Related
I am fairly new to Flutter and I find myself in a hole I have dug.
What I want to accomplish is quite simple. I have a global variable. Let's say var PlayerPointsToAdd. Then I have this Stateful Widget that should update itself everytime this global variable is not zero. I want to be able to call the addPointsToPlayer() from another widget.
So far my understanding of the Stateful widgets end at the setState((){}) which I can only call inside the Stateful widget itself.
The code below is inside a container inside a Stateful widget. Whenever this button is pressed, it calls the setState according to the new list that PlayerScoreBoard.add() returns.
...
child: RaisedButton(
onPressed: () {
setState(() {
if (PointsToAddPlayer != 0) {
for (int i = 0; i < PointsToAddPlayer; i++) {
PlayerScoreBoard.add(new Icon(
Icons.check_circle_outline,
size: 11));
}
PointsToAddPlayer = 0;
}
I have been reading the documentation and watching the Flutter videos on YouTube so please do not dismiss the question. I am genuinely having trouble understanding this.
I figure I have to use Streams to do this. They seem a bit complicated, but if that's the only way I will use them. Does this mean that I should create a global variable, preferably a class with variables in it and make it stream it's variables to every other widget that needs to update when the variables update. So therefore this ScoreBoard widget must listen to the Stream coming from the global class and update itself. Will this allow me to change the variables in a global class and update the respective widgets to their new states automatically? And does this mean that every widget that relies on variables outside itself must be listening to streams? I would really appreciate an ELI5 to the relationship between Stateful Widgets and Streams.
And because this global data will be updated many times over and over again, using a Future is not possible.
Thank you very much.
I mistakenly said a global class variable. I meant to say something like a Map, where I can have something like gameData.PlayerPoints, or gameData.turn.
You can try ValueListenableBuilder.
import 'package:flutter/material.dart';
final playerPointsToAdd =
ValueNotifier<int>(0); //TODO 1st: ValueNotifier declaration
void main() => runApp(TestApp());
class TestApp extends StatelessWidget {
#override
Widget build(BuildContext context) => MaterialApp(
routes: {
'/': (context) => HomePage(),
},
);
}
//TODO you can use StatelessWidget either
class HomePage extends StatefulWidget {
#override
_HomePageState createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
#override
Widget build(BuildContext context) => Scaffold(
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
RaisedButton(
// TODO 3rd: change the value
onPressed: () => playerPointsToAdd.value++,
child: Text('Increase'),
),
ValueListenableBuilder(
//TODO 2nd: listen playerPointsToAdd
valueListenable: playerPointsToAdd,
builder: (context, value, widget) {
//TODO here you can setState or whatever you need
return Text(
//TODO e.g.: create condition with playerPointsToAdd's value
value == 0
? 'playerPointsToAdd equals 0 right now'
: value.toString());
},
),
RaisedButton(
onPressed: () => playerPointsToAdd.value--,
child: Text('Decrease'),
),
],
),
),
);
}
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();
});
}
}
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();
});
}
}
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();
});
}
}
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();
});
}
}