Suppose I have a ScrollController _scrollController;
Is there a way to check ability to scroll during build function?
I tried somthing like this:
Widget build(BuildContext context) {
final hasScroll = MediaQuery.of(context).size.height < scrollController.position.maxScrollExtent;
But as I understand it does't work because _scrollController does't attach during build() function.
Yes, you can't use ScrollController until it is attached to the ListView. You need to use WidgetsBindingObserver's SchedulerBinding's addPostFrameCallback method. Here is the simple example demonstrating that.
class _HomePageState extends State<HomePage> with WidgetsBindingObserver {
ScrollController _scrollController;
#override
void initState() {
super.initState();
_scrollController = ScrollController();
SchedulerBinding.instance.addPostFrameCallback((duration) {
// prints true if scrollable else false
print("isScrollable = ${_scrollController.position.maxScrollExtent != 0}");
});
}
Widget build() {
return ListView.builder(
controller: _scrollController,
...
}
}
Related
If I were using a StatefulWidget, then I would be listening to a Stream for example inside the initState method. Where would I do the equivalent in a StatelessWidget (like to use Bloc with streams for state management)? I could do it in the build method but since these are repetitively I wondered if there is a more efficient way than checking for existent listeners like below. I know that this is a redundant and useless example but it's just to show the problem.
import "package:rxdart/rxdart.dart";
import 'package:flutter/material.dart';
final counter = BehaviorSubject<int>();
final notifier = ValueNotifier<int>(0);
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
if (!counter.hasListener)
counter.listen((value) => notifier.value += value);
return MaterialApp(
home: Scaffold(
body: Center(
child:FlatButton(
onPressed: () => counter.add(1),
child: ValueListenableBuilder(
valueListenable: notifier,
builder: (context, value, child) => Text(
value.toString()
),
),
)
),
)
);
}
}
There is no clean way to have a StatelessWidget listen to a Listenable/Stream.
You will always need a StatefulWidget.
On the other hand, you can use composition to write that StatefulWidget just once, and be done with it.
Common examples for that pattern are widgets such as ValueListenableBuilder, StreamBuilder, or AnimatedBuilder. But it is possible to do the same thing, for listening too.
You'd use it this way:
class Foo extends StatelessWidget {
Foo({Key key, this.counter}): super(key: key);
final ValueListenable<int> counter;
#override
Widget build(BuildContext context) {
return ValueListenableListener(
valueListenable: counter,
onChange: (value) {
// TODO: do something
},
child: Something(),
);
}
}
Where ValueListenableListener is implemented this way:
class ValueListenableListener<T> extends StatefulWidget {
const ValueListenableListener(
{Key key, this.valueListenable, this.onChange, this.child})
: super(key: key);
final ValueListenable<T> valueListenable;
final ValueChanged<T> onChange;
final Widget child;
#override
_ValueListenableListenerState createState() =>
_ValueListenableListenerState();
}
class _ValueListenableListenerState extends State<ValueListenableListener> {
#override
void initState() {
super.initState();
widget.valueListenable?.addListener(_listener);
_listener();
}
#override
void didUpdateWidget(ValueListenableListener oldWidget) {
super.didUpdateWidget(oldWidget);
if (oldWidget.valueListenable != widget.valueListenable) {
oldWidget.valueListenable?.removeListener(_listener);
widget.valueListenable?.addListener(_listener);
_listener();
}
}
#override
void dispose() {
widget.valueListenable?.removeListener(_listener);
super.dispose();
}
void _listener() {
widget.onChange?.call(widget.valueListenable.value);
}
#override
Widget build(BuildContext context) {
return widget.child;
}
}
You shouldn't. Not handling variables that might have their values modified is the very purpose of a Stateless widget:
A stateless widget never changes.
UPDATE:
I think this is a problem of understanding Flutter's state management concepts. This new recommended way by the Flutter team should clear some confusions.
You could do something like this:
class ExampleWidget extends StatelessWidget {
bool _initialized = false;
#override
Widget build(BuildContext context) {
if (!_initialized) {
_initialized = true;
// Add listeners here only once
}
return Container();
}
}
But you shouldn't! In fact, your IDE will give you a warning, because this is not the way to go with Stateless widget as it is marked as #immutable. If you need to use lifecycle methods (like initState()) you should make it a Stateful widget. There's no big deal.
This is achievable with flutter_bloc package. The code to be run in initstate can be added inside BlocListener on whatever state you want.
BlocProvider(
create: (BuildContext context) =>
CategoryBlock()..add(LoadCategories()),
child: BlocListener<CategoryBlock, CategoryStates>(
listener: (context, state) {
//Example to add a listener for listview
if (state is LoadCategoriesSuccess) {
itemPositionsListener.itemPositions.addListener(() {
print(itemPositionsListener.itemPositions.value);
});
}
}
You could have your streams being instantiated in a StatefulWidget and then passed down to your StatelessWidgets as an option, so the parent widget would only have a role of controlling the lifecycle of the stream while the child would be using the stream to update the view.
Regarding the earlier answer:
There's no problem in using StreamBuilders inside your StatelessWidgets since the StreamBuilder itself is a a Widget that extends from StatefulWidget and will take care of it's own state and dispose correctly on its own.
Logging into our Flutter app opens to dashboard that has a Scaffold with a Drawer full of menu items.
I'd like to perform some A/B testing with having the Drawer open on page load or at least animating the Drawer being opened immediately on load.
I'm aware of Scaffold.of(context).openDrawer() but I'm not sure where to place this code so that it will run immediately after the build() method. I'm also not aware of any fields on either Drawer or Scaffold which would load with the Drawer open.
Thanks for your time and help.
You need to wait after the first frame is loaded.
_onLayoutDone(_) {
//your logic here
}
#override
void initState() {
WidgetsBinding.instance.addPostFrameCallback(_onLayoutDone);
super.initState();
}
I wrote a post about this, you can take a look if you want : https://medium.com/#diegoveloper/flutter-widget-size-and-position-b0a9ffed9407
Override initState.
#override
void initState() {
super.initState();
// use this
Timer.run(() => Scaffold.of(context).openDrawer());
}
Store a state variable to hide and show drawer - isDrawerBeingShown.
Based on the state variable toggle the state of drawer. It is set to false by default so it will be displayed for the first time.
void _showDrawer(BuildContext context) async it must be marked as async so that it runs after build method.
Create showDrawerUtility method to show drawer on demand when ever required.
Edit:
Use GlobalKey
GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey();
class MainScreen extends StatefulWidget {
MainScreen({Key key }) : super(key: key);
#override
State<MainScreen> createState() => new MainScreenState();
}
class MainScreenState extends State<MainScreen> {
bool isDrawerBeingShown;
#override
void initState() {
super.initState();
isDrawerBeingShown = false;
_showDrawer(context);
}
void _showDrawer(BuildContext context) async {
if(!isDrawerBeingShown) {
_scaffoldKey.currentState.openDrawer();
setState(() => isDrawerBeingShown = true);
}
}
#override
Widget build(BuildContext context) { // build method goes here}
}
follow my code
import 'package:easy_debounce/easy_debounce.dart';
import 'package:flutter/material.dart';
GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey();
class openDrawerOnLoadPage extends StatefulWidget {
openDrawerOnLoadPage({Key? key}) : super(key: key);
#override
_openDrawerOnLoadPageState createState() => _openDrawerOnLoadPageState();
}
class _openDrawerOnLoadPageState extends State<openDrawerOnLoadPage> {
late bool isDrawerBeingShown;
#override
void initState() {
super.initState();
isDrawerBeingShown = false;
_showDrawer(context);
}
void _showDrawer(BuildContext context) async {
if (!isDrawerBeingShown) {
EasyDebounce.debounce('openDrawer', Duration(milliseconds: 100),
() async {
_scaffoldKey.currentState!.openDrawer();
setState(() => isDrawerBeingShown = true);
});
}
}
#override
Widget build(BuildContext context) {
return Scaffold(
key: _scaffoldKey,
);
}
}
Can someone explain me where I should define a scroll controller? I have chat list view which is the body of a scrollable view. I want to be able to control the scrolling behaviour from MainView but don't know how to pass the controller down to _ChatListView. Any ideas?
mainview.dart
class MainView extends StatelessWidget {
...
// is this the correct place?
final ScrollController scrollController = ScrollController();
#override
Widget build(BuildContext context) {
return new Scaffold(
body: new ChatListView()
);
}
}
chatlistview.dart
class ChatListView extends StatefulWidget {
#override
_ChatListView createState() => _ChatListView();
}
class _ChatListView extends State< ChatListView > {
Widget build(BuildContext context) {
return ListView.builder(
controller: scrollController,
);
}
}
Add a constructor and pass the controller as parameter
class MainView extends StatelessWidget {
...
// is this the correct place?
final ScrollController scrollController = ScrollController();
#override
Widget build(BuildContext context) {
return new Scaffold(
body: new ChatListView(scrollController: scrollController)
);
}
}
class ChatListView extends StatefulWidget {
ChatListView({#required this.scrollController});
final ScrollController scrollController;
#override
_ChatListView createState() => _ChatListView();
}
class _ChatListView extends State< ChatListView > {
Widget build(BuildContext context) {
return ListView.builder(
controller: widget.scrollController,
);
}
}
I would like to be able to run functions once a Widget has finished building/loading but I am unsure how.
My current use case is to check if a user is authenticated and if not, redirect to a login view. I do not want to check before and push either the login view or the main view, it needs to happen after the main view has loaded.
Is there anything I can use to do this?
You could use
https://github.com/slightfoot/flutter_after_layout
which executes a function only one time after the layout is completed.
Or just look at its implementation and add it to your code :-)
Which is basically
void initState() {
super.initState();
WidgetsBinding.instance
.addPostFrameCallback((_) => yourFunction(context));
}
UPDATE: Flutter v1.8.4
Both mentioned codes are working now:
Working:
WidgetsBinding.instance
.addPostFrameCallback((_) => yourFunction(context));
Working
import 'package:flutter/scheduler.dart';
SchedulerBinding.instance.addPostFrameCallback((_) => yourFunction(context));
Best ways of doing this,
1. WidgetsBinding
WidgetsBinding.instance.addPostFrameCallback((_) {
print("WidgetsBinding");
});
2. SchedulerBinding
SchedulerBinding.instance.addPostFrameCallback((_) {
print("SchedulerBinding");
});
It can be called inside initState, both will be called only once after Build widgets done with rendering.
#override
void initState() {
// TODO: implement initState
super.initState();
print("initState");
WidgetsBinding.instance.addPostFrameCallback((_) {
print("WidgetsBinding");
});
SchedulerBinding.instance.addPostFrameCallback((_) {
print("SchedulerBinding");
});
}
both above codes will work the same as both use the similar binding framework.
For the difference find the below link.
https://medium.com/flutterworld/flutter-schedulerbinding-vs-widgetsbinding-149c71cb607f
There are 3 possible ways:
1) WidgetsBinding.instance.addPostFrameCallback((_) => yourFunc(context));
2) Future.delayed(Duration.zero, () => yourFunc(context));
3) Timer.run(() => yourFunc(context));
As for context, I needed it for use in Scaffold.of(context) after all my widgets were rendered.
But in my humble opinion, the best way to do it is this:
void main() async {
WidgetsFlutterBinding.ensureInitialized(); //all widgets are rendered here
await yourFunc();
runApp( MyApp() );
}
Flutter 1.2 - dart 2.2
According with the official guidelines and sources if you want to be certain that also the last frame of your layout was drawned you can write for example:
import 'package:flutter/scheduler.dart';
void initState() {
super.initState();
if (SchedulerBinding.instance.schedulerPhase == SchedulerPhase.persistentCallbacks) {
SchedulerBinding.instance.addPostFrameCallback((_) => yourFunction(context));
}
}
If you are looking for ReactNative's componentDidMount equivalent, Flutter has it. It's not that simple but it's working just the same way. In Flutter, Widgets do not handle their events directly. Instead they use their State object to do that.
class MyWidget extends StatefulWidget{
#override
State<StatefulWidget> createState() => MyState(this);
Widget build(BuildContext context){...} //build layout here
void onLoad(BuildContext context){...} //callback when layout build done
}
class MyState extends State<MyWidget>{
MyWidget widget;
MyState(this.widget);
#override
Widget build(BuildContext context) => widget.build(context);
#override
void initState() => widget.onLoad(context);
}
State.initState immediately will be called once upon screen has finishes rendering the layout. And will never again be called even on hot reload if you're in debug mode, until explicitly reaches time to do so.
In flutter version 1.14.6, Dart version 28.
Below is what worked for me, You simply just need to bundle everything you want to happen after the build method into a separate method or function.
#override
void initState() {
super.initState();
print('hello girl');
WidgetsBinding.instance
.addPostFrameCallback((_) => afterLayoutWidgetBuild());
}
The PostFrameCallback fires before the screen has fully painted. Therefore Devv's answer above was helpful with the added delay to allow the screen to paint.
#override
void initState() {
super.initState();
WidgetsBinding.instance.addPostFrameCallback((_) {
Future.delayed(Duration(seconds: 3), () => yourFunction());
});
}
Try SchedulerBinding,
SchedulerBinding.instance
.addPostFrameCallback((_) => setState(() {
isDataFetched = true;
}));
if you having issue with new SDK and old answer you can try my solution.I have tested it on v3.0.4
WidgetsBinding.instance.endOfFrame.then(
(_) {
if (mounted) {
// do some suff
// you can get width height of specific widget based on GlobalKey
};
},
);
If you don't want to use WidgetsBinding or SchedulerBinding:
Use Future or Timer (easy-peasy)
Future<void> _runsAfterBuild() async {
// This code runs after build ...
}
#override
Widget build(BuildContext context) {
Future(_runsAfterBuild); // <-- Use Future or Timer
return Container();
}
Await a dummy Future
Future<void> _runsAfterBuild() async {
await Future((){}); // <-- Dummy await
// This code runs after build ...
}
#override
Widget build(BuildContext context) {
_runsAfterBuild();
return Container();
}
my english is poor
forgive me
import 'package:flutter/material.dart';
class TestBox extends StatefulWidget {
final Color color;
final Duration delay;
const TestBox({
Key? key,
this.color = Colors.red,
this.delay = const Duration(seconds: 5),
}) : super(key: key);
#override
_TestBoxState createState() => _TestBoxState();
}
class _TestBoxState extends State<TestBox> {
String? label;
#override
void initState() {
initialMembers();
super.initState();
}
void initialMembers() async {
label = await fetchLabel();
if (mounted) setState(() {});
/// don't worry
/// if `(!mounted)`, means wen `build` calld
/// the label already has the newest value
}
Future<String> fetchLabel() async {
await Future.delayed(widget.delay);
print('fetchLabel call');
return 'from fetchLabel()';
}
#override
Widget build(BuildContext context) {
return AnimatedContainer(
margin: EdgeInsets.symmetric(vertical: 12),
duration: Duration(milliseconds: 500),
width: 220,
height: 120,
color: label == null ? Colors.white : widget.color,
child: Center(
child: Text(label ?? 'fetching...'),
),
);
}
}
Column(
children: [
TestBox(
delay: Duration(seconds: 1),
color: Colors.green,
),
TestBox(
delay: Duration(seconds: 3),
color: Colors.yellow,
),
TestBox(
delay: Duration(seconds: 5),
color: Colors.red,
),
],
),
I have a Stateful widget where I use html_editor_enhanced plugin widget. This is the only way to set initial message in it.
class _SendChatMessageState extends State<SendChatMessage> {
final _htmlController = HtmlEditorController();
#override
void initState() {
super.initState();
Future.delayed(const Duration(seconds: 3), () {
_htmlController.setText(widget.chatMessage.message ?? '');
});
}
I tried addPostFrameCallback but it didn't work because a JavaScript generates exception "HTML editor is still loading, please wait before evaluating this JS ..."
another solution that worked pretty well for me is wrapping the function you want to call by Future.delayed() as showen below:
#override
void initState() {
super.initState();
WidgetsBinding.instance.addPostFrameCallback((_) {
Future.delayed(Duration(seconds: 3), () => yourFunction());
});
}
According to this: sample code
I created my own implementation of TabController:
void main() {
runApp(new MyApp());
}
class MyApp extends StatefulWidget {
#override
_MyAppState createState() => new _MyAppState();
}
class _MyAppState extends State<MyApp> {
TabController _tabController;
#override
void initState() {
super.initState();
_tabController = new TabController(vsync: this, length: choices.length);
}
#override
void dispose() {
_tabController.dispose();
super.dispose();
}
#override
Widget build(BuildContext context) {
return new MaterialApp(
home: new Scaffold(
bottomNavigationBar: new Material(
color: Colors.blue,
child: new TabBar(
controller: _tabController,
isScrollable: false,
tabs: choices.map((Choice choice) {
return new Tab(
text: null,
icon: new Icon(choice.icon),
);
}).toList(),
),
),
appBar: new AppBar(
title: const Text('Swap'),
),
body: new TabBarView(
controller: _tabController,
children: choices.map((Choice choice) {
return new Padding(
padding: const EdgeInsets.all(16.0),
child: new ChoiceCard(choice: choice),
);
}).toList(),
),
),
);
}
}
In line: _tabController = new TabController(vsync: this, length: choices.length); I got error this message:
error: The argument type '_MyAppState' can't be assigned to the parameter type 'TickerProvider'. (argument_type_not_assignable at [swap] lib/main.dart:24)
What is wrong with my code?
Add with TickerProviderStateMixin to the end of your State’s class declaration.
Simply add with TickerProviderStateMixin at the end of extends state class as follows:
class _MyAppState extends State<MyApp> with TickerProviderStateMixin {
//...
}
As Answered earlier adding the mixin, TickerProviderStateMixin should do the job or you can also use SingleTickerProviderStateMixin if you only need a Single Ticker.
But what is Does TickerProviders really do?
vsync takes a TickerProvider as an argument , that's why we use SingleTickerProviderStateMixin and as the named describes TickerProvider provides Ticker which simply means it tells our app about the Frame update(or Screen Update), so that our AnimationController can generate a new value and we can redraw the animated widget.
Question is very generic, so need to describe more
Vsync used for
vsync is the property that represents the TickerProvider (i.e., Tick
is similar to clock's tick which means that at every certain duration
TickerProvider will render the class state and redraw the object.)
vsync property is required only on that constructor which requires to render its class state at every certain off-set time when we need to render our components or widgets to redraw and reflect the UI.
vsync can be used with the classes which require certain transition or animation to re-render to draw different objects.
Internal Implementation
TabController({ int initialIndex = 0, #required this.length, #required TickerProvider vsync })
: assert(length != null && length >= 0),
assert(initialIndex != null && initialIndex >= 0 && (length == 0 || initialIndex < length)),
_index = initialIndex,
_previousIndex = initialIndex,
_animationController = AnimationController.unbounded(
value: initialIndex.toDouble(),
vsync: vsync,
);
TabController uses AnimationController internally for the rendering of the tab bar state
Add TickerProviderStateMixin at the end of class state
Here is the full example
#override
_HomeState createState() => _HomeState();
}
class _HomeState extends State<Home> with TickerProviderStateMixin {
MotionTabController? _tabController;
#override
void initState() {
super.initState();
_tabController = new MotionTabController(initialIndex: 1, vsync: this);
}
#override
void dispose() {
super.dispose();
_tabController!.dispose();
}
#override
Widget build(BuildContext context) {
// TODO: implement build
throw UnimplementedError();
}
}
The above answers are correct but you have to declare a tabbar in class and initialize the tabbar from iniState, else the vsync variable doesn't accept 'this'
Following code may help you.
class _MatchesState extends State<Matches> with SingleTickerProviderStateMixin {
TabController? tabController;
#override
void initState() {
tabController = TabController(
length: 2,
vsync: this,
initialIndex: 0,
);
super.initState();
}
In GetX
I found a solution just add with SingleGetTickerProviderMixin to be the full code as the below one:
import 'package:get/get.dart';
import 'package:flutter/material.dart';
class ControllerViewModel extends GetxController with GetSingleTickerProviderStateMixin {
AnimationController _controller;
#override
void onInit() {
// TODO: implement onInit
super.onInit();
_controller = AnimationController(
vsync: this,
duration: const Duration(
milliseconds: 2500,
),
);
}
}
Add any of these SingleTickerProviderStateMixin/ TickerProviderStateMixin mixins at the end of the statement like below:
Eg:
class _ListingViewState extends State with SingleTickerProviderStateMixin { }
just extend with SingleTickerProviderStateMixin in class
you can see here full code