Is there a way to use setState with StatelessWidget?
I know that I could be used with StatefulWidget and using a State, but I don't know if there's a way to use it with StatelessWidget.
I think that's a direct question and it doesn't need code to be shown.
If you could help me, I will appreciate it.
here is an example of code that makes it possible for a StatelessWidget to update itself, its from an article of Didier Boelens.
https://www.didierboelens.com/2019/09/flutter-internals/
The following useless code makes possible for a StatelessWidget to
update itself (as if it was a StatefulWidget but without using any
setState()), by using the BuildContext …
void main(){
runApp(MaterialApp(home: TestPage(),));
}
class TestPage extends StatelessWidget {
// final because a Widget is immutable (remember?)
final bag = {"first": true};
#override
Widget build(BuildContext context){
return Scaffold(
appBar: AppBar(title: Text('Stateless ??')),
body: Container(
child: Center(
child: GestureDetector(
child: Container(
width: 50.0,`enter code here`
height: 50.0,
color: bag["first"] ? Colors.red : Colors.blue,
),
onTap: (){
bag["first"] = !bag["first"];
//
// This is the trick
//
(context as Element).markNeedsBuild();
}
),
),
),
);
}
}
Between us, when you are invoking the setState() method, the latter
ends up doing the very same thing: _element.markNeedsBuild().
No. That's the whole point of StatelessWidget: It doesn't have a state.
Only StatefulWidget has a state, and therefore only it has a setState.
Related
This seems like a bug, but I wanted to solicit feedback first. In my simple demo app below, you can see where the SliverChildBuilderDelegate is generating Text widgets as the SliverList. When you scroll up the text widgets overlap with the header. If you wrap the Text widget in a Material widget then that solves the problem (perhaps it's because of Material's clipping capability?). Mixing Material and Cupertino widgets isn't the best, however, particularly if you want to use a CupertinoTheme.
Any suggestions about what's going on and what to do about it is appreciated.
thanks
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return CupertinoApp(
title: 'Cupertino Demo',
home: MyHomePage(),
);
}
}
class MyHomePage extends StatelessWidget {
#override
Widget build(BuildContext context) {
return CupertinoPageScaffold(
child: SafeArea(
child: CustomScrollView(
shrinkWrap: true,
slivers: [
CupertinoSliverNavigationBar(
largeTitle: Text('Cupertino Demo'),
),
SliverList(
delegate: SliverChildBuilderDelegate(
(_, index) => Material(child: Text('This is list item number $index')),
),
)
],
),
),
);
}
}
Because of the shrinkWrap:true, you can read more here:
https://github.com/flutter/flutter/issues/28197
I don't see why would you need that, so simply just remove and it will work.
Okay, I am sure this is probably an easy to answer question, but after much searching I can't put my finger on the solution.
How does one in Flutter/Dart create a variable in app XYZ that maintains its' value during the time a user moves away from the app, uses another app, allows the phone to go to sleep, etc., then opens the XYZ app again.
I've created a tiny little app that demonstrates the issue as shown below.
The static variable Globals.UID can be incremented upwards, but if the app is moved away from (what term describes using another app correctly?) then as noted the phone goes to sleep, when the user comes back to the app the Globals.UID variable is reset back to -1.
Sometimes the reset is not immediate and I have to let the IOS phone sleep for perhaps 5 minutes to recreate the behavior.
Basically I need Globals.UID to retain its' value until the app is actually exited.
Any insight would truly be appreciated.
import 'package:flutter/material.dart';
import 'package:test_flutter/Globals.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++;
Globals.UID ++;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
Text ( "Global variable is:")
, Text ( Globals.UID.toString() )
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}
And in a different file called Globals.dart
class Globals {
static int UID = -1;
}
Thank you!
Ultimately, the solution appears to be solved mostly by using the shared_preferences library. That solution brings with it a good many documented problems, but to maintain data while the app is placed in the background, or even killed by perhaps the user, shared_preferences appears to be the way to go.
Actually this happens to when you don't import with reference. This is a known issues which is already fixed, may be not landed yet. You can check the issue here.
Please try import package_name/globals.dart instead of just import globals.dart if you are doing so.
Hope that helps!
I have a TextFormField that reloads the current screen when I tap on it to enter text. When I tap on the formfield the software keyboard is displayed briefly before the entire screen reloads and renders all the widgets again. I am running the app on an Android device.
Container(
child: Form(
key: _formKey,
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
TextFormField(
validator: (value) {
if (value.isEmpty) {
return 'Your input cannot be empty';
}
},
),
Padding(
padding: const EdgeInsets.symmetric(vertical: 16.0),
child: RaisedButton(
onPressed: () {
if (_formKey.currentState.validate()) {
print('validated');
}
},
child: Text('Save'),
),
),
],
),
),
margin: EdgeInsets.only(top:8.0),
),
The problem is that the controller of the TextFormField is rebuild when you click on the field, and that's the reason of your issue.
So to solve that, did you try to create a Statefull widget and then creating a TextEditingController in the State of this widget and passing it as an argument to the TextFormField ?
I had the same Problem. this was my code
class MainPage extends StatefulWidget {
#override
_MainPageState createState() => _MainPageState();
}
class _MainPageState extends State<MainPage> {
Model model = Model();
#override
Widget build(BuildContext context) {
GlobalKey<FormState> _formKey = GlobalKey<FormState>();
var mediaWidth = MediaQuery.of(context).size.width / 2.0;
return Scaffold(
...
and I solved this problem by declaring the _formKey outside of build method. and this worked for me.
class MainPage extends StatefulWidget {
#override
_MainPageState createState() => _MainPageState();
}
class _MainPageState extends State<MainPage> {
Model model = Model();
GlobalKey<FormState> _formKey = GlobalKey<FormState>();
#override
Widget build(BuildContext context) {
var mediaWidth = MediaQuery.of(context).size.width / 2.0;
return Scaffold(
...
hope it will help you
Yes, that happens because when the keyboard appears, the flutter scaffold gets resize to the current available screen size. So, we can easily handle this by preventing the scaffold size change. I suggest to set scaffold resizeToAvoidBottomInset property false. If it's true the body and the scaffolds floating widgets should size themselves to avoid the onscreen keyboard whose height is defined by the ambient MediaQuery's, MediaQueryData,viewInsets bottom property.
Solution:
resizeToAvoidBottomInset: false,
Complete example:
#override
Widget build(BuildContext context) {
setDisplayData();
return Scaffold(
resizeToAvoidBottomInset: false,
appBar: getAppBar(),
body: OrientationBuilder(
builder: (context, orientation) {
return orientation == Orientation.portrait
? _buildVerticalLayout()
: _buildHorizontalLayout();
},
),
);
Check if you are using MediaQueries wrongly in your project, I had similar issue and it stopped when I changed the MediaQuery
in my case:
Size _size = MediaQuery.of(context).size;
removing this piece of code fixed my app.
When TextFormField focused the size of screen will changed because of the appearance of keyboard, that cause rebuild of state, you cant prevent re-build of state.
Instead of trying prevent re-build state, you need to solve problems which happen when state do re-build, one of common problem is declaration and initialization variables inside build(BuildContext context){ ... }' function.
The main problem, when you need to get some data related of context (like size of screen), in this case I prefer to pass this value from parent Widget...
For example this code will cause problem when re-build state:
#override
Widget build(BuildContext context) {
double? _screenHeight = MediaQuery.of(context).size.height;
return Container();
}
To solve problem get _screenHeight from parent, to know how to do that look at https://stackoverflow.com/a/50289032/2877427
Hi,
Below code example is taken from when we create a new project in flutter.
Question:
_MyHomePageState will rerender as soon as _incrementCounter is pressed. This will rerender appBar, body, and FloatingActionButton (build method which is costly). Am I right? If right, the only thing that should rerender is a body with text.
How can I separate Widgets that needs re-rendering only? Means separating Stateless and Stateful in the below code. I hope you get my point?
Thanks
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter Demo',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new MyHomePage(title: 'Flutter Hot reload'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
_counter++;
});
}
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
),
body: new Center(
child: new Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
new Text(
'You have pushed the button this many times:',
),
new Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
),
floatingActionButton: new FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: new Icon(Icons.add),
),
);
}
}
Flutter will determine which Widgets need to be recreated or rebuilt, and only recreate / rebuild those.
IIRC, on each setState() (or animation tick, or other re-render), Flutter traverses the Widget tree and compares the before and after state of each Widget tree branch. A stateful widget is only rebuilt if its state has actually changed, and only changed stateless Widgets will be recreated. So in your case, _MyHomePageState is rebuilt, but only the second Text widget is recreated.
See
https://www.youtube.com/watch?v=dkyY9WCGMi0&t=171s
for a far more detailed explanation of Widgets (and underlying elements and render boxes).
I am trying to change the color of the element the user clicked on using a GestureDetector:
new GestureDetector(
onTap: (){
// Change the color of the container beneath
},
child: new Container(
width: 80.0,
height: 80.0,
margin: new EdgeInsets.all(10.0),
color: Colors.orange,
),
),
The problem is that I can't use setState inside of onTap. Otherwise I would have created a color variable. Any suggestions?
You can use setState() inside of onTap. In fact, that's exactly the right thing to do in this situation. If you are having trouble calling setState(), make sure your widget is stateful (see the interactivity tutorial).
You might also want to check out FlatButton or InkWell as more material-y ways to capture touches. If you really want a GestureDetector, read up on HitTestBehavior to make sure you're configuring it correctly.
Here's an example that changes to a random color every time it's clicked.
import 'dart:math';
import 'package:flutter/material.dart';
void main() {
runApp(new MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter Demo',
home: new MyHome(),
);
}
}
class MyHome extends StatefulWidget {
#override
State createState() => new _MyHomeState();
}
class _MyHomeState extends State<MyHome> {
final Random _random = new Random();
Color _color = Colors.orange;
#override
Widget build(BuildContext context) {
return new Scaffold(
body: new Center(
child: new GestureDetector(
onTap: () {
// Change the color of the container beneath
setState(() {
_color = new Color.fromRGBO(
_random.nextInt(256),
_random.nextInt(256),
_random.nextInt(256),
1.0
);
});
},
child: new Container(
width: 80.0,
height: 80.0,
margin: new EdgeInsets.all(10.0),
color: _color,
),
),
),
);
}
}