In my Flutter app i need being able to react on incoming fcm messages which can instruct the app to go a different navigation tab by having a corresponding key/value pair in its data payload.
Currently the selected index is stored in the stateful widget which also hosts the bottom navigation bar:
#override
Widget build(BuildContext context) {
presentTicketsModel = ref.watch(presentTicketsModelProvider);
contractsModel = ref.watch(contractsModelProvider);
return Scaffold(
body: PersistentTabs(
currentTabIndex: index,
screenWidgets: buildPages(),
),
bottomNavigationBar: NavigationBar(
height: 60,
selectedIndex: index,
onDestinationSelected: (index) => setState(() {
this.index = index;
}),
destinations: _buildNavigationDestinations(),
),
);
}
With the new challenge i thought about moving that state index into a separate object and use Riverpod's StateNotifierProvider to provide that state object, as it is described in the official doc (https://riverpod.dev/docs/providers/state_notifier_provider).
What i don't get is: How can the following service class (which listens for incoming fcm messages) get hold of that state object and update the index in order that the watching view class gets notified and can switch to the targeted navigation tab?
class PushNotificationService {
final fcm = FirebaseMessaging.instance;
Future initialise() async {
print('initialising push notification service...');
}
/// foreground handler
FirebaseMessaging.onMessage.listen((RemoteMessage message) {
print('onMessage: $message');
// here the state change would be done
});
/// handler if the app has been opened from a background state
FirebaseMessaging.onMessageOpenedApp.listen((RemoteMessage message) {
print('onMessageOpenedApp: $message');
});
}
}
Or asked differently: How does this service class get passed a ProviderReference in order to access the mentioned state object and change the index value?
The service currently is registered with GetIt as a lazy singleton:
GetIt locator = GetIt.instance;
void setupLocator() {
locator.registerLazySingleton(PushNotificationService.new);
}
The initialise method can have a Ref parameter like so:
class PushNotificationService {
final fcm = FirebaseMessaging.instance;
Future initialise(WidgetRef ref) async {
print('initialising push notification service...');
}
/// foreground handler
FirebaseMessaging.onMessage.listen((RemoteMessage message) {
// Use ref.read here -> ref.read();
});
/// handler if the app has been opened from a background state
FirebaseMessaging.onMessageOpenedApp.listen((RemoteMessage message) {
print('onMessageOpenedApp: $message');
});
}
}
Then you can pass a ref when calling initialise.
PushNotificationService.initialise(ref);
If you'd be calling initialise in a widget, use WidgetRef instead of Ref
EDIT: Where to pass ref (notice that we're using WidgetRef now)
Follow these steps
Make MyApp a ConsumerStatefulWidget
Call PushNotificationService.initialise(ref); in initState
Full code:
void main() {
runApp(const ProviderScope(child: MyApp()));
}
class MyApp extends ConsumerStatefulWidget {
const MyApp({Key? key}) : super(key: key);
#override
ConsumerState<ConsumerStatefulWidget> createState() => _MyAppState();
}
class _MyAppState extends ConsumerState<MyApp> {
#override
initState(){
PushNotificationService.initialise(ref);
super.initState();
}
class _MyAppState extends ConsumerState<MyApp> {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: "App",
home: Home(),
);
}
}
I have a StateNotifier whose state I want to use in a widget.
As far as I know, if you watch a StateNotifierProvider with ref.watch in a build, the widget gets rebuilt every time the state changes.
Now, In the StateNotifier I have a DatabaseService instance to call api requests and set the state accordingly and in a ConsumerWidget I watch the state.
The thing is, I want to call a fetch method defined in StateNotifier the first time the widget builds, so I can display the data retrieved from the database.
Something like this
class MyStateNotifier extends StateNotifier<CustomState> {
final DatabaseService databaseService;
MyStateNotifier(this.databaseService) : super(CustomStateInit());
Future<void> fetchData() async {
state = CustomStateLoading();
final result = await databaseService.apiRequest();
state = CustomStateSuccess(result);
}
}
final stateNotifierProvider = StateNotifierProvider((ref) => MyStateNotifier());
and in the widget
class MyWidget extends ConsumerWidget {
// I have to call the MyStateNotifier fetchData() method to get the data
Widget build(BuildContext context, WidgetRef ref) {
final data = ref.watch(stateNotifierProvider);
return Column(
children: data.map((e) => Text(e)).toList()
);
}
}
To call fetchData() once you watch your stateNotifierProvider you need to call it in the StateNotifier's constructor like this:
MyStateNotifier(this.databaseService) : super(CustomStateInit()){fetchData();}
I am writing a Flutter app that uses a Global Function to handle Pushy.me notifications. This function needs to update a stateful widget's state.
I have tried a Global Key to access the widgets current state but it did nothing. I have tried an Eventify emitter, the emit and the listener didnt seem to line up.
import 'package:eventify/eventify.dart';
EventEmitter emitter = new EventEmitter();
GlobalKey<_WrapperScreenState> _key = GlobalKey<_WrapperScreenState>();
void backgroundNotificationListener(Map<String, dynamic> data) {
// Print notification payload data
print('Received notification: $data');
// Notification title
String notificationTitle = 'MyApp';
// Attempt to extract the "message" property from the payload: {"message":"Hello World!"}
String notificationText = data['message'] ?? 'Hello World!';
Pushy.notify(notificationTitle, notificationText, data);
emitter.emit('updateList',null,"");
try{
print(_key.currentState.test);
}
catch(e){
print(e);
}
// Clear iOS app badge number
Pushy.clearBadge();
}
class WrapperScreen extends StatefulWidget {
#override
_WrapperScreenState createState() => _WrapperScreenState();
}
You can try using events for this, using a StreamController or the event bus package. Your stateful widget would listen on a global event bus, and you can fire an event with the necessary information for the widget itself to use to update the state.
Something like this (using the event bus package):
// main.dart
EventBus eventBus = EventBus();
class MyEvent {}
void somewhereGlobal() {
// trigger widget state change from global location
eventBus.fire(MyEvent());
}
void main() {
...
}
// my_stateful_widget.dart
...
class _MyWidgetState extends State<MyWidget> {
#override
void initState() {
super.initState();
eventBus.on<MyEvent>().listen((event) {
// update widget state
print("update widget state");
});
}
...
I am new to flutter and I am woking on sockets which keeps streaming data which I need to update on a appbar. So I have to stateful widgets
class _HomePageState extends State<HomePage> {
TickerAppBar appBar = TickerAppBar();
//Some declarations
#override
void initState() {
// TODO: implement initState
super.initState();
connectionSetup();
}
connectionSetup() async {
this.socket.connect(connectionString);
this.socket.sendSubscription(subscriptionString);
await processData();
}
Future<Null> processData() async{
subscription = socket.streamController.stream.listen(
(data) {
appBar.setStreamData(data); //Passing data into appbar widget
//rest of the code of current widget
}
);
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: appBar,
//rest of the code
)
}
}
class TickerAppBar extends StatefulWidget implements PreferredSizeWidget {
String streamData;
_TickerAppBarState child = _TickerAppBarState();
setStreamData(data){
this.streamData = data;
child.processData(); //To invoke the function every time the data is passed
}
}
class _TickerAppBarState extends State<TickerAppBar>{
Future<String> processData() async{
//Cannot set state here since its a constructor and the widget is not mounted.
}
}
So how I can set the data every time the data is passed into tickerappbar? so that my custom appbar displays new data which comes in every second.
I would suggest you follow the BLoC pattern, in which you can define your streams at one place and subscribe to the said stream from any widget, once you have the data ready, you can write to the stream and it will be shared on all the subscribed widgets.
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 event and perform some action ( DB call, service request, etc ).
The controller can call showActivity or set message to have the view show progress with a message.
Because a Stateful Widget does not expose its 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 this:
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.
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 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.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 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)
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'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
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.
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();
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
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.
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
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.