The following code is from an example from "dartpad.dev" web site. Within the class MyHomePage extends StatefulWidget { ..., in constructor part, I can't understand what's going on. I've been studying dart for a reasonable time, but still it's hard for me to figure out what's happenning. required this.title is OK, however why there are Key? key and : super(key: key) ?
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
debugShowCheckedModeBanner: false,
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
final String title;
const MyHomePage({
Key? key,
required this.title,
}) : super(key: key);
#override
State<MyHomePage> createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
const Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
);
}
}
Related
all. I am trying to add a ListView.builder to a Column that is inside a SingleChildScrollView. However, I am getting an exception, likely due to the fact that there is no constraint for the ListView.builder. Here is my code:
import 'package:flutter/material.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({Key? key, required this.title}) : super(key: key);
final String title;
#override
State<MyHomePage> createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: SingleChildScrollView(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
ListView.builder(
itemBuilder: (context, index) => const Text('a'),
itemCount: 2,
),
const Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
);
}
}
If I use a Container and set a defined height, the code above works. However, I am trying to get the ListView.builder to not have a fixed size. I've tried using the Expanded widget and I still get this error. Is there a way to make this work without a defined height? Thanks
In the column widget add mainAxisSize:MainAxisSize.min and in List view.builder add shrinkWrap:true and physics:NeverScrollablePhysics(). That should solve the issue and instead of center widget use SafeArea or a container with specific height.
Here's your working code
import 'package:flutter/material.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({Key? key, required this.title}) : super(key: key);
final String title;
#override
State<MyHomePage> createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: SingleChildScrollView(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
ListView.builder(
shrinkWrap:true,// -> Add this here
physics:NeverScrollablePhysics(),// -> And this one
itemBuilder: (context, index) => const Text('a'),
itemCount: 2,
),
const Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
);
}
In my Flutter project, I have the default Flutter code in home screen to update a counter by floating action button press. Here's the code of that class-
main.dart
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key? key, required this.title}) : super(key: key);
String title;
#override
State<MyHomePage> createState() => MyHomePageState();
}
class MyHomePageState extends State<MyHomePage> {
int counter = 0;
void _incrementCounter() {
setState(() {
counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
const Text(
'You have pushed the button this many times:',
),
Text(
'$counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: UpdateWidget().showButton()
),
);
}
}
Now, I just wanted to update the counter(state variable) without making it static from a new class named UpdateWidget.
I have tried updating the variable like this way:
UpdateWidget.dart
class UpdateWidget extends StatefulWidget {
UpdateWidget({Key? key}) : super(key: key);
#override
_UpdateWidgetState createState() => _UpdateWidgetState();
GlobalKey<MyHomePageState> homeKey = GlobalKey();
Widget showButton() {
return FlatButton(
onPressed: () {
MyHomePage(key: homeKey, title: 'Hello',);
debugPrint('pressed');
homeKey.currentState?.counter = 5;
}, child: const Text('A'),
);
}
}
class _UpdateWidgetState extends State<UpdateWidget> {
double defaultScreenWidth = 400.0;
double defaultScreenHeight = 810.0;
#override
Widget build(BuildContext context) {
return const Scaffold(
body: Center(
child: Text(''),
),
);
}
}
In, UpdateWidget class, inside FlatButton onPressed block, the print message is showing but it is not updating the counter variable like the way I wanted.
So, I need a way to update this variable and show that in UI.
I just started learning to code with Flutter & Dart. I could not fully understand the working principle of Flutter. There are no errors on the code I shared below. But I am not getting the result I want. On the code, the counter starts from 0. And there is a text on the counter that shows whether this counter is even or odd. This is done when the button is clicked. But the button only works once. The counter increases but the text does not change.
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
debugShowCheckedModeBanner: false,
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
final String title;
const MyHomePage({
Key? key,
required this.title,
}) : super(key: key);
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
String evenNumber = "Even Number";
String oddNumber = "Odd Number";
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(_counter / 2 == 0 ? evenNumber : oddNumber),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
);
}
}
Change this line:
_counter / 2 == 0 ? evenNumber : oddNumber
To:
_counter % 2 == 0 ? evenNumber : oddNumber
The '%' operator evaluates the remainder that you need. Not the half (_counter/2) of counter value.
You do not get your desired result because you are using using the normal division instead of modulo. With / you only check if the result is 0 and with % you can check if the number is even. To fix this change your Text widget to the following.
Text(_counter % 2 == 0 ? evenNumber : oddNumber),
Your full code should look like this now.
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
debugShowCheckedModeBanner: false,
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
final String title;
const MyHomePage({
Key? key,
required this.title,
}) : super(key: key);
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
String evenNumber = "Even Number";
String oddNumber = "Odd Number";
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Text(_counter / 2 == 0 ? evenNumber : oddNumber),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: const Icon(Icons.add),
),
);
}
}
I am having a problem with the NotificationListener in flutter. I've build a simple testing app because I am struggling with it.
After clicking on the FlatButton the Notification should be dispatched and then caught by the NotificationListener in onNotification.
So the expected console output would be:
"TestNot"
"Bubble"
But all I am getting is "TestNot".
So the notification is not caught by the listener.
Any idea what I could be doing wrong?
Thank you :-)
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyNotification extends Notification {
final String title;
const MyNotification({this.title});
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: NotificationListener<MyNotification>(
onNotification: (notification) {
print("Bubble");
return true;
},
child: Center(
child: Column(
// horizontal).
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
FlatButton(onPressed: () {print("TestNot"); MyNotification(title: "TestNot").dispatch(context);}, child: Text("TestNot")),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
)),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
When you need a child to notify its parent, you can use NotificationListener.
But, when you need communications to be implemented inversely, in other words, a parent to notify its children, you can use ValueListenableBuilder
A nice doc about it available here:
https://medium.com/flutter-community/flutter-notifications-bubble-up-and-values-go-down-c1a499d22d5f
"Flutter, notifications ‘bubble up’ and values ‘go down’"
You cannot receive the notification at the same level of where it was dispatched. Please refer to docs : https://api.flutter.dev/flutter/widgets/NotificationListener-class.html
NotificationListener class :
A widget that listens for Notifications bubbling up the tree.
I've updated your code to make it work.
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyNotification extends Notification {
final String title;
const MyNotification({this.title});
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: NotificationListener<MyNotification>(
onNotification: (MyNotification notification) {
print("Bubble");
return true;
},
child: Center(
child: Column(
// horizontal).
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
MyChild(),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
)),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
class MyChild extends StatelessWidget {
#override
Widget build(BuildContext context) {
return FlatButton(
onPressed: () {
print("TestNot");
MyNotification(title: "TestNot").dispatch(context);
},
child: Text("TestNot"));
}
}
Error saying 1 required argument(s) expected but 0 found. On every flutter project the Column gives an error. Opened up a new flutter project on VS code and still gives the same error.
have updated dart, uninstalled flutter and dart on VS and installed again, my flutter doctor runs completely fine.
import package:flutter/material.dart
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
child: Column(
mainAxisAlignment: MainAxisAlignment.center ,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}
1 required argument(s) expected, but 0 found. Try adding the additional required arguments.
Here is the full code, replace yours with mine.
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}