutter ( 7180): TestState.INIT
I/flutter ( 7180): TestState.FINISH

but when i add some delay before states :

import 'package:bloc/bloc.dart';

enum TestState {
  INIT, LOADING, LOCAL_SUCCESS, REMOTE_SUCCESS, FINISH
}

enum TestEvent {
  Get
}

class TestBloc extends Bloc<TestEvent, TestState> {
  @override
  TestState get initialState => TestState.INIT;

  @override
  Stream<TestState> mapEventToState(TestEvent event) async* {
    if(event == TestEvent.Get) {
      yield TestState.LOADING;
      await Future.delayed(Duration(milliseconds: 100));
      yield TestState.LOCAL_SUCCESS;
      await Future.delayed(Duration(milliseconds: 100));
      yield TestState.REMOTE_SUCCESS;
      await Future.delayed(Duration(milliseconds: 100));
      yield TestState.FINISH;
    }
  }
}

output :

I/flutter ( 7180): TestState.INIT
I/flutter ( 7180): TestState.LOADING
I/flutter ( 7180): TestState.LOCAL_SUCCESS
I/flutter ( 7180): TestState.REMOTE_SUCCESS
I/flutter ( 7180): TestState.FINISH

note : this bug exists only in flutter package, because output of this :

bloc.state.listen((state) {
  print(state);
});

is ok

[felangel]

Hi @DJafari 👋
Thanks for opening an issue!

I believe this is a duplicate of #395.
TL;DR this is working as expected. You're simply yielding states faster than flutter can rebuild the UI. The same thing would happen if you used setState and changed the state back-to-back.

Hope that helps 👍