I have tried to use the Flutter camera plugin (0.2.1) in combination with a PageView and a BottomNavigationBar, but everytime the page gets switched, a few frames get skipped and the UI freezes for a second.
I've simplified my codebase for this example:
import 'package:flutter/material.dart';
import 'package:camera/camera.dart';
void main() => runApp(new Pages());
class Pages extends StatefulWidget {
#override
_PagesState createState() => _PagesState();
}
class _PagesState extends State<Pages> {
PageController _pageController;
int _page = 0;
#override
void initState() {
_pageController = new PageController();
super.initState();
}
void navTapped(int page) {
_pageController.animateToPage(page,
duration: const Duration(milliseconds: 300), curve: Curves.ease);
}
void onPageChanged(int page) {
setState(() {
this._page = page;
});
}
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: new Text("CameraTest"),
),
body: PageView(
children: <Widget>[Feed(), Camera(), Profile()],
controller: _pageController,
onPageChanged: onPageChanged,
),
bottomNavigationBar: new BottomNavigationBar(
items: [
new BottomNavigationBarItem(
icon: new Icon(Icons.home), title: new Text("Feed")),
new BottomNavigationBarItem(
icon: new Icon(Icons.camera), title: new Text("Capture")),
new BottomNavigationBarItem(
icon: new Icon(Icons.person), title: new Text("Profile"))
],
onTap: navTapped,
currentIndex: _page,
),
),
);
}
}
class Camera extends StatefulWidget {
#override
_CameraState createState() => _CameraState();
}
class _CameraState extends State<Camera> {
List<CameraDescription> _cameras;
CameraController _controller;
initCameras() async{
_cameras = await availableCameras();
_controller = new CameraController(_cameras[0], ResolutionPreset.medium);
await _controller.initialize();
setState(() {});
}
#override
void initState() {
initCameras();
super.initState();
}
#override
void dispose() {
_controller?.dispose();
super.dispose();
}
#override
Widget build(BuildContext context) {
if (_controller == null || !_controller.value.isInitialized) {
return new Center(
child: new Text("Waiting for camera...", style: TextStyle(color: Colors.grey),),
);
}
return new AspectRatio(
aspectRatio: _controller.value.aspectRatio,
child: new CameraPreview(_controller));
}
}
//just placeholder widgets
class Feed extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Center(child: new Text("Feed"));
}
}
class Profile extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Center(child: new Text("Profile"));
}
}
There are basically three pages with the middle one showing a camera-preview
(how it's supposed to look), but on switching to the camera and back from it this happens. This is really annoying since it ruins the user experience and is not smooth at all. The lag appears when calling initCameras() or when disposing the camera-controller. I tried using initCameras() in combination with a FutureBuilder, which didn't help at all, and running the method in a seperate isolate, but platform calls seem to be only allowed on the main isolate. It seems a bit weird to me since opening the camera doesn't need too much cpu power, so an async method should be fine. I am aware there is an image-picker plugin, but I want to have the preview in the app directly. I have also considered to run initCameras() on app start, but i don't want to have the camera running all the time when the user is just using another page of the app.
Is there any way to improve upon initCameras() or perhaps use a different implementation to fix the stuttering? I wouldn't even care if it takes a second to load, but i don't want any frame skips.
I followed the example on the bottom of the camera page.
Tested on physical devices as well as emulators on different Android versions.
I have solved a similar issue by adding a delay before initializing the camera:
Future.delayed(Duration(seconds: 1), () {
initCameras();
});
This way the initCameras() is called after the page navigation animation is completed. You can show a CircularProgressIndicator() to make this delay more user friendly.
I am sure there is a more neat way to work around the issue, but this seems to be the simplest solution.
If you want to use image path in the different page than store image path as the global variable, and use it where you want.
Related
In my flutter app, screen A has no AppBar.
So I call SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle.dark) in build.
After another screen B which has an AppBar was pushed and then popped,
screen A has light status bar.
I'd like the system UI to return to the original setting when the screen is popped.
The reason behind this is the fact that your new screen will have its own lifecycle and thus, might use another color for the status bar.
You can call SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle.dark) in your initState method but that won't trigger after a stacked screen is popped. There are two problems here, you can, however, call that back again after returning from a screen pop(). Simple enough right? Almost there.
When you press the back button on the AppBar widget, will return immediately from your Navigator.of(context).push(someroute), even if the navigation animation is still being rendered from the stacked screen.
To handle this, you can add a little "tweak" that will set the status bar color again after 500 milseconds, that should be enough for the animation to fully complete. So, you'll want something more or less like this:
class HomeScreen extends StatefulWidget {
_HomeScreenState createState() => _HomeScreenState();
}
class _HomeScreenState extends State<HomeScreen> {
#override
void initState() {
_updateAppbar();
super.initState();
}
void _updateAppbar() {
SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle.dark);
}
#override
Widget build(BuildContext context) {
return Container(
child: RaisedButton(
child: Text('Navigate to second screen'),
onPressed: () => Navigator.of(context)
.push(MaterialPageRoute(builder: (BuildContext context) => SecondScreen()))
.whenComplete(() => Future.delayed(Duration(milliseconds: 500)).then((_) => _updateAppbar()))));
}
}
class SecondScreen extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(),
);
}
}
Although this works, I'm still curious to know if someone knows a better way to handle this though and keep a status bar color binding to each screen.
maybe you can wrap the whole page widget with AnnotatedRegion like this:
AnnotatedRegion(
value: _currentStyle,
child: Center(
child: ElevatedButton(
child: const Text('Change Color'),
onPressed: _changeColor,
),
),
);
you can follow the full example here:
https://api.flutter.dev/flutter/services/SystemChrome/setSystemUIOverlayStyle.html
maybe that works
SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle.transparent)
Add this package to your project Need Resume and extends your screen state to ResumableState
import 'package:need_resume/need_resume.dart';
class WelcomeScreen extends StatefulWidget {
final String title;
const WelcomeScreen({Key key, this.title}) : super(key: key);
#override
_WelcomeScreenState createState() => _WelcomeScreenState();
}
class _WelcomeScreenState extends ResumableState<WelcomeScreen> {
#override
void onResume() {
SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle.dark);
super.onResume();
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Column(
children: [],
),
);
}
}
This solutions works as expected with very little changes in the code.
I'm trying to change the state from a different widget in Flutter. For example, in the following example I set the state after a few seconds.
Here is the code for that:
class _MyAppState extends State<MyApp> {
int number = 1;
#override
void initState() {
super.initState();
new Future.delayed(new Duration(seconds: 5)).then((_) {
this.setState(() => number = 2);
print("Changed");
});
}
#override
Widget build(BuildContext context) {
return new Scaffold(
body: new Center(
child: new FlatButton(
color: Colors.blue,
child: new Text("Next Page"),
onPressed: () {
Navigator.of(context).push(new MaterialPageRoute(
builder: (BuildContext context) => new StatefulBuilder(builder: (BuildContext context, setState) =>new MySecondPage(number))
));
},
),
),
);
}
}
I tried using an InheritedWidget, but that won't work unless I wrap it around my top level widget, which is not feasible for what I'm trying to do (the code above is a simplification of what I'm trying to achieve).
Any ideas on what the best way of achieving this is in Flutter?
Avoid this whenever possible. It makes these widgets depends on each others and can make things harder to maintain in the long term.
What you can do instead, is having both widgets share a common Listenable or something similar such as a Stream. Then widgets interact with each other by submitting events.
For easier writing, you can also combine Listenable/Stream with respectively ValueListenableBuilder and StreamBuilder which both do the listening/update part for you.
A quick example with Listenable.
class MyHomePage extends StatelessWidget {
final number = new ValueNotifier(0);
#override
Widget build(BuildContext context) {
return Scaffold(
body: ValueListenableBuilder<int>(
valueListenable: number,
builder: (context, value, child) {
return Center(
child: RaisedButton(
onPressed: () {
number.value++;
},
child: MyWidget(number),
),
);
},
),
);
}
}
class MyWidget extends StatelessWidget {
final ValueListenable<int> number;
MyWidget(this.number);
#override
Widget build(BuildContext context) {
return new Text(number.value.toString());
}
}
Notice here how we have our UI automatically updating when doing number.value++ without ever having to call setState.
Actually the most effective way to do this is using BLoC package in flutter and implement it from the top of the widget tree so all inheriting widgets can use the same bloc. If you have worked with Android before - it works like Android Architecture Components - you separate data and state management from the UI - so you do not setState in the UI, but instead use the block to manage state. So you can set and access the same data - from any widget that inherits from the top widget where the bloc is implemented, for more complex apps, it is very useful.
This is where you can find the package: https://pub.dev/packages/flutter_bloc#-readme-tab-
Write-up: https://www.didierboelens.com/2018/08/reactive-programming-streams-bloc/
And a great tutorial on youtube https://www.youtube.com/watch?v=hTExlt1nJZI&list=PLB6lc7nQ1n4jCBkrirvVGr5b8rC95VAQ5&index=7
I'm currently working on building a Flutter app that will preserve states when navigating from one screen, to another, and back again when utilizing BottomNavigationBar. Just like it works in the Spotify mobile application; if you have navigated down to a certain level in the navigation hierarchy on one of the main screens, changing screen via the bottom navigation bar, and later changing back to the old screen, will preserve where the user were in that hierarchy, including preservation of the state.
I have run my head against the wall, trying various different things without success.
I want to know how I can prevent the pages in pageChooser(), when toggled once the user taps the BottomNavigationBar item, from rebuilding themselves, and instead preserve the state they already found themselves in (the pages are all stateful Widgets).
import 'package:flutter/material.dart';
import './page_plan.dart';
import './page_profile.dart';
import './page_startup_namer.dart';
void main() => runApp(new Recipher());
class Recipher extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new Pages();
}
}
class Pages extends StatefulWidget {
#override
createState() => new PagesState();
}
class PagesState extends State<Pages> {
int pageIndex = 0;
pageChooser() {
switch (this.pageIndex) {
case 0:
return new ProfilePage();
break;
case 1:
return new PlanPage();
break;
case 2:
return new StartUpNamerPage();
break;
default:
return new Container(
child: new Center(
child: new Text(
'No page found by page chooser.',
style: new TextStyle(fontSize: 30.0)
)
),
);
}
}
#override
Widget build(BuildContext context) {
return new MaterialApp(
home: new Scaffold(
body: pageChooser(),
bottomNavigationBar: new BottomNavigationBar(
currentIndex: pageIndex,
onTap: (int tappedIndex) { //Toggle pageChooser and rebuild state with the index that was tapped in bottom navbar
setState(
(){ this.pageIndex = tappedIndex; }
);
},
items: <BottomNavigationBarItem>[
new BottomNavigationBarItem(
title: new Text('Profile'),
icon: new Icon(Icons.account_box)
),
new BottomNavigationBarItem(
title: new Text('Plan'),
icon: new Icon(Icons.calendar_today)
),
new BottomNavigationBarItem(
title: new Text('Startup'),
icon: new Icon(Icons.alarm_on)
)
],
)
)
);
}
}
For keeping state in BottomNavigationBar, you can use IndexedStack
#override
Widget build(BuildContext context) {
return Scaffold(
bottomNavigationBar: BottomNavigationBar(
onTap: (index) {
setState(() {
current_tab = index;
});
},
currentIndex: current_tab,
items: [
BottomNavigationBarItem(
...
),
BottomNavigationBarItem(
...
),
],
),
body: IndexedStack(
children: <Widget>[
PageOne(),
PageTwo(),
],
index: current_tab,
),
);
}
Late to the party, but I've got a simple solution. Use the PageView widget with the AutomaticKeepAliveClinetMixin.
The beauty of it that it doesn't load any tab until you click on it.
The page that includes the BottomNavigationBar:
var _selectedPageIndex;
List<Widget> _pages;
PageController _pageController;
#override
void initState() {
super.initState();
_selectedPageIndex = 0;
_pages = [
//The individual tabs.
];
_pageController = PageController(initialPage: _selectedPageIndex);
}
#override
void dispose() {
_pageController.dispose();
super.dispose();
}
#override
Widget build(BuildContext context) {
...
body: PageView(
controller: _pageController,
physics: NeverScrollableScrollPhysics(),
children: _pages,
),
bottomNavigationBar: BottomNavigationBar(
...
currentIndex: _selectedPageIndex,
onTap: (selectedPageIndex) {
setState(() {
_selectedPageIndex = selectedPageIndex;
_pageController.jumpToPage(selectedPageIndex);
});
},
...
}
The individual tab:
class _HomeState extends State<Home> with AutomaticKeepAliveClientMixin<Home> {
#override
bool get wantKeepAlive => true;
#override
Widget build(BuildContext context) {
//Notice the super-call here.
super.build(context);
...
}
}
I've made a video about it here.
Use AutomaticKeepAliveClientMixin to force your tab content to not be disposed.
class PersistantTab extends StatefulWidget {
#override
_PersistantTabState createState() => _PersistantTabState();
}
class _PersistantTabState extends State<PersistantTab> with AutomaticKeepAliveClientMixin {
#override
Widget build(BuildContext context) {
return Container();
}
// Setting to true will force the tab to never be disposed. This could be dangerous.
#override
bool get wantKeepAlive => true;
}
To make sure your tab does get disposed when it doesn't require to be persisted, make wantKeepAlive return a class variable. You must call updateKeepAlive() to update the keep alive status.
Example with dynamic keep alive:
// class PersistantTab extends StatefulWidget ...
class _PersistantTabState extends State<PersistantTab>
with AutomaticKeepAliveClientMixin {
bool keepAlive = false;
#override
void initState() {
doAsyncStuff();
}
Future doAsyncStuff() async {
keepAlive = true;
updateKeepAlive();
// Keeping alive...
await Future.delayed(Duration(seconds: 10));
keepAlive = false;
updateKeepAlive();
// Can be disposed whenever now.
}
#override
bool get wantKeepAlive => keepAlive;
#override
Widget build(BuildContext context) {
super.build();
return Container();
}
}
Instead of returning new instance every time you run pageChooser, have one instance created and return the same.
Example:
class Pages extends StatefulWidget {
#override
createState() => new PagesState();
}
class PagesState extends State<Pages> {
int pageIndex = 0;
// Create all the pages once and return same instance when required
final ProfilePage _profilePage = new ProfilePage();
final PlanPage _planPage = new PlanPage();
final StartUpNamerPage _startUpNamerPage = new StartUpNamerPage();
Widget pageChooser() {
switch (this.pageIndex) {
case 0:
return _profilePage;
break;
case 1:
return _planPage;
break;
case 2:
return _startUpNamerPage;
break;
default:
return new Container(
child: new Center(
child: new Text(
'No page found by page chooser.',
style: new TextStyle(fontSize: 30.0)
)
),
);
}
}
#override
Widget build(BuildContext context) {
return new MaterialApp(
home: new Scaffold(
body: pageChooser(),
bottomNavigationBar: new BottomNavigationBar(
currentIndex: pageIndex,
onTap: (int tappedIndex) { //Toggle pageChooser and rebuild state with the index that was tapped in bottom navbar
setState(
(){ this.pageIndex = tappedIndex; }
);
},
items: <BottomNavigationBarItem>[
new BottomNavigationBarItem(
title: new Text('Profile'),
icon: new Icon(Icons.account_box)
),
new BottomNavigationBarItem(
title: new Text('Plan'),
icon: new Icon(Icons.calendar_today)
),
new BottomNavigationBarItem(
title: new Text('Startup'),
icon: new Icon(Icons.alarm_on)
)
],
)
)
);
}
}
Or you can make use of widgets like PageView or Stack to achieve the same.
Hope that helps!
Use “IndexedStack Widget” with “Bottom Navigation Bar Widget” to keep state of Screens/pages/Widget
Provide list of Widget to IndexedStack and index of widget you want to show because IndexedStack show single widget from list at one time.
final List<Widget> _children = [
FirstClass(),
SecondClass()
];
Scaffold(
body: IndexedStack(
index: _selectedPage,
children: _children,
),
bottomNavigationBar: BottomNavigationBar(
........
........
),
);
The most convenient way I have found to do so is using PageStorage widget along with PageStorageBucket, which acts as a key value persistent layer.
Go through this article for a beautiful explanation -> https://steemit.com/utopian-io/#tensor/persisting-user-interface-state-and-building-bottom-navigation-bars-in-dart-s-flutter-framework
Do not use IndexStack Widget, because it will instantiate all the tabs together, and suppose if all the tabs are making a network request then the callbacks will be messed up the last API calling tab will probably have the control of the callback.
Use AutomaticKeepAliveClientMixin for your stateful widget it is the simplest way to achieve it without instantiating all the tabs together.
My code had interfaces that were providing the respective responses to the calling tab I implemented it the following way.
Create your stateful widget
class FollowUpsScreen extends StatefulWidget {
FollowUpsScreen();
#override
State<StatefulWidget> createState() {
return FollowUpsScreenState();
}
}
class FollowUpsScreenState extends State<FollowUpsScreen>
with AutomaticKeepAliveClientMixin<FollowUpsScreen>
implements OperationalControls {
#override
Widget build(BuildContext context) {
//do not miss this line
super.build(context);
return .....;
}
#override
bool get wantKeepAlive => true;
}
This solution is based on CupertinoTabScaffold's implementation which won't load screens unnecessary.
import 'package:flutter/material.dart';
enum MainPage { home, profile }
class BottomNavScreen extends StatefulWidget {
const BottomNavScreen({super.key});
#override
State<BottomNavScreen> createState() => _BottomNavScreenState();
}
class _BottomNavScreenState extends State<BottomNavScreen> {
var currentPage = MainPage.home;
#override
Widget build(BuildContext context) {
return Scaffold(
body: PageSwitchingView(
currentPageIndex: MainPage.values.indexOf(currentPage),
pageCount: MainPage.values.length,
pageBuilder: _pageBuilder,
),
bottomNavigationBar: BottomNavigationBar(
currentIndex: MainPage.values.indexOf(currentPage),
onTap: (index) => setState(() => currentPage = MainPage.values[index]),
items: const [
BottomNavigationBarItem(
label: 'Home',
icon: Icon(Icons.home),
),
BottomNavigationBarItem(
label: 'Profile',
icon: Icon(Icons.account_circle),
),
],
),
);
}
Widget _pageBuilder(BuildContext context, int index) {
final page = MainPage.values[index];
switch (page) {
case MainPage.home:
return ...
case MainPage.profile:
return ...
}
}
}
/// A widget laying out multiple pages with only one active page being built
/// at a time and on stage. Off stage pages' animations are stopped.
class PageSwitchingView extends StatefulWidget {
const PageSwitchingView({
super.key,
required this.currentPageIndex,
required this.pageCount,
required this.pageBuilder,
});
final int currentPageIndex;
final int pageCount;
final IndexedWidgetBuilder pageBuilder;
#override
State<PageSwitchingView> createState() => _PageSwitchingViewState();
}
class _PageSwitchingViewState extends State<PageSwitchingView> {
final List<bool> shouldBuildPage = <bool>[];
#override
void initState() {
super.initState();
shouldBuildPage.addAll(List<bool>.filled(widget.pageCount, false));
}
#override
void didUpdateWidget(PageSwitchingView oldWidget) {
super.didUpdateWidget(oldWidget);
// Only partially invalidate the pages cache to avoid breaking the current
// behavior. We assume that the only possible change is either:
// - new pages are appended to the page list, or
// - some trailing pages are removed.
// If the above assumption is not true, some pages may lose their state.
final lengthDiff = widget.pageCount - shouldBuildPage.length;
if (lengthDiff > 0) {
shouldBuildPage.addAll(List<bool>.filled(lengthDiff, false));
} else if (lengthDiff < 0) {
shouldBuildPage.removeRange(widget.pageCount, shouldBuildPage.length);
}
}
#override
Widget build(BuildContext context) {
return Stack(
fit: StackFit.expand,
children: List<Widget>.generate(widget.pageCount, (int index) {
final active = index == widget.currentPageIndex;
shouldBuildPage[index] = active || shouldBuildPage[index];
return HeroMode(
enabled: active,
child: Offstage(
offstage: !active,
child: TickerMode(
enabled: active,
child: Builder(
builder: (BuildContext context) {
return shouldBuildPage[index] ? widget.pageBuilder(context, index) : Container();
},
),
),
),
);
}),
);
}
}
proper way of preserving tabs state in bottom nav bar is by wrapping the whole tree with PageStorage() widget which takes a PageStorageBucket bucket as a required named parameter and for those tabs to which you want to preserve its state pas those respected widgets with PageStorageKey(<str_key>) then you are done !! you can see more details in this ans which i've answered few weeks back on one question : https://stackoverflow.com/a/68620032/11974847
there's other alternatives like IndexedWidget() but you should beware while using it , i've explained y we should be catious while using IndexedWidget() in the given link answer
good luck mate ..
I have to create an app for psychological reaction time testing. For that purpose I have to control exactly when an image is
visible and exactly measure the delay between the onset of the visibility of the image and the onset of the reaction (e.g. tap event).
How can I achieve that in Flutter? Specifically, what is the most efficient way to show and hide several different images in the same place and how can I exactly know the onset of the tap event in relation to the onset of the real and full visibility to the user taking into consideration the frame rate of the device? Is there way to get low-level control over that process. Flutter seems to expose a high-level api, usually.
I have made an attempt that is possibly exactly what you are looking for, my logic is as follows:
Add a listener to the image being presented.
Using Stopwatch class, I notify my object to start counting time once the image is displayed.
When clicking on the correct answer, I stop my Stopwatch to stop counting.
Save my current score, and carry on to the next question.
Note:
In this example, for the sake of simplicity, I did not make an account for which answer is correct and which is not.
I create a new StatelessWidget to hold each question, using PageView.builder might be a good use here as well.
Simple Example:
import 'dart:async';
import 'package:flutter/material.dart';
int score = 0;
class TimeTest extends StatefulWidget {
Widget nextQuestionWidget; //to navigate
String question;
NetworkImage questionImage;
List<String> answers;
TimeTest(
{this.questionImage, this.question, this.answers, this.nextQuestionWidget });
#override
_TimeTestState createState() => new _TimeTestState();
}
class _TimeTestState extends State<TimeTest> {
final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
bool _loading = true;
Stopwatch timer = new Stopwatch();
#override
void initState() {
widget.questionImage.resolve(new ImageConfiguration()).addListener((_, __) {
if (mounted) {
setState(() {
_loading = false;
});
timer.start();
}
});
}
#override
Widget build(BuildContext context) {
return new Scaffold(
key: _scaffoldKey,
appBar: new AppBar(title: new Text("Time Test"),),
body: new Container(
alignment: FractionalOffset.center,
margin: const EdgeInsets.symmetric(vertical: 15.0),
child: new Column(
children: <Widget>[
new Text(widget.question),
new Divider(height: 15.0, color: Colors.blueAccent,),
new CircleAvatar(backgroundImage: widget.questionImage,
backgroundColor: Colors.transparent,),
new Container(height: 15.0,),
new Column(
children: new List.generate(widget.answers.length, (int index) {
return new FlatButton(
onPressed: () {
///TODO
///add conditions for correct or incorrect answers
///and some manipulation on the score
timer.stop();
score = score + timer.elapsedMilliseconds;
print(score);
_scaffoldKey.currentState.showSnackBar(new SnackBar(
content: new Text(
"Your answered this question in ${timer
.elapsedMilliseconds}ms")));
///Hold on before moving to the next question
new Future.delayed(const Duration(seconds: 3), () {
Navigator.of(context).push(new MaterialPageRoute(
builder: (_) => widget.nextQuestionWidget));
});
}, child: new Text(widget.answers[index]),);
}),
)
],
),),
);
}
}
class QuestionOne extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new TimeTest(question: "Which animal is in this photo?",
questionImage: new NetworkImage(
"http://cliparting.com/wp-content/uploads/2016/09/Tiger-free-to-use-clipart.png"),
answers: ["Lion", "Tiger", "Cheetah"],
nextQuestionWidget: new QuestionTwo(),);
}
}
class QuestionTwo extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new TimeTest(question: "Which bird is in this photo?",
questionImage: new NetworkImage(
"http://www.clker.com/cliparts/P/q/7/9/j/q/eagle-hi.png"),
answers: ["Hawk", "Eagle", "Falcon"],
nextQuestionWidget: new ResultPage(),);
}
}
class ResultPage extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(title: new Text("Result"),),
body: new Center(
child: new Text("CONGRATULATIONS! Your score is $score milliseconds"),),
);
}
}
void main() {
runApp(new MaterialApp(home: new QuestionOne()));
}
Is there a way to force Flutter to redraw all widgets (e.g. after locale change)?
Your Widget should have a setState() method, everytime this method is called, the widget is redrawn.
Documentation : Widget setState()
Old question, but here is the solution:
In your build method, call the rebuildAllChildren function and pass it the context:
#override
Widget build(BuildContext context) {
rebuildAllChildren(context);
return ...
}
void rebuildAllChildren(BuildContext context) {
void rebuild(Element el) {
el.markNeedsBuild();
el.visitChildren(rebuild);
}
(context as Element).visitChildren(rebuild);
}
This will visit all children and mark them as needing to rebuild.
If you put this code in the topmost widget in your widgets tree, it will rebuild everything.
Also note you must order that specific widget to rebuild. Also you could have some boolean so that the rebuild of that widget only rebuilds all of its children when you really need it (it's an expensive operation, of course).
IMPORTANT: This is a hack, and you should only do this if you know what you are doing, and have strong reason to do so. One example where this is necessary is in my internationalization package: i18_extension. As Collin Jackson explained in his answer, you are really not supposed to do this in general.
This type of use case, where you have data that children can read but you don't want to explicitly pass the data to the constructor arguments of all your children, usually calls for an InheritedWidget. Flutter will automatically track which widgets depend on the data and rebuild the parts of your tree that have changed. There is a LocaleQuery widget that is designed to handle locale changes, and you can see how it's used in the Stocks example app.
Briefly, here's what Stocks is doing:
Put a callback on root widget (in this case, StocksApp) for handling locale changes. This callback does some work and then returns a customized instance of LocaleQueryData
Register this callback as the onLocaleChanged argument to the MaterialApp constructor
Child widgets that need locale information use LocaleQuery.of(context).
When the locale changes, Flutter only redraws widgets that have dependencies on the locale data.
If you want to track something other than locale changes, you can make your own class that extends InheritedWidget, and include it in the hierarchy near the root of your app. Its parent should be a StatefulWidget with key set to a GlobalKey that accessible to the children. The State of the StatefulWidget should own the data you want to distribute and expose methods for changing it that call setState. If child widgets want change the State's data, they can use the global key to get a pointer to the State (key.currentState) and call methods on it. If they want to read the data, they can call the static of(context) method of your subclass of InheritedWidget and that will tell Flutter that these widgets need to rebuilt whenever your State calls setState.
Refreshing the whole widget tree might be expensive and when you do it in front of the users eyes that wouldn't seem sweet.
so for this purpose flutter 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,
),
);
And also never forget the power of setState(() {});
I explain how to create a custom 'AppBuilder' widget in this post.
https://hillelcoren.com/2018/08/15/flutter-how-to-rebuild-the-entire-app-to-change-the-theme-or-locale/
You can use the widget by wrapping your MaterialApp with it, for example:
Widget build(BuildContext context) {
return AppBuilder(builder: (context) {
return MaterialApp(
...
);
});
}
You can tell the app to rebuild using:
AppBuilder.of(context).rebuild();
Simply Use:
Navigator.popAndPushNamed(context,'/screenname');
Whenever you need to refresh :)
What might work for your use case is using the Navigator to reload the page. I do this when switching between "real" and "demo" mode in my app. Here's an example :
Navigator.of(context).push(
new MaterialPageRoute(
builder: (BuildContext context){
return new SplashPage();
}
)
);
You can replace "new SplashPage()" in the above code with whatever main widget (or screen) you would like to reload. This code can be called from anywhere you have access to a BuildContext (which is most places in the UI).
Just use a Key on one of your high-level widgets, everything below this will lose state:
Key _refreshKey = UniqueKey();
void _handleLocalChanged() => setState((){
_refreshKey = UniqueKey()
});
Widget build(BuildContext context){
return MaterialApp(
key: _refreshKey ,
...
)
}
You could also use a value key like:
return MaterialApp(
key: ValueKey(locale.name)
...
);
Why not just have Flutter.redrawAllWidgetsBecauseISaidSo();? –
TimSim
There kinda is:
Change to key to redraw statefull child widgets.
Jelena Lecic explained it good enough for me on medium.
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key? key}) : super(key: key);
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
var _forceRedraw; // generate the key from this
void _incrementCounter() {
setState(() {
_counter++;
_forceRedraw = Object();
});
}
#override
void initState() {
_forceRedraw = Object();
super.initState();
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
MyStatefullTextWidget(
key: ValueKey(_forceRedraw),
counter: _counter,
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}
class MyStatefullTextWidget extends StatefulWidget {
final int counter;
const MyStatefullTextWidget({
required this.counter,
Key? key,
}) : super(key: key);
#override
_MyStatefullTextWidgetState createState() => _MyStatefullTextWidgetState();
}
class _MyStatefullTextWidgetState extends State<MyStatefullTextWidget> {
#override
Widget build(BuildContext context) {
return Text(
'You have pushed the button this many times:${widget.counter}',
);
}
}
Simply Use:
Navigator.popAndPushNamed(context,'/xxx');
I my case it was enough to reconstruct the item.
Changed:
return child;
}).toList(),
To:
return SetupItemTypeButton(
type: child.type,
icon: child.icon,
active: _selected[i] == true,
onTap: ...,
);
}).toList(),
class SetupItemTypeButton extends StatelessWidget {
final dynamic type;
final String icon;
estureTapCallback onTap;
SetupItemTypeButton({Key? key, required this.type, required this.icon, required this.onTap}) : super(key: key);
#override
Widget build(BuildContext context) {
return Container();
}
}
class SetupItemsGroup extends StatefulWidget {
final List<SetupItemTypeButton> children;
final Function(int index)? onSelect;
SetupItemsGroup({required this.children, this.onSelect});
#override
State<SetupItemsGroup> createState() => _SetupItemsGroupState();
}
class _SetupItemsGroupState extends State<SetupItemsGroup> {
final Map<int, bool> _selected = {};
#override
Widget build(BuildContext context) {
int index = 0;
return Container(
child: GridView.count(
children: widget.children.map((child) {
return SetupItemTypeButton(
type: child.type,
icon: child.icon,
active: _selected[i] == true,
onTap: () {
if (widget.onSelect != null) {
int i = index++;
child.active = _selected[i] == true;
setState(() {
_selected[i] = _selected[i] != true;
child.onTap();
widget.onSelect!(i);
});
}
},
);
}).toList(),
),
);
}
}