rebloc

rebloc

A state management library for Flutter that combines aspects of Redux and BLoC (this readme assumes some familiarity with both). It's a personal project by redbrogdon, rather than an official library from the Flutter team. You can find it on pub, Dart's package manager.

Adding rebloc to your project

Add this line to the project dependencies in your pubspec.yaml:

rebloc: ^0.4.0

And use this import:

import 'package:rebloc/rebloc.dart';

What's going on here

Rebloc is an attempt to smoosh together two popular Flutter state management approaches: Redux and BLoC. It defines a Redux-y single direction data flow that involves actions, middleware, reducers, and a store. Rather than using functional programming techniques to compose reducers and middleware from parts and wire everything up, however, it uses BLoCs.

The store defines a dispatch stream that accepts new actions and produces state objects in response. In between, BLoCs are wired into the stream to function as middleware, reducers, and afterware. Afterware is essentially a second chance for Blocs to perform middleware-like tasks after the reducers have had their chance to update the app state. The stream for a simple, two-BLoC store might look like this, for example:

Dispatch ->
  BloC #1 middleware ->
  BLoC #2 middleware ->
  BLoC #1 reducer ->
  BLoC #2 reducer ->
  resulting state object is emitted by store ->
  BLoC #1 afterware ->
  BLoC #2 afterware ->
  Done

There are two ways to implement BLoCs. The first is a basic Bloc<StateType> interface that allows direct access to the dispatch stream (meaning you can transform it, expand it, and do all sorts of other streamy goodness). The other is an abstract class, SimpleBloc<StateType>, that hides away interaction with the stream and provides a simple, functional interface.

Middleware and afterware methods can perform side effects like calling out to REST endpoints and dispatching new actions, but reducers should work as pure functions in keeping with Redux core principles. Middleware and afterware are also allowed to cancel (or "swallow") actions.

Why does this exist?

State management is an open question for Flutter developers. I like the freedom of BLoCs, because a streaming interface can often turn what would be weird async patterns into easily understood transforms, maps, expands, and so on. On the other hand, I also like that the Redux pattern can offer things like easily composed reducers and middleware, great support for cross-cutting concerns like logging, and the potential for time-travel debugging if interest merits building those sorts of tools.

Thus I'd like to see if I can combine the two and get the parts I like from both. Also, it's a chance to test out some widgets...

ViewModelSubscriber

Included in this library is a widget called ViewModelSubscriber. It looks for an InheritedWidget called StoreProvider above it to find a stream of app state objects to subscribe to. Then it converts each object that comes through the stream into a view model object and builds widgets with it. This is similar to a StreamBuilder, but with the benefit that a ViewModelSubscriber will ignore any state objects that don't cause a change to its view model.

What this means in practice is that if you're building a piece of UI that depends on one part of your overall app state, it will only be rebuilt and redrawn if that one particular bit of data changes. If you've got a list of complicated records, for example, you can use ViewModelSubscriber widgets to avoid rebuilding the entire list just because one field in one record changed.

DispatchSubscriber

DispatchSubscriber is a ViewModelSubscriber without the view model. If you have a piece of UI that just needs to dispatch an Action and doesn't need any actual data from the Store, DispatchSubscriber is your huckleberry.

FirstBuildDispatcher

FirstBuildDispatcher automatically dispatches an Action to an ancestor Store the first time it's built. If you'd like to refresh some data from the network any time a particular widget is displayed, for example, FirstDispatchBuilder can help.

Basic example

Imagine an app that just needs to track a list of int as its only state. You might have a class to represent the state of the app that looks like this:

class AppState {
  List<int> numbers;

  AppState(this.numbers);
}

You might want to add a new number to the list, so you create an Action class to trigger such a state change:

class AddNumberAction extends Action {
  final int newNumber;

  const AddNumberAction(this.newNumber);
}

To process that action, you'll need a BLoC, so you create one:

class NumbersBloc extends SimpleBloc<AppState> {
  @override
  AppState reducer(AppState state, Action action) {
    if (action is AddNumberAction) {
      final newList = List<int>.from(state.numbers)..add(action.newNumber);
      return AppState(newList);
    }

    return state;
  }
}

And you'll need a store to wire all this up for you, so you create one in main and use a StoreProvider to propagate it down the tree:

void main() {
  final store = Store<AppState>(
    initialState: AppState([1, 2, 3]),
    blocs: [
      NumbersBloc(),
    ],
  );

  runApp(
    StoreProvider<AppState>(
      store: store,
      child: HomeScreen(),
    ),
  );
}

To build widgets with the data and dispatch new actions, you add a ViewModelSubscriber to HomeScreen that looks like this:

ViewModelSubscriber<AppState, int>(
  converter: (state) => state.numbers.reduce((sum, val) => sum + val),
  builder: (context, dispatcher, model) => Center(
    child: Column(
      children: [
        Text('Sum of all the numbers: $model'),
        RaisedButton(
          child: Text('Add 3'),
          onPressed: () => dispatcher(AddNumberAction(3)),
        ),
        RaisedButton(
          child: Text('Add 0'),
          onPressed: () => dispatcher(AddNumberAction(0)),
        ),
      ],
    ),
  ),
)

Now when your user taps on one of the buttons, an action is dispatched to the store. It's processed by the reducer and a new AppState object is created. If the user hit the 'Add 3' button, the ViewModelSubscriber will be rebuilt and show the new sum. If the user hit the 'Add 0' button, the ViewModelSubscriber will not be rebuilt, since the sum wasn't changed (even though there's a new number in the list).

If you'd like to do something fancy like log every action to the console, you can add a middleware function to the BLoC:

@override
FutureOr<Action> middleware(
    DispatchFunction dispatcher, AppState state, Action action) {
  if (action is AddNumberAction) {
    print('Adding a number: ${action.newNumber}.');
  }

  return action;
}

Example app

An example app is included in the repo to illustrate what real-world use of rebloc might look like. It includes two demos: a simple, "hit a button to dispatch an action" screen and a list-based demo that shows a Bloc dispatching a steady stream of actions on its own.

Feedback

I'm interested in whatever feedback other devs in the Flutter community may have, whether it's "This is awesome" or "This design is bad and you should feel bad." Feel free to file issues and feature requests, tweet at @RedBrogdon, and come join the conversation at the Flutter Dev Google group.