I want to use the _count variable in another widget.
I can't access _count variable with provider
class Hamburger extends StatefulWidget {
const Hamburger({super.key});
#override
State<Hamburger> createState() => _HamburgerState();
}
class _HamburgerState extends State<Hamburger> {
int _count = 1;
The issue is that you are trying to access _count in a separate class without providing context to the Provider. To access the value of _count using Provider, you need to wrap the _HamburgerState widget in a ChangeNotifierProvider and expose _count as a listenable property.
This is an example:
import 'package:flutter/widgets.dart';
import 'package:provider/provider.dart';
class Hamburger extends StatefulWidget {
const Hamburger({Key key}) : super(key: key);
#override
State<Hamburger> createState() => _HamburgerState();
}
class _HamburgerState extends State<Hamburger> {
int _count = 1;
void incrementCount() {
setState(() {
_count++;
});
}
#override
Widget build(BuildContext context) {
return ChangeNotifierProvider.value(
value: this,
child: Container(),
);
}
}
class CountNotifier extends ChangeNotifier {
int _count;
int get count => _count;
set count(int value) {
_count = value;
notifyListeners();
}
}
Related
home.dart
class HomePage extends StatefulWidget {
#override
_HomePageState createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
late Directory? appDir;
late List<String>? records;
record.dart
class Records extends StatefulWidget {
final List<String> records;
const Records({
Key? key,
required this.records,
}) : super(key: key);
#override
_RecordsState createState() => _RecordsState();
}
app.dart
class App extends GetView<AppController>{
const App({Key? key}) : super(key: key);
static const PrimaryColor1 = const Color(0xFF708FF8);
#override
Widget build(BuildContext context){
return Scaffold(
body: Obx(() {
switch(RouteName.values[controller.currentIndex.value]) {
case RouteName.Home:
return Home();
break;
case RouteName.Record:
return Record(records: records);
I need to return Record(records: records) in app.dart. The variable records is in home.dart. Is there a way to use the variable records in app.dart?
+) I tried changing it as follows, but 'recordnull' was printed.
home.dart
class video_Body extends StatefulWidget {
video_Body({Key? key, required this.index}) : super(key: key);
#override
_video_Body createState() => _video_Body();
int index;
static List<String>? records;
}
class _video_Body extends State<video_Body> {
List<String>? get record => video_Body.records;
app.dart
if(video_Body.records == null) print('recordnull');
return Record(records: video_Body.records);
There are several ways to do what you want to do, here are the two simplest ones I could think of:
1. Store the variable somewhere else.
It is hard to explain how to do this because I have very little information about what the home page is doing to the list of records, but the general idea is declare the list of records on the app.dart file and pass the value of the list into the Home class.
To do this, you will probably have to declare some sort of callback for when the Home class wants to edit the list, I will give you an example:
In this example, we have a button widget that displays a number, and when you press the button it increases the value of the number, and we want to access the current value of the number.
First I will show you the equivalent of your problem:
Button:
class MyButton extends StatefulWidget {
...
}
class _MyButtonState extends State<MyButton> {
int value = 0;
#override
Widget build(BuildContext) {
return ElevatedButton(
child: Text(value.toString()),
onPressed: () => setState(() => value++),
);
}
}
Home:
class Home extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Column(
children: [
MyButton(),
Text(buttonText), // how do we access the button text?????
]
);
}
}
And now, with the solution:
Button:
class MyButton extends StatelessWidget {
MyButton({required this.value, required this.onChange});
final void Function(int) onChange;
final int value;
#override
Widget build(BuildContext) {
return ElevatedButton(
child: Text(value.toString()),
onPressed: () => onChange(value +1),
);
}
}
Home:
class Home extends StatefulWidget {
...
}
class _HomeState extends State<Home> {
int value = 0;
#override
Widget build(BuildContext context) {
return Column(
children: [
MyButton(onChange: (v) => setState(() => value=v), value: value),
Text(value.toString()),
]
);
}
}
Of course, this solution may not work for you depending on the situation.
2. Use a static class
The idea here is to move the variable you want to access into a static class, first you need to make a class with a static variable you want to access:
class RecordsService {
static List<String>? records;
}
Because it is static, you can access it from anywhere, so on home, make the following changes:
Before:
late List<String>? records;
After:
List<String>? get records => RecordService.records;
and to access the value on app:
return Record(records: RecordService.records);
Hopefully one of these solutions solves your use case.
I want to set the location as visited while using StatefulWidget.
How do I make it just once?
If I run setLocationAsVisited function in _LocationScreenState , then will it run forever (60fps)?
class LocationScreen extends StatefulWidget {
final location;
LocationScreen({
#required this.location,
});
#override
_LocationScreenState createState() => _LocationScreenState();
}
class _LocationScreenState extends State<LocationScreen> {
#override
Widget build(BuildContext context) {
setLocationAsVisited(widget.location.id);
-----------
}
}
setLocationAsVisited(int visitedID) async {
SharedPreferences prefs = await SharedPreferences.getInstance();
await prefs.setInt('lastID', (visitedID));
}
You should make use of the initState method which runs once when the widget is first build.
class LocationScreen extends StatefulWidget {
final location;
LocationScreen({
#required this.location,
});
#override
_LocationScreenState createState() => _LocationScreenState();
}
class _LocationScreenState extends State<LocationScreen> {
// The init state function.
#override
void initState() {
setLocationAsVisited(widget.location.id);
// Make sure to call super.initState();
super.initState();
});
#override
Widget build(BuildContext context) {
// Return the widgets.
return ...
I am trying to refresh the parent widget from sub children widget. Actually, there are a number of widgets in between like A uses B and B uses C. I would like to refresh A widget on an event in C widget.I researched a lot but couldn't find an exact answer. A code snipped will be really helpful. Thanks in advance
There are a few solutions:
A pass a callback that does a setState to B, which then pass it to C:
class A extends StatefulWidget {
#override
_AState createState() => _AState();
}
class _AState extends State<A> {
#override
Widget build(BuildContext context) {
return B(
onSomething: () => setState(() {}),
);
}
}
class B extends StatelessWidget {
final VoidCallback onSomething;
const B({Key key, this.onSomething}) : super(key: key);
#override
Widget build(BuildContext context) {
return C(onSomething: onSomething);
}
}
class C extends StatelessWidget {
final VoidCallback onSomething;
const C({Key key, this.onSomething}) : super(key: key);
#override
Widget build(BuildContext context) {
return RaisedButton(
onPressed: onSomething,
);
}
}
use NotificationListener in A, and dispatch a Notification from C:
class MyNotification extends Notification {}
class A extends StatefulWidget {
#override
_AState createState() => _AState();
}
class _AState extends State<A> {
#override
Widget build(BuildContext context) {
return NotificationListener<MyNotification>(
onNotification: (_) {
setState(() {});
},
child: B(),
);
}
}
class C extends StatelessWidget {
#override
Widget build(BuildContext context) {
return RaisedButton(
onPressed: () {
MyNotification().dispatch(context);
},
);
}
}
Why do you want to refresh? You updated any data and wanted the new ones to be displayed?
You could try to use Provider widget. With it you can modify any data and notify everyone interested in that data that it changed.
In your setup you could put the provider on the A widget, on the C widget you could get the value, updated and notify everyone. When doing that, A widget will automatically rebuild with the updated information.
The code would be something like this:
class AppState with ChangeNotifier {
AppState();
YourData _data;
void setData(YourData data) {
_data = data;
notifyListeners();
}
}
ChangeNotifierProvider.value(
value: AppState(),
child: WidgetA()
)
WidgetC() {
Provider.of<AppState>(context).setData(yourChangedDataHere);
}
I'm wondering what the recommended way of passing data to a stateful widget, while creating it, is.
The two styles I've seen are:
class ServerInfo extends StatefulWidget {
Server _server;
ServerInfo(Server server) {
this._server = server;
}
#override
State<StatefulWidget> createState() => new _ServerInfoState(_server);
}
class _ServerInfoState extends State<ServerInfo> {
Server _server;
_ServerInfoState(Server server) {
this._server = server;
}
}
This method keeps a value both in ServerInfo and _ServerInfoState, which seems a bit wasteful.
The other method is to use widget._server:
class ServerInfo extends StatefulWidget {
Server _server;
ServerInfo(Server server) {
this._server = server;
}
#override
State<StatefulWidget> createState() => new _ServerInfoState();
}
class _ServerInfoState extends State<ServerInfo> {
#override
Widget build(BuildContext context) {
widget._server = "10"; // Do something we the server value
return null;
}
}
This seems a bit backwards as the state is no longer stored in _ServerInfoSate but instead in the widget.
Is there a best practice for this?
Don't pass parameters to State using it's constructor.
You should only access the parameters using this.widget.myField.
Not only editing the constructor requires a lot of manual work ; it doesn't bring anything. There's no reason to duplicate all the fields of Widget.
EDIT :
Here's an example:
class ServerIpText extends StatefulWidget {
final String serverIP;
const ServerIpText ({ Key? key, this.serverIP }): super(key: key);
#override
_ServerIpTextState createState() => _ServerIpTextState();
}
class _ServerIpTextState extends State<ServerIpText> {
#override
Widget build(BuildContext context) {
return Text(widget.serverIP);
}
}
class AnotherClass extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Center(
child: ServerIpText(serverIP: "127.0.0.1")
);
}
}
Best way is don't pass parameters to State class using it's constructor. You can easily access in State class using widget.myField.
For Example
class UserData extends StatefulWidget {
final String clientName;
final int clientID;
const UserData(this.clientName,this.clientID);
#override
UserDataState createState() => UserDataState();
}
class UserDataState extends State<UserData> {
#override
Widget build(BuildContext context) {
// Here you direct access using widget
return Text(widget.clientName);
}
}
Pass your data when you Navigate screen :
Navigator.of(context).push(MaterialPageRoute(builder: (context) => UserData("WonderClientName",132)));
Another answer, building on #RĂ©miRousselet's anwser and for #user6638204's question, if you want to pass initial values and still be able to update them in the state later:
class MyStateful extends StatefulWidget {
final String foo;
const MyStateful({Key key, this.foo}): super(key: key);
#override
_MyStatefulState createState() => _MyStatefulState(foo: this.foo);
}
class _MyStatefulState extends State<MyStateful> {
String foo;
_MyStatefulState({this.foo});
#override
Widget build(BuildContext context) {
return Text(foo);
}
}
For passing initial values (without passing anything to the constructor)
class MyStateful extends StatefulWidget {
final String foo;
const MyStateful({Key key, this.foo}): super(key: key);
#override
_MyStatefulState createState() => _MyStatefulState();
}
class _MyStatefulState extends State<MyStateful> {
#override
void initState(){
super.initState();
// you can use this.widget.foo here
}
#override
Widget build(BuildContext context) {
return Text(foo);
}
}
Flutter's stateful widgets API is kinda awkward: storing data in Widget in order to access it in build() method which resides in State object đŸ¤¦ If you don't want to use some of bigger state management options (Provider, BLoC), use flutter_hooks (https://pub.dev/packages/flutter_hooks) - it is a nicer and cleaner substitute for SatefullWidgets:
class Counter extends HookWidget {
final int _initialCount;
Counter(this._initialCount = 0);
#override
Widget build(BuildContext context) {
final counter = useState(_initialCount);
return GestureDetector(
// automatically triggers a rebuild of Counter widget
onTap: () => counter.value++,
child: Text(counter.value.toString()),
);
}
}
#RĂ©mi Rousselet, #Sanjayrajsinh, #Daksh Shah is also better. but I am also defined this is in from starting point.that which parameter is which value
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
String name = "Flutter Demo";
String description = "This is Demo Application";
#override
Widget build(BuildContext context) {
return MaterialApp(
home: MainActivity(
appName: name,
appDescription: description,
),
);
}
}
class MainActivity extends StatefulWidget {
MainActivity({Key key, this.appName, this.appDescription}) : super(key: key);
var appName;
var appDescription;
#override
_MainActivityState createState() => _MainActivityState();
}
class _MainActivityState extends State<MainActivity> {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.appName),
),
body: Scaffold(
body: Center(
child: Text(widget.appDescription),
),
),
);
}
}
The best practice is to define the stateful widget class as immutable which means defining all dependencies (arrival parameter) as final parameters. and getting access to them by widget.<fieldName> in the state class. In case you want to change their values like reassigning you should define the same typed properties in your state class and re-assign them in the initState function. it is highly recommended not to define any not-final property in your stateful widget class and make it a mutable class. something like this pattern:
class SomePage extends StatefulWidget{
final String? value;
SomePage({this.value});
#override
State<SomePage> createState() => _SomePageState();
}
class _SomePageState extends State<SomePage> {
String? _value;
#override
void initState(){
super.initState();
setState(() {
_value = widget.value;
});
}
#override
Widget build(BuildContext context) {
return Text(_value);
}
}
To pass data to stateful widget, first of all, create two pages. Now from the first page open the second page and pass the data.
class PageTwo extends StatefulWidget {
final String title;
final String name;
PageTwo ({ this.title, this.name });
#override
PageTwoState createState() => PageTwoState();
}
class PageTwoStateState extends State<PageTwo> {
#override
Widget build(BuildContext context) {
return Text(
widget.title,
style: TextStyle(
fontSize: 18, fontWeight: FontWeight.w700),
),
}
}
class PageOne extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialButton(
text: "Open PageTwo",
onPressed: () {
var destination = ServicePage(
title: '<Page Title>',
provider: '<Page Name>',
);
Navigator.push(context,
MaterialPageRoute(builder: (context) => destination));
},);
}
}
I'm wondering what the recommended way of passing data to a stateful widget, while creating it, is.
The two styles I've seen are:
class ServerInfo extends StatefulWidget {
Server _server;
ServerInfo(Server server) {
this._server = server;
}
#override
State<StatefulWidget> createState() => new _ServerInfoState(_server);
}
class _ServerInfoState extends State<ServerInfo> {
Server _server;
_ServerInfoState(Server server) {
this._server = server;
}
}
This method keeps a value both in ServerInfo and _ServerInfoState, which seems a bit wasteful.
The other method is to use widget._server:
class ServerInfo extends StatefulWidget {
Server _server;
ServerInfo(Server server) {
this._server = server;
}
#override
State<StatefulWidget> createState() => new _ServerInfoState();
}
class _ServerInfoState extends State<ServerInfo> {
#override
Widget build(BuildContext context) {
widget._server = "10"; // Do something we the server value
return null;
}
}
This seems a bit backwards as the state is no longer stored in _ServerInfoSate but instead in the widget.
Is there a best practice for this?
Don't pass parameters to State using it's constructor.
You should only access the parameters using this.widget.myField.
Not only editing the constructor requires a lot of manual work ; it doesn't bring anything. There's no reason to duplicate all the fields of Widget.
EDIT :
Here's an example:
class ServerIpText extends StatefulWidget {
final String serverIP;
const ServerIpText ({ Key? key, this.serverIP }): super(key: key);
#override
_ServerIpTextState createState() => _ServerIpTextState();
}
class _ServerIpTextState extends State<ServerIpText> {
#override
Widget build(BuildContext context) {
return Text(widget.serverIP);
}
}
class AnotherClass extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Center(
child: ServerIpText(serverIP: "127.0.0.1")
);
}
}
Best way is don't pass parameters to State class using it's constructor. You can easily access in State class using widget.myField.
For Example
class UserData extends StatefulWidget {
final String clientName;
final int clientID;
const UserData(this.clientName,this.clientID);
#override
UserDataState createState() => UserDataState();
}
class UserDataState extends State<UserData> {
#override
Widget build(BuildContext context) {
// Here you direct access using widget
return Text(widget.clientName);
}
}
Pass your data when you Navigate screen :
Navigator.of(context).push(MaterialPageRoute(builder: (context) => UserData("WonderClientName",132)));
Another answer, building on #RĂ©miRousselet's anwser and for #user6638204's question, if you want to pass initial values and still be able to update them in the state later:
class MyStateful extends StatefulWidget {
final String foo;
const MyStateful({Key key, this.foo}): super(key: key);
#override
_MyStatefulState createState() => _MyStatefulState(foo: this.foo);
}
class _MyStatefulState extends State<MyStateful> {
String foo;
_MyStatefulState({this.foo});
#override
Widget build(BuildContext context) {
return Text(foo);
}
}
For passing initial values (without passing anything to the constructor)
class MyStateful extends StatefulWidget {
final String foo;
const MyStateful({Key key, this.foo}): super(key: key);
#override
_MyStatefulState createState() => _MyStatefulState();
}
class _MyStatefulState extends State<MyStateful> {
#override
void initState(){
super.initState();
// you can use this.widget.foo here
}
#override
Widget build(BuildContext context) {
return Text(foo);
}
}
Flutter's stateful widgets API is kinda awkward: storing data in Widget in order to access it in build() method which resides in State object đŸ¤¦ If you don't want to use some of bigger state management options (Provider, BLoC), use flutter_hooks (https://pub.dev/packages/flutter_hooks) - it is a nicer and cleaner substitute for SatefullWidgets:
class Counter extends HookWidget {
final int _initialCount;
Counter(this._initialCount = 0);
#override
Widget build(BuildContext context) {
final counter = useState(_initialCount);
return GestureDetector(
// automatically triggers a rebuild of Counter widget
onTap: () => counter.value++,
child: Text(counter.value.toString()),
);
}
}
#RĂ©mi Rousselet, #Sanjayrajsinh, #Daksh Shah is also better. but I am also defined this is in from starting point.that which parameter is which value
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
String name = "Flutter Demo";
String description = "This is Demo Application";
#override
Widget build(BuildContext context) {
return MaterialApp(
home: MainActivity(
appName: name,
appDescription: description,
),
);
}
}
class MainActivity extends StatefulWidget {
MainActivity({Key key, this.appName, this.appDescription}) : super(key: key);
var appName;
var appDescription;
#override
_MainActivityState createState() => _MainActivityState();
}
class _MainActivityState extends State<MainActivity> {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.appName),
),
body: Scaffold(
body: Center(
child: Text(widget.appDescription),
),
),
);
}
}
The best practice is to define the stateful widget class as immutable which means defining all dependencies (arrival parameter) as final parameters. and getting access to them by widget.<fieldName> in the state class. In case you want to change their values like reassigning you should define the same typed properties in your state class and re-assign them in the initState function. it is highly recommended not to define any not-final property in your stateful widget class and make it a mutable class. something like this pattern:
class SomePage extends StatefulWidget{
final String? value;
SomePage({this.value});
#override
State<SomePage> createState() => _SomePageState();
}
class _SomePageState extends State<SomePage> {
String? _value;
#override
void initState(){
super.initState();
setState(() {
_value = widget.value;
});
}
#override
Widget build(BuildContext context) {
return Text(_value);
}
}
To pass data to stateful widget, first of all, create two pages. Now from the first page open the second page and pass the data.
class PageTwo extends StatefulWidget {
final String title;
final String name;
PageTwo ({ this.title, this.name });
#override
PageTwoState createState() => PageTwoState();
}
class PageTwoStateState extends State<PageTwo> {
#override
Widget build(BuildContext context) {
return Text(
widget.title,
style: TextStyle(
fontSize: 18, fontWeight: FontWeight.w700),
),
}
}
class PageOne extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialButton(
text: "Open PageTwo",
onPressed: () {
var destination = ServicePage(
title: '<Page Title>',
provider: '<Page Name>',
);
Navigator.push(context,
MaterialPageRoute(builder: (context) => destination));
},);
}
}