Related
I have tackeled without any success with problem...
My main page is stateful class with tabbar with two tabs. First tab has some text from global variables and couple of textfields that also are prefilled with global variables.
Second tab has a button and ontap it calls setstate that changes variables, that are used on first tab and then animates to first tab.
My problem is that first tabs text doesnt change to new value. At the same time textfields will have new values. If i add print command before returning text on first tab, code will print out new values, but state for text is not set, at the same time textfields state will be set.
Its not possible at moment to add code, but i hope i described mu problem good enough.
Thank You!
I tryed many things and now i got strange working solution that makes what i want.
If i just set new variables and after that let tabcontroller to animate dirst page, pages state will not be set, but if i add small delay, then it works like i want. If anyone could explain why, i would be really thankful.
onPressed: () {
setProduct();
Timer(Duration(milliseconds: 100), animateToFirstPage);
}
There is a really elaborate explanation in this answer.
Bottom line, there is a race condition between setState and animateTo, and he suggests breaking it so:
onPressed: () {
setProduct();
WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
animateToFirstPage;
})
}
Verified it worked for me, and without creepy .sleep solutions
Use a simple state management solution. Where both tabs can listen and modify the values you want. Without code is hard to demonstrate. But you can't simply change the state of a widget from another widget, using provider would be easier.
To update and listen to the change, use StreamController and StreamBuilder. You can put the first tab in a widget combined with SingleTickerProviderStateMixin to prevent it from reloading as well. I created a simple app for demonstration:
Full example:
main.dart
import 'dart:async';
import 'dart:math';
import 'package:flutter/material.dart';
void main() {
runApp(MaterialApp(home: MyApp()));
}
class MyApp extends StatefulWidget {
#override
_MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> with SingleTickerProviderStateMixin {
final StreamController _streamController = StreamController<String>();
TabController _tabController;
#override
void initState() {
_tabController = TabController(length: 2, vsync: this);
super.initState();
}
#override
void dispose() {
_streamController.close();
super.dispose();
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Sample App'),
),
bottomNavigationBar: TabBar(
controller: _tabController,
labelColor: Colors.red,
tabs: [Tab(text: 'Tab 1'), Tab(text: 'Tab 2')],
),
body: TabBarView(
controller: _tabController,
children: [
FirstTab(_streamController),
Container(
child: Center(
child: TextButton(
child: Text('Press me'),
onPressed: () {
final _someText =
'Random number: ' + Random().nextInt(100).toString();
_streamController.add(_someText);
_tabController.animateTo(0);
},
),
),
),
],
),
);
}
}
class FirstTab extends StatefulWidget {
final StreamController _streamController;
FirstTab(this._streamController);
#override
_FirstTabState createState() => _FirstTabState();
}
class _FirstTabState extends State<FirstTab>
with AutomaticKeepAliveClientMixin {
#override
bool get wantKeepAlive => true;
#override
Widget build(BuildContext context) {
super.build(context);
return Container(
child: Center(
child: StreamBuilder<String>(
initialData: 'Empty text',
stream: widget._streamController.stream,
builder: (context, snapshot) {
return Text(snapshot.data);
}),
),
);
}
}
I have widget that is unmounted on page change.
I am using flutter web.
I looked at all the methods here but I can't seem to find the one.
I tried
#override
void dispose() {
print("dispose?");
super.dispose();
}
#override
void deactivate() {
print("deactivate");
super.deactivate();
}
and neither is called.
It is a very simple thing I need to detect when the widget is not being "built" (rendered). I don't think having something to dtect the page change is the solution, it seems overkill.
Thank you
Adding code sample
import 'package:flutter/material.dart';
void main() {
runApp(MaterialApp(
title: 'Named Routes Demo',
initialRoute: '/',
routes: {
'/': (context) => FirstScreen(),
'/second': (context) => SecondScreen(),
},
));
}
class FirstScreen extends StatefulWidget {
FirstScreen({Key key}) : super(key: key);
#override
_FirstScreenState createState() => _FirstScreenState();
}
class _FirstScreenState extends State<FirstScreen> {
#override
void dispose() {
print("dispose");
super.dispose();
}
#override
void deactivate() {
print("deactivate");
super.deactivate();
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('First Screen'),
),
body: Center(
child: ElevatedButton(
child: Text('Launch screen'),
onPressed: () {
Navigator.pushNamed(context, '/second');
},
),
),
);
}
}
class SecondScreen extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Second Screen"),
),
body: Center(
child: ElevatedButton(
onPressed: () {
// Navigate back to the first screen by popping the current route
// off the stack.
Navigator.pop(context);
},
child: Text('Go back!'),
),
),
);
}
}
It seems neither deactive nor dispose are called when navigating to the second screen and the first screen widget stops being part of the render tree. So what to call to detect that Screen 1 leaves the render tree?
if the Widget has not mounted then return it. But you have to do it before any setState method
if (!mounted) return;
setState(() {});
or
if (mounted) {
//your code
};
setState(() {});
You can read more about it here: https://api.flutter.dev/flutter/widgets/State/mounted.html
Anyways the only thing beeing called before the build is the initState. Everything youre doing inside this is before the build.
initstate: https://api.flutter.dev/flutter/widgets/EditableTextState/initState.html
In the code example you added, screen 1 is never unmounted, thus those callbacks are not fired. Screen 2 is rendered on top of screen 1, and both are mounted.
You can confirm that screen 1 is always mounted, by adding a periodic callback in its state, add let it print out whether it's still mounted, for example:
import 'dart:async';
// in _FirstScreenState, add:
#override
void initState() {
Timer.periodic(Duration(seconds: 1), (timer) {
print("screen 1 is still mounted? $mounted");
});
super.initState();
}
You will see that, it is always mounted, even if screen 2 is rendered on top of screen 1.
Also, you can do the same deactivate and dispose logic for screen 2, and you will see that, when navigating back from screen 2 to screen 1, screen 2 is truly unmounted, and both of the callbacks work correctly. Thus again proving the problem isn't at those callbacks, it's that screen 1 wasn't unmounted.
So yeah... before you posted the code sample, I thought you ran into some error about widget being unmounted (defunct). But in this case, since you are using buttons and thus know exactly when page transition occurs, maybe you can just do whatever business logic you need to do, in the button callback?
For various reasons, sometimes the build method of my widgets is called again.
I know that it happens because a parent updated. But this causes undesired effects.
A typical situation where it causes problems is when using FutureBuilder this way:
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: httpCall(),
builder: (context, snapshot) {
// create some layout here
},
);
}
In this example, if the build method were to be called again, it would trigger another HTTP request. Which is undesired.
Considering this, how to deal with the unwanted build? Is there any way to prevent a build call?
The build method is designed in such a way that it should be pure/without side effects. This is because many external factors can trigger a new widget build, such as:
Route pop/push
Screen resize, usually due to keyboard appearance or orientation change
The parent widget recreated its child
An InheritedWidget the widget depends on (Class.of(context) pattern) change
This means that the build method should not trigger an http call or modify any state.
How is this related to the question?
The problem you are facing is that your build method has side effects/is not pure, making extraneous build calls troublesome.
Instead of preventing build calls, you should make your build method pure, so that it can be called anytime without impact.
In the case of your example, you'd transform your widget into a StatefulWidget then extract that HTTP call to the initState of your State:
class Example extends StatefulWidget {
#override
_ExampleState createState() => _ExampleState();
}
class _ExampleState extends State<Example> {
Future<int> future;
#override
void initState() {
future = Future.value(42);
super.initState();
}
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: future,
builder: (context, snapshot) {
// create some layout here
},
);
}
}
I know this already. I came here because I really want to optimize rebuilds
It is also possible to make a widget capable of rebuilding without forcing its children to build too.
When the instance of a widget stays the same; Flutter purposefully won't rebuild children. It implies that you can cache parts of your widget tree to prevent unnecessary rebuilds.
The easiest way is to use dart const constructors:
#override
Widget build(BuildContext context) {
return const DecoratedBox(
decoration: BoxDecoration(),
child: Text("Hello World"),
);
}
Thanks to that const keyword, the instance of DecoratedBox will stay the same even if the build was called hundreds of times.
But you can achieve the same result manually:
#override
Widget build(BuildContext context) {
final subtree = MyWidget(
child: Text("Hello World")
);
return StreamBuilder<String>(
stream: stream,
initialData: "Foo",
builder: (context, snapshot) {
return Column(
children: <Widget>[
Text(snapshot.data),
subtree,
],
);
},
);
}
In this example when StreamBuilder is notified of new values, subtree won't rebuild even if the StreamBuilder/Column does.
It happens because, thanks to the closure, the instance of MyWidget didn't change.
This pattern is used a lot in animations. Typical uses are AnimatedBuilder and all transitions such as AlignTransition.
You could also store subtree into a field of your class, although less recommended as it breaks the hot-reload feature.
You can prevent unwanted build calling, using these way
Create child Statefull class for individual small part of UI
Use Provider library, so using it you can stop unwanted build method calling
In these below situation build method call
After calling initState
After calling didUpdateWidget
when setState() is called.
when keyboard is open
when screen orientation changed
If Parent widget is build then child widget also rebuild
Flutter also has ValueListenableBuilder<T> class . It allows you to rebuild only some of the widgets necessary for your purpose and skip the expensive widgets.
you can see the documents here ValueListenableBuilder flutter docs
or just the sample code below:
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:'),
ValueListenableBuilder(
builder: (BuildContext context, int value, Widget child) {
// This builder will only get called when the _counter
// is updated.
return Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: <Widget>[
Text('$value'),
child,
],
);
},
valueListenable: _counter,
// The child parameter is most helpful if the child is
// expensive to build and does not depend on the value from
// the notifier.
child: goodJob,
)
],
),
),
floatingActionButton: FloatingActionButton(
child: Icon(Icons.plus_one),
onPressed: () => _counter.value += 1,
),
);
One of the easiest ways to avoid unwanted reBuilds that are caused usually by calling setState() in order to update only a specific Widget and not refreshing the whole page, is to cut that part of your code and wrap it as an independent Widget in another Stateful class.
For example in following code, Build method of parent page is called over and over by pressing the FAB button:
import 'package:flutter/material.dart';
void main() {
runApp(TestApp());
}
class TestApp extends StatefulWidget {
#override
_TestAppState createState() => _TestAppState();
}
class _TestAppState extends State<TestApp> {
int c = 0;
#override
Widget build(BuildContext context) {
print('build is called');
return MaterialApp(home: Scaffold(
appBar: AppBar(
title: Text('my test app'),
),
body: Center(child:Text('this is a test page')),
floatingActionButton: FloatingActionButton(
onPressed: (){
setState(() {
c++;
});
},
tooltip: 'Increment',
child: Icon(Icons.wb_incandescent_outlined, color: (c % 2) == 0 ? Colors.white : Colors.black)
)
));
}
}
But if you separate the FloatingActionButton widget in another class with its own life cycle, setState() method does not cause the parent class Build method to re-run:
import 'package:flutter/material.dart';
import 'package:flutter_app_mohsen/widgets/my_widget.dart';
void main() {
runApp(TestApp());
}
class TestApp extends StatefulWidget {
#override
_TestAppState createState() => _TestAppState();
}
class _TestAppState extends State<TestApp> {
int c = 0;
#override
Widget build(BuildContext context) {
print('build is called');
return MaterialApp(home: Scaffold(
appBar: AppBar(
title: Text('my test app'),
),
body: Center(child:Text('this is a test page')),
floatingActionButton: MyWidget(number: c)
));
}
}
and the MyWidget class:
import 'package:flutter/material.dart';
class MyWidget extends StatefulWidget {
int number;
MyWidget({this.number});
#override
_MyWidgetState createState() => _MyWidgetState();
}
class _MyWidgetState extends State<MyWidget> {
#override
Widget build(BuildContext context) {
return FloatingActionButton(
onPressed: (){
setState(() {
widget.number++;
});
},
tooltip: 'Increment',
child: Icon(Icons.wb_incandescent_outlined, color: (widget.number % 2) == 0 ? Colors.white : Colors.black)
);
}
}
I just want to share my experience of unwanted widget build mainly due to context but I found a way that is very effective for
Route pop/push
So you need to use Navigator.pushReplacement() so that the context of the previous page has no relation with the upcoming page
Use Navigator.pushReplacement() for navigating from the first page to Second
In second page again we need to use Navigator.pushReplacement()
In appBar we add -
leading: IconButton(
icon: Icon(Icons.arrow_back),
onPressed: () {
Navigator.pushReplacement(
context,
RightToLeft(page: MyHomePage()),
);
},
)
In this way we can optimize our app
You can do something like this:
class Example extends StatefulWidget {
#override
_ExampleState createState() => _ExampleState();
}
class _ExampleState extends State<Example> {
Future<int> future;
#override
void initState() {
future = httpCall();
super.initState();
}
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: future,
builder: (context, snapshot) {
// create some layout here
},
);
}
void refresh(){
setState((){
future = httpCall();
});
}
}
i'm brand new to Flutter.
I'm trying to open a panel pressing a button and than closing it by pressing a button on that panel.
I've managed to do it by writing the code in the same page.
What i can't do is splitting the code and keep everything working.
What I'm actually doing is calling a variable in the State of a widget that is initialized False and then with an if statement i'm calling: or an empty container or the panel i want.
When i press the button i call SetState(){} and the variable changes to true to let the panel appears, then in the panel there's button that do opposite thing.
Assuming that what i'm doing it is correct. How to i keep doing this with the panel refactored in a new page?
I've red something about streams and inherited widgets but i haven't completely understood
If I understand correctly, you want to notify a StatefullWidget from another StatefullWidget. There are several approaches on this one but since you've mentioned Streams, I'll try to post an example and explain a bit this scenario.
So basically, you can consider the streams like a pipe linked to a faucet in one end and the other end it's added into a cup (the end can be split in multiple ends and put in multiple cups, "broadcast streams").
Now, the cup is the listener (subscriber) and waits for water to drop trough the pipe.
The faucet is the emitter, and it will emit water droplets when the faucet is opened.
The faucet can be opened when the other end is put into a cup, this is a smart faucet with some cool sensors, (the emitter will start emitting events when a subscriber is "detected).
The droplets are actual events that are happening in the the app.
Also you must remember to close the faucet in order to avoid a massive leak from your cup into the kitchen floor. (you must cancel the subscribers when you've done handling events to avoid a leak).
Now for your particular case here's the code snippet that kind of illustrate the above metaphor:
class ThePannel extends StatefulWidget { // this is the cup
final Stream<bool> closeMeStream; // this is the pipe
const ThePannel({Key key, this.closeMeStream}) : super(key: key);
#override
_ThePannelState createState() => _ThePannelState(closeMeStream);
}
class _ThePannelState extends State<ThePannel> {
bool _closeMe = false;
final Stream<bool> closeMeStream;
StreamSubscription _streamSubscription;
_ThePannelState(this.closeMeStream);
#override
void initState() {
super.initState();
_streamSubscription = closeMeStream.listen((shouldClose) { // here we listen for new events coming down the pipe
setState(() {
_closeMe = shouldClose; // we got a new "droplet"
});
});
}
#override
void dispose() {
_streamSubscription.cancel(); // THIS IS QUITE IMPORTANT, we have to close the faucet
super.dispose();
}
#override
Widget build(BuildContext context) {
return Stack(
children: <Widget>[
SomeWidgetHere(shouldClose: _closeMe),
RaisedButton(
onPressed: () {
setState(() {
_closeMe = true;
});
},
)
],
);
}
}
class SomeWidgetThatUseThePreviousOne extends StatefulWidget { // this one is the faucet, it will emit droplets
#override
_SomeWidgetThatUseThePreviousOneState createState() =>
_SomeWidgetThatUseThePreviousOneState();
}
class _SomeWidgetThatUseThePreviousOneState
extends State<SomeWidgetThatUseThePreviousOne> {
final StreamController<bool> thisStreamWillEmitEvents = StreamController(); // this is the end of the pipe linked to the faucet
#override
Widget build(BuildContext context) {
return Stack(
children: <Widget>[
ThePannel(closeMeStream: thisStreamWillEmitEvents.stream), // we send the other end of the pipe to the cup
RaisedButton(
child: Text("THIS SHOULD CLOSE THE PANNEL"),
onPressed: () {
thisStreamWillEmitEvents.add(true); // we will emit one droplet here
},
),
RaisedButton(
child: Text("THIS SHOULD OPEN THE PANNEL"),
onPressed: () {
thisStreamWillEmitEvents.add(false); // we will emit another droplet here
},
)
],
);
}
#override
void dispose() {
thisStreamWillEmitEvents.close(); // close the faucet from this end.
super.dispose();
}
}
I hope that my analogy will help you understand a bit the streams concept.
If you want to open an dialog (instead of what you call a "panel") you can simply give the selected data back when you close the dialog again.
You can find a good tutorial here: https://medium.com/#nils.backe/flutter-alert-dialogs-9b0bb9b01d28
you can navigate and return data from another screen like that :
import 'package:flutter/material.dart';
void main() {
runApp(MaterialApp(
title: 'Returning Data',
home: HomeScreen(),
));
}
class HomeScreen extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Returning Data Demo'),
),
body: Center(child: SelectionButton()),
);
}
}
class SelectionButton extends StatelessWidget {
#override
Widget build(BuildContext context) {
return RaisedButton(
onPressed: () {
_navigateAndDisplaySelection(context);
},
child: Text('Pick an option, any option!'),
);
}
// A method that launches the SelectionScreen and awaits the result from
// Navigator.pop!
_navigateAndDisplaySelection(BuildContext context) async {
// Navigator.push returns a Future that will complete after we call
// Navigator.pop on the Selection Screen!
final result = await Navigator.push(
context,
MaterialPageRoute(builder: (context) => SelectionScreen()),
);
// After the Selection Screen returns a result, hide any previous snackbars
// and show the new result!
Scaffold.of(context)
..removeCurrentSnackBar()
..showSnackBar(SnackBar(content: Text("$result")));
}
}
class SelectionScreen extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Pick an option'),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Padding(
padding: const EdgeInsets.all(8.0),
child: RaisedButton(
onPressed: () {
// Close the screen and return "Yep!" as the result
Navigator.pop(context, 'Yep!');
},
child: Text('Yep!'),
),
),
Padding(
padding: const EdgeInsets.all(8.0),
child: RaisedButton(
onPressed: () {
// Close the screen and return "Nope!" as the result
Navigator.pop(context, 'Nope.');
},
child: Text('Nope.'),
),
)
],
),
),
);
}
}
for more details about navigation:
https://flutter.dev/docs/cookbook/navigation/returning-data
For various reasons, sometimes the build method of my widgets is called again.
I know that it happens because a parent updated. But this causes undesired effects.
A typical situation where it causes problems is when using FutureBuilder this way:
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: httpCall(),
builder: (context, snapshot) {
// create some layout here
},
);
}
In this example, if the build method were to be called again, it would trigger another HTTP request. Which is undesired.
Considering this, how to deal with the unwanted build? Is there any way to prevent a build call?
The build method is designed in such a way that it should be pure/without side effects. This is because many external factors can trigger a new widget build, such as:
Route pop/push
Screen resize, usually due to keyboard appearance or orientation change
The parent widget recreated its child
An InheritedWidget the widget depends on (Class.of(context) pattern) change
This means that the build method should not trigger an http call or modify any state.
How is this related to the question?
The problem you are facing is that your build method has side effects/is not pure, making extraneous build calls troublesome.
Instead of preventing build calls, you should make your build method pure, so that it can be called anytime without impact.
In the case of your example, you'd transform your widget into a StatefulWidget then extract that HTTP call to the initState of your State:
class Example extends StatefulWidget {
#override
_ExampleState createState() => _ExampleState();
}
class _ExampleState extends State<Example> {
Future<int> future;
#override
void initState() {
future = Future.value(42);
super.initState();
}
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: future,
builder: (context, snapshot) {
// create some layout here
},
);
}
}
I know this already. I came here because I really want to optimize rebuilds
It is also possible to make a widget capable of rebuilding without forcing its children to build too.
When the instance of a widget stays the same; Flutter purposefully won't rebuild children. It implies that you can cache parts of your widget tree to prevent unnecessary rebuilds.
The easiest way is to use dart const constructors:
#override
Widget build(BuildContext context) {
return const DecoratedBox(
decoration: BoxDecoration(),
child: Text("Hello World"),
);
}
Thanks to that const keyword, the instance of DecoratedBox will stay the same even if the build was called hundreds of times.
But you can achieve the same result manually:
#override
Widget build(BuildContext context) {
final subtree = MyWidget(
child: Text("Hello World")
);
return StreamBuilder<String>(
stream: stream,
initialData: "Foo",
builder: (context, snapshot) {
return Column(
children: <Widget>[
Text(snapshot.data),
subtree,
],
);
},
);
}
In this example when StreamBuilder is notified of new values, subtree won't rebuild even if the StreamBuilder/Column does.
It happens because, thanks to the closure, the instance of MyWidget didn't change.
This pattern is used a lot in animations. Typical uses are AnimatedBuilder and all transitions such as AlignTransition.
You could also store subtree into a field of your class, although less recommended as it breaks the hot-reload feature.
You can prevent unwanted build calling, using these way
Create child Statefull class for individual small part of UI
Use Provider library, so using it you can stop unwanted build method calling
In these below situation build method call
After calling initState
After calling didUpdateWidget
when setState() is called.
when keyboard is open
when screen orientation changed
If Parent widget is build then child widget also rebuild
Flutter also has ValueListenableBuilder<T> class . It allows you to rebuild only some of the widgets necessary for your purpose and skip the expensive widgets.
you can see the documents here ValueListenableBuilder flutter docs
or just the sample code below:
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:'),
ValueListenableBuilder(
builder: (BuildContext context, int value, Widget child) {
// This builder will only get called when the _counter
// is updated.
return Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: <Widget>[
Text('$value'),
child,
],
);
},
valueListenable: _counter,
// The child parameter is most helpful if the child is
// expensive to build and does not depend on the value from
// the notifier.
child: goodJob,
)
],
),
),
floatingActionButton: FloatingActionButton(
child: Icon(Icons.plus_one),
onPressed: () => _counter.value += 1,
),
);
One of the easiest ways to avoid unwanted reBuilds that are caused usually by calling setState() in order to update only a specific Widget and not refreshing the whole page, is to cut that part of your code and wrap it as an independent Widget in another Stateful class.
For example in following code, Build method of parent page is called over and over by pressing the FAB button:
import 'package:flutter/material.dart';
void main() {
runApp(TestApp());
}
class TestApp extends StatefulWidget {
#override
_TestAppState createState() => _TestAppState();
}
class _TestAppState extends State<TestApp> {
int c = 0;
#override
Widget build(BuildContext context) {
print('build is called');
return MaterialApp(home: Scaffold(
appBar: AppBar(
title: Text('my test app'),
),
body: Center(child:Text('this is a test page')),
floatingActionButton: FloatingActionButton(
onPressed: (){
setState(() {
c++;
});
},
tooltip: 'Increment',
child: Icon(Icons.wb_incandescent_outlined, color: (c % 2) == 0 ? Colors.white : Colors.black)
)
));
}
}
But if you separate the FloatingActionButton widget in another class with its own life cycle, setState() method does not cause the parent class Build method to re-run:
import 'package:flutter/material.dart';
import 'package:flutter_app_mohsen/widgets/my_widget.dart';
void main() {
runApp(TestApp());
}
class TestApp extends StatefulWidget {
#override
_TestAppState createState() => _TestAppState();
}
class _TestAppState extends State<TestApp> {
int c = 0;
#override
Widget build(BuildContext context) {
print('build is called');
return MaterialApp(home: Scaffold(
appBar: AppBar(
title: Text('my test app'),
),
body: Center(child:Text('this is a test page')),
floatingActionButton: MyWidget(number: c)
));
}
}
and the MyWidget class:
import 'package:flutter/material.dart';
class MyWidget extends StatefulWidget {
int number;
MyWidget({this.number});
#override
_MyWidgetState createState() => _MyWidgetState();
}
class _MyWidgetState extends State<MyWidget> {
#override
Widget build(BuildContext context) {
return FloatingActionButton(
onPressed: (){
setState(() {
widget.number++;
});
},
tooltip: 'Increment',
child: Icon(Icons.wb_incandescent_outlined, color: (widget.number % 2) == 0 ? Colors.white : Colors.black)
);
}
}
I just want to share my experience of unwanted widget build mainly due to context but I found a way that is very effective for
Route pop/push
So you need to use Navigator.pushReplacement() so that the context of the previous page has no relation with the upcoming page
Use Navigator.pushReplacement() for navigating from the first page to Second
In second page again we need to use Navigator.pushReplacement()
In appBar we add -
leading: IconButton(
icon: Icon(Icons.arrow_back),
onPressed: () {
Navigator.pushReplacement(
context,
RightToLeft(page: MyHomePage()),
);
},
)
In this way we can optimize our app
You can do something like this:
class Example extends StatefulWidget {
#override
_ExampleState createState() => _ExampleState();
}
class _ExampleState extends State<Example> {
Future<int> future;
#override
void initState() {
future = httpCall();
super.initState();
}
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: future,
builder: (context, snapshot) {
// create some layout here
},
);
}
void refresh(){
setState((){
future = httpCall();
});
}
}