Controlling State from outside of a StatefulWidget

Question

I'm trying to understand the best practice for controlling a StatefulWidget's state outside of that Widgets State.

I have the following interface defined.

abstract class StartupView {
  Stream<String> get onAppSelected;

  set showActivity(bool activity);
  set message(String message);
}

I would like to create a StatefulWidget StartupPage that implements this interface. I expect the Widget to do the following:

1. When a button is pressed it would send an event over the onAppSelected stream. A controller would listen to this even and perform some action ( db call, service request etc ).

2. The controller can call showActivity or set message to have the view show progress with a message.

Because a Stateful Widget does not expose it's State as a property, I don't know the best approach for accessing and modifying the State's attributes.

The way I would expect to use this would be something like:

Widget createStartupPage() {
    var page = new StartupPage();
    page.onAppSelected.listen((app) {
      page.showActivity = true;
      //Do some work
      page.showActivity = false;
    });
  }

I've thought about instantiating the Widget by passing in the state I want it to return in createState() but that feels wrong.

Some background on why we have this approach: We currently have a Dart web application. For view-controller separation, testability and forward thinking towards Flutter we decided that we would create an interface for every view in our application. This would allow a WebComponent or a Flutter Widget to implement this interface and leave all of the controller logic the same.

Answer 1

Have you considered lifting the state to the parent widget? It is a common, though less ideal than Redux, way to manage state in React as far as I know, and this repository shows how to apply the concept to a Flutter app.

Answer 2

You can expose the state's widget with a static method, a few of the flutter examples do it this way and I've started using it as well:

class StartupPage extends StatefulWidget {
  static _StartupPageState of(BuildContext context) => context.ancestorStateOfType(const TypeMatcher<_StartupPageState>());

  @override
  _StartupPageState createState() => new _StartupPageState();
}

class _StartupPageState extends State<StartupPage> {
  ...
}

You can then access the state by calling StartupPage.of(context).doSomething();.

The caveat here is that you need to have a BuildContext with that page somewhere in its tree.

Answer 3

There are multiple ways to interact with other stateful widgets.

1. ancestorStateOfType

The first and most straightforward is through context.ancestorStateOfType method.

Usually wrapped in a static method of the Stateful subclass like this :

class MyState extends StatefulWidget {
  static of(BuildContext context, {bool root = false}) => root
      ? context.rootAncestorStateOfType(const TypeMatcher<_MyStateState>())
      : context.ancestorStateOfType(const TypeMatcher<_MyStateState>());

  @override
  _MyStateState createState() => _MyStateState();
}

class _MyStateState extends State<MyState> {
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

This is how Navigator works for example.

Pro:

• Easiest solution

Con:

• Tempted to access State properties or manually call setState
• Requires to expose State subclass

Don't use this method when you want to access a variable. As your widget may not reload when that variable change.

2. Listenable, Stream and/or InheritedWidget

Sometimes instead of a method, you may want to access some properties. The thing is, you most likely want your widgets to update whenever that value changes over time.

In this situation, dart offer Stream and Sink. And flutter adds on the top of it InheritedWidget and Listenable such as ValueNotifier. They all do relatively the same thing: subscribing to a value change event when coupled with a StreamBuilder/context.inheritFromWidgetOfExactType/AnimatedBuilder.

This is the go-to solution when you want your State to expose some properties. I won't cover all the possibilities but here's a small example using InheritedWidget :

First, we have an InheritedWidget that expose a count :

class Count extends InheritedWidget {
  static of(BuildContext context) =>
      context.inheritFromWidgetOfExactType(Count);

  final int count;

  Count({Key key, @required Widget child, @required this.count})
      : assert(count != null),
        super(key: key, child: child);

  @override
  bool updateShouldNotify(Count oldWidget) {
    return this.count != oldWidget.count;
  }
}

Then we have our State that instantiate this InheritedWidget

class _MyStateState extends State<MyState> {
  int count = 0;

  @override
  Widget build(BuildContext context) {
    return Count(
      count: count,
      child: Scaffold(
        body: CountBody(),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            setState(() {
              count++;
            });
          },
        ),
      ),
    );
  }
}

Finally, we have our CountBody that fetch this exposed count

class CountBody extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text(Count.of(context).count.toString()),
    );
  }
}

Pros:

• More performant than ancestorStateOfType
• Stream alternative is dart only (works with web) and is strongly integrated in the language (keywords such as await for or async*)
• Automic reload of the children when the value change

Cons:

• More boilerplate
• Stream can be complicated

3. Notifications

Instead of directly calling methods on State, you can send a Notification from your widget. And make State subscribe to these notifications.

An example of Notification would be :

class MyNotification extends Notification {
  final String title;

  const MyNotification({this.title});
}

To dispatch the notification simply call dispatch(context) on your notification instance and it will bubble up.

MyNotification(title: "Foo")..dispatch(context)

Any given widget can listen to notifications dispatched by their children using NotificationListener<T> :

class _MyStateState extends State<MyState> {
  @override
  Widget build(BuildContext context) {
    return NotificationListener<MyNotification>(
      onNotification: onTitlePush,
      child: Container(),
    );
  }

  void onTitlePush(MyNotification notification) {
    print("New item ${notification.title}");
  }
}

An example would be Scrollable, which can dispatch ScrollNotification including start/end/overscroll. Then used by Scrollbar to know scroll information without having access to ScrollController

Pros:

• Cool reactive API. We don't directly do stuff on State. It's State that subscribes to events triggered by its children
• More than one widget can subscribe to that same notification
• Prevents children from accessing unwanted State properties

Cons:

• May not fit your use-case
• Requires more boilerplate