Controlling State from outside of a StatefulWidget
FlutterFlutter Problem Overview
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:
-
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 ).
-
The controller can call
showActivity
orset 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.
Flutter Solutions
Solution 1 - Flutter
There are multiple ways to interact with other stateful widgets.
1. findAncestorStateOfType
The first and most straightforward is through context.findAncestorStateOfType
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.findRootAncestorStateOfType<_MyStateState>()
: context.findAncestorStateOfType<_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 callsetState
- 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.dependOnInheritedWidgetOfExactType
/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.dependOnInheritedWidgetOfExactType<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
findAncestorStateOfType
- Stream alternative is dart only (works with web) and is strongly integrated in the language (keywords such as
await for
orasync*
) - 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)
> Note: you need put above line of code inside a class, otherwise no context, can NOT call notification.
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(),
);
}
bool onTitlePush(MyNotification notification) {
print("New item ${notification.title}");
// true meaning processed, no following notification bubbling.
return true;
}
}
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'sState
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
Solution 2 - Flutter
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.
Solution 3 - Flutter
There is another common used approach to have access to State's properties/methods:
class StartupPage extends StatefulWidget {
StartupPage({Key key}) : super(key: key);
@override
StartupPageState createState() => StartupPageState();
}
// Make class public!
class StartupPageState extends State<StartupPage> {
int someStateProperty;
void someStateMethod() {}
}
// Somewhere where inside class where `StartupPage` will be used
final startupPageKey = GlobalKey<StartupPageState>();
// Somewhere where the `StartupPage` will be opened
final startupPage = StartupPage(key: startupPageKey);
Navigator.push(context, MaterialPageRoute(builder: (_) => startupPage);
// Somewhere where you need have access to state
startupPageKey.currentState.someStateProperty = 1;
startupPageKey.currentState.someStateMethod();
Solution 4 - Flutter
I do:
class StartupPage extends StatefulWidget {
StartupPageState state;
@override
StartupPageState createState() {
this.state = new StartupPageState();
return this.state;
}
}
class DetectedAnimationState extends State<DetectedAnimation> {
And outside just startupPage.state
Solution 5 - Flutter
You can use eventify
> This library provide mechanism to register for event notifications with emitter > or publisher and get notified in the event of an event.
You can do something like:
// Import the library
import 'package:eventify/eventify.dart';
final EventEmitter emitter = new EventEmitter();
var controlNumber = 50;
List<Widget> buttonsGenerator() {
final List<Widget> buttons = new List<Widget>();
for (var i = 0; i < controlNumber; i++) {
widgets.add(new MaterialButton(
// Generate 10 Buttons afterwards
onPressed: () {
controlNumber = 10;
emitter.emit("updateButtonsList", null, "");
},
);
}
}
class AState extends State<ofYourWidget> {
@override
Widget build(BuildContext context) {
List<Widget> buttons_list = buttonsGenerator();
emitter.on('updateButtonsList', null, (event, event_context) {
setState(() {
buttons_list = buttonsGenerator();
});
});
}
...
}
I can't think of anything which can't be achieved by event driven programming. You are limitless!
> "Freedom cannot be bestowed — it must be achieved." > - Elbert Hubbard
Solution 6 - Flutter
While trying to solve a similar problem, I discovered that ancestorStateOfType()
and TypeMatcher
have been deprecated. Instead, one has to use findAncestorStateOfType()
. However as per the documentation, "calling this method is relatively expensive". The documentation for the findAncestorStateOfType()
method can be found here.
In any case, to use findAncestorStateOfType()
, the following can be implemented (this is a modification of the correct answer using the findAncestorStateOfType()
method):
class StartupPage extends StatefulWidget {
static _StartupPageState of(BuildContext context) => context.findAncestorStateOfType<_StartupPageState>();
@override
_StartupPageState createState() => new _StartupPageState();
}
class _StartupPageState extends State<StartupPage> {
...
}
The state can be accessed in the same way as described in the correct answer (using StartupPage.of(context).yourFunction()
). I wanted to update the post with the new method.
Solution 7 - Flutter
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.