Observable state #

Flutter's State Manager for Reactive Apps in a Centralized and Predictable container.

Using React? Check out!

But this time #

🧰 State changes are handled using encapsulation on plain-old objects.
✔️ You can freely test your state and its changes, like pure Dart; because IT IS pure Dart.
🎯 Notify only the specific States that are observing the triggered change, NOT the entire Widget tree.
💧 No Streams or new Widgets, it is purely State and setState(); like Vanilla, but in a Centralized and Predictable state container. Your Container!

How it works #

It's a sweet, sweet sugar on top of the battle-tested Observer pattern. Your state is stored in a list of Observables of a given subject (see Change below) and it's setState() is called only when your Model explicit tells to notify about that change.

It isn't BLoC, but you still can maintain, test and visualize your Business Logic away from the UI.
It isn't Flux (Redux/Rx/Stream), but you still can control data in an unidirectional flow.

Get started #

The fact is: state is hard! Probably the hardest thing (after naming, of course). This is why things like BLoC, Flux/Redux and ScopedModel appears; to help you solve that.

And that is why observable_state is here too.

Talk is cheap. Show me the code — Torvalds, Linus

Installing #

dependencies:
  observable_state: ^0.1.0

State modeling #

class MyState {
  int counter;
}

From a simple model like that, to add observable_state super-powers you just need to:

class MyState extends Observable<Changes> {
  int counter;
}

Then make sure you declare what changes to your state are:

enum Changes {
  increment,
}

Even better, remember that changes are handled by encapsulation:

enum Changes {
  increment,
}

class MyState extends Observable<Changes> {
  int _counter;
  int get counter => _counter;

  void increment() {
    _counter++;
  }
}

How to notify about the `increment` change? #

Observable state borrows the same Flutter API for local state (setState()) with a slight difference: the notify argument:

enum Changes {
  increment,
}

class MyState extends Observable<Changes> {
  int _counter;
  int get counter => _counter;

  void increment() {
    setState(
      () => _counter++,
      notify: Changes.increment,
    );
  }
}

That is it. Your Observable is notifying about MyState changes.

How to listen to then? #

Here comes the StateObserver.

StatelessWidgets are state-less as the name suggests. We are not going to mess with them adding Streams or whatever. Actually, even the StatefulWidget remains the same. We are going to super-power the State only!

class MyStatefulWidget extends StatefulWidget {
  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends StateObserver<MyStatefulWidget, MyState, Changes> {
  @override
  List<Changes> get changes => [Changes.increment];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Observable state #${state.counter}')),
      body: Center(child: Text('Counter: ${state.counter}')),
      floatingActionButton: Builder(
        builder: (context) {
          return FloatingActionButton(
            child: Icon(Icons.add),
            onPressed: () {
              state.increment();
            },
          );
        },
      ),
    );
  }
}

Note that we are extending StateObserver instead of State and we are given a list of changes we are interest in, like Changes.increment. StateObserver already got the state so we can get counter and we can call increment() on it as well. Whenever increment() is called, since it notifies about Changes.increment, whoever Observer (like a StateObserver) is observing this change, it will automatically calls its inner setState() method, then rebuilding it.

Where does the `state` comes from? #

The last piece: ObservableProvider.

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ObservableProvider(
      state: MyState(),
      child: MaterialApp(home: MyStatefulWidget()),
    );
  }
}

It's an InheritedWidget responsible to provide our state from its context.

Fine-grained control #

The example above works great, but because it is very simple. When using things like FutureBuilders and/or Animations. No longer will be a good idea to have your entire State UI rebuilt after a change. You don't need to rebuild the entire Scaffold and its AppBar just because of the body.

That is why Observable state comes with 3 more constructions for a fine-grained control over what should be rebuilt on changes.

ObserverBuilder
bind() & bindWidget()
observableStateOf()

ObserverBuilder #

Works like any other builder in Flutter (StreamBuilder, FutureBuilder, AnimatedBuilder etc), each builder is specialized on a task and the ObserverBuilder job is to provide the Observable state from the ObserverProvider and listen for changes:

ObserverBuilder<MyState, Changes>(
  changes: [Changes.increment],
  builder: (context, state) => Text(state.counter.toString())
);

Now just this little piece of Text will rebuild after Changes.increment is notified.

bind & bindWidget #

They work almost like ObserverBuilder, but they don't listen for changes. They just provide (or binds) the current state to a function (bind) or a builder bindWidget. This is useful when you need just the state and don't need to listen for changes. bind() is special, because it returns a void Function(), this is useful to immediately pass to on* listeners, like:

RaisedButton(
  onPressed: bind<MyState>(context, (state) => state.increment())
  child: const Icon(Icons.add),
);

AppBar(
  title: bindWidget<MyState>(context, (context, state) => Text(state.title)),
);