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.
Add this line to the project dependencies in your pubspec.yaml
:
rebloc: ^0.0.7
And use this import:
import 'package:rebloc/rebloc.dart';
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 and reducers. The stream for a simple, two-BLoC store might look like this, for example:
Dispatch ->
BloC #1 middleware ->
BLoC #2 middleware ->
the action is converted into an accumulator (action and state together) ->
BLoC #1 reducer ->
BLoC #2 reducer ->
resulting state object is emitted by store
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 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 are also allowed to cancel actions.
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...
Also 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.
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;
}
Two examples are included (a basic one, and a list-based one) so you can see the library in action. It's also used in the (currently being built) Voxxed Days conference app.
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.