Modern implementation of the Original BLoC

by Flutter Awesome

stream_bloc

Modern implementation of the Original BLoC

About

This package contains a modern (bloc package version 8.0.0+) implementation of the Original, Stream/generator-based BLoC with several modifications and convenience extras.

Motivation

After the 7.2.0 version update, the bloc has changed. Generators and the signature method mapEventToState were replaced with method-based pattern-matching using the on method and its argument handler. This approach is defiantly usable and solves a particular bug of the Dart language itself, but it lacks a few things.

  1. Power of streams. They can be transformed natively using a vast choice of transformers, but with the new bloc, streams are hidden under the hood and are not “first-class citizens”.
  2. Power of generators. They allow to asynchronously return multiple values, including other streams. The new version emulates their behavior using higher-order functions.
  3. New on approach makes freezed basically useless. It is possible to register just a single handler, but it negates the whole point of the on handlers.

This package brings back Original bloc with all the benefits, whilst maintaining 100% compatibility with bloc and flutter_bloc packages. The StreamBlocs can be used with all flutter_bloc widgets; they implement the same interfaces.

Overview

If you are familiar with the bloc before the 8.0.0/7.2.0 you are familiar with StreamBloc – the central class of this package. Documentation for previous bloc’s versions can be used for this packages besides a few modifications that are listed in the next section.

StreamBloc uses a central event-processing method called mapEventToStates to convert a single Event to a Stream of States that are emitted asynchronously. Official Bloc can be directly translated to StreamBloc as described below.

It is highly advised to use freezed package. The following example is a demonstration and should not be considered a “Best practice” for StreamBloc

abstract class CounterEvent {} // Shared counter event type

class Increment implements CounterEvent {} // Increment counter event

class Decrement implements CounterEvent {} // Decrement counter event

class OnCounterBloc extends Bloc<CounterEvent, int> { // Official Bloc – `on`s
  OnCounterBloc() : super(0) {
    on<Increment>((event, emit) => emit(state + 1));
    on<Decrement>((event, emit) => emit(state - 1));
  }
}

class StreamCounterBloc extends StreamBloc<CounterEvent, int> { // StreamBloc – `mapEventToStates`
  StreamCounterBloc() : super(0);

  @override
  Stream<int> mapEventToStates(CounterEvent event) async* {
    if (event is Increment) {
      yield state + 1;
    } else if (event is Decrement) {
      yield state - 1;
    }
  }
}

Modifications

There are three main differences from the Original bloc.

  1. mapEventToState is renamed to mapEventToStates. The method returns an asynchronous sequence of states – not a single state.

  2. It is not possible to emit a new state without it being a response to a certain event. StreamBloc does not implement Emittable and does not have an emit method. The original bloc has its method because both Cubit and Bloc are descendants of the same base class, but emit should not be used within a Bloc. It is marked as visible for testing, but it is always a good idea to test a whole instead of its parts.

  3. Bloc can emit identical states consequentially. The output stream of the StreamBloc is not distinct because of two main reasons

    • flutter_bloc‘s BlocListener/BlocConsumer may be interested in any new emitted state, even if the state had not changed
    • stream.map(...)/stream.where(...) (essentially BlocBuilder and/or BlocSelector) applied to stream.distinct() removes the guarantee of uniques event in the stream, making the distinct redundant; it should be applied last, not first.

Extras

The package also offers a single convenience mixin BlocLifecycleMixin which makes Bloc-to-Bloc communications easier. It offers four methods: listenToStream, listenToStreamable, reactToStream and reactToStreamable. Below is an example of its usage.

enum EventA { eventA }

class BlocA extends StreamBloc<EventA, int> {
  BlocA() : super(0);

  @override
  Stream<int> mapEventToStates(EventA event) => Stream.value(1);
}

enum EventB { eventB }

class BlocB extends StreamBloc<EventB, int> with BlocLifecycleMixin<EventB> {
  BlocB(BlocA blocA) : super(0) {
    /// Will print every new state of this Bloc to the console.
    listenToStream(stream, print);

    /// Will add [EventB.eventB] to this bloc every time BlocA emits any state.
    reactToStreamable<int>(blocA, (blocAState) => EventB.eventB);
  }

  @override
  Stream<int> mapEventToStates(EventB event) => Stream.value(1);
}

All methods return an instance of StreamSubscription which can be canceled by hand if desired, but it is optional – it will be canceled any way on the closing of a Bloc that mixes in BlocLifecycleMixin

GitHub

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