mapEventToState not called if dispatched events quickly about bloc

[felangel]

@QoLTech thanks for reporting this!

Can you please put together a super simple app that exhibits this behavior?

I'm unable to reproduce this with the following code:

import 'package:bloc/bloc.dart';

abstract class CounterEvent {}

class Increment extends CounterEvent {
  @override
  String toString() => 'Increment';
}

class Decrement extends CounterEvent {
  @override
  String toString() => 'Decrement';
}

class CounterBloc extends Bloc<CounterEvent, int> {
  @override
  int get initialState => 0;

  @override
  Stream<int> mapEventToState(int currentState, CounterEvent event) async* {
    if (event is Increment) {
      yield currentState + 1;
    }
    if (event is Decrement) {
      yield currentState - 1;
    }
  }
}

class SimpleBlocDelegate implements BlocDelegate {
  @override
  void onTransition(Transition transition) {
    print(transition.toString());
  }
}

void main() {
  BlocSupervisor().delegate = SimpleBlocDelegate();

  final counterBloc = CounterBloc();

  for (int i = 0; i < 100; i++) {
    counterBloc.dispatch(Increment());
  }
}

[QoLTech]

I've got my code that I'm currently working on here. It's slightly more complicated than a sample app, but I can easily reproduce the error.

After building and running, just hit connect and after connecting, you'll be on the messaging page. You'll have to use this to send messages. To reproduce, I just rapidly send messages with a single digit (1-9). When I do this, I see 1-9 printed to the console which is from the listener for messages and I only see an extra 9 which is from the Bloc mapEventToState.

Let me know if you need anything else.

[felangel]

@QoLTech I finally had a chance to take a look and the reason why this is happening is because you are overriding transform in the MessageBloc and debouncing. If you remove the transform override it will work as you expect.

[felangel]

In addition, I noticed that you have your own bloc_builder and bloc_provider. I'm not sure if this is intentional but if you want you can add flutter_bloc to your dependencies and you'll have access to BlocBuilder and BlocProvider without needed to copy the code into your own project.

[QoLTech]

@QoLTech I finally had a chance to take a look and the reason why this is happening is because you are overriding transform in the MessageBloc and debouncing. If you remove the transform override it will work as you expect.

That's great, thank you. Could you explain or link me to an explanation of how debouncing works?

In addition, I noticed that you have your own bloc_builder and bloc_provider. I'm not sure if this is intentional but if you want you can add flutter_bloc to your dependencies and you'll have access to BlocBuilder and BlocProvider without needed to copy the code into your own project.

Yes, I didn't realize that package was also on pub. I will make sure to add it as a dependency, rather than copy it into my own projects.

[felangel]

@QoLTech you can learn about denounce here.

Let me know if you have any other questions. 👍

[QoLTech]

@QoLTech you can learn about denounce here.

Let me know if you have any other questions.

At first I thought the time had something to do with it, but couldn't figure out what exactly. Can I ask what the benefit of something like this is? If a button gets tapped in quick succession? If so, why do it this way rather than checking the dispatched events for equality?

[felangel]

@QoLTech it’s useful if you’re implementing something like a search and you don’t want to make a network request after every key press because it will put unnecessary load on the backend.

You can debounce with a duration of something like 500 milliseconds and events that are dispatched less than 500 milliseconds apart won’t be processed. This allows you to only hit your backend after a user has stopped typing.

[QoLTech]

@QoLTech it’s useful if you’re implementing something like a search and you don’t want to make a network request after every key press because it will put unnecessary load on the backend.

You can debounce with a duration of something like 500 milliseconds and events that are dispatched less than 500 milliseconds apart won’t be processed. This allows you to only hit your backend after a user has stopped typing.

That's great, thank you.