Related
I have created a widget with a scaffold and called a widget in it as
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.white,
appBar: AppBar(
title: Text("Everything Store"),
),
body: Register(),
);
}
}
the Register component doesn't have a scaffold in it, but when I try to navigate from the Register widget to another one that also doesn't have a scaffold too, so I used
onPressed: () => {
Navigator.of(context)
.push(MaterialPageRoute(builder: (context) => login_page()))
},
I got an error as " No Material widget found. "
So is there a way to have one scaffold or should I make a scaffold for each widget?
The scaffold is one of the main widgets that helps you build up the UI screen on a device but a screen doesn't necessarily need it. The error that you are getting it might be because you are not passing the Widget class correctly.
Try to replace login_page() with the class name like so Register().
I have a big app and there's multi files with same scaffold tap action to hide the keyboard.
Scaffold(
backgroundColor: Colors.transparent,
body: SafeArea(
child: GestureDetector(
onTap: () {
FocusScope.of(context).requestFocus(new FocusNode());
},
child: Container(
///Textfields
),
),
),
);
Is there way to edit Scaffold widget for the whole app?
short answer, you can not customize your scaffold for the whole app because each scaffold has its own body, but you can customize the theme of the app such as the text colour or the font for the whole app on the main screen using the materialApp widget.
-If you are willing to make something fixed in the whole app like the same app bar or the navbar for the whole app that's possible and you have to create a separate screen for it.
-or if you are using a customized widget a lot, Flutter let you extract it, give it a name and use it again without rewriting the whole code again but by just writing the name you chose for it
In that case you can create your custom scaffold wrapper.
class ScaffoldWrapper extends StatelessWidget {
const ScaffoldWrapper({Key? key, required this.child}) : super(key: key);
final Widget child;
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.transparent,
body: SafeArea(
child: GestureDetector(
onTap: () => FocusScope.of(context).requestFocus(FocusNode()),
child: child,
),
),
);
}
}
Now you can replace all occurrence of your mentioned code block with this..
ScaffoldWrapper(child: Container()); // your widget contents
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();
});
}
}