I think hot reloading is equivalent to calling setState but the following code contradicts this.
My State subclass:
class _FooPageState extends State<FooPage> {
#override
void initState() {
super.initState();
Timer.periodic(Duration(seconds: 1), (_) {
// Calling setState every 1 second.
setState(() {});
});
}
#override
Widget build(BuildContext context) => CustomPaint(painter: FooPainter());
}
My custom painter:
class FooPainter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) => print('paint');
#override
bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}
As you can see after 1s I'm calling setState, however, the print statement in paint method isn't invoked every second. But as soon as I do hot reload using IDE shortcut, it does print the statement.
Note: I know I can return true from shouldRepaint to make the print statement work but that's not the question.
Related
I am under the impression that using AutomaticKeepAliveClientMixin would prevent the states dispose() callback from being called when the Widget isn't visible anymore.
However, I have a situation where dispose() and initState() get called every time I hide/show a Widget, even though I implemented AutomaticKeepAliveClientMixin correctly.
class IdleScreenState extends State<IdleScreen> with AutomaticKeepAliveClientMixin {
#override
void initState() {
super.initState();
print('IdleScreen initState');
}
#override
void dispose() {
print('IdleScreen dispose');
super.dispose();
}
#override
Widget build(BuildContext context) {
super.build(context);
// ...build the page...
}
#override
bool get wantKeepAlive => true;
}
This is how I hide/show this Widget
class MainScreen extends State<MainScreen> with AutomaticKeepAliveClientMixin {
#override
Widget build(BuildContext context) {
super.build(context);
return somecondition ? IdleScreen() : OtherScreen();
}
#override
bool get wantKeepAlive => true;
}
Every time this Widget (screen) is shown, initState()gets called, and every time I hide it, dispose() gets called. It's as if the AutomaticKeepAliveClientMixin has no effect. All other similar issues I could find seem to be due to either missing the wantKeepAlive => true or the super.build(context), but they are 100% there in the code.
I tried supplying a GlobalKey for IdleScreen as well, but that didn't have any effect.
However, if I use an IndexedStack or Offstage to hide/show the widget, it works as expected (initState() and dispose() don't get called when hiding/showing the widget).
IndexedStack(
index: somecondition ? 0 : 1,
children: [
IdleScreen(),
OtherScreen()
],
),
Maybe I'm mistaken, but isn't the whole purpose of AutomaticKeepAliveClientMixin to not have to manually keep the widget around using this technique?
This is in a web project, if that matters.
The type argument T is the type of the StatefulWidget subclass of the State into which this class is being mixed.
you have to pass the widget class name like this..
class IdleScreenState extends State<IdleScreen>
with AutomaticKeepAliveClientMixin <IdleScreen> {...
I am seeing this mounted syntax. What is it for? Could you give me sample?
TL;DR: A widget is mounted if it has state. If the widget is no longer mounted, i.e it has been closed or disposed, its state can no longer be updated. Therefore, we check if a widget is mounted to determine its state can still be updated.
Mounting is the process of creating the state of a StatefulWidget and attaching it to a BuildContext.
Take the following example:
class Example extends StatefulWidget {
#override
_ExampleState createState() => _ExampleState();
}
class _ExampleState extends State<Example> {
#override
Widget build(BuildContext context) {
return Container(
);
}
}
The widget is assigned its state (_ExampleState) when the createState() method is called.
As soon as it is assigned its state, the widget becomes mounted.
Why is that important?
When a widget is unmounted in the dispose method of a StatefulWidget, it loses its state. This happens when it is no longer in the tree. I.e, it is has been closed, or no longer exists.
#override
void unmount() {
super.unmount();
state.dispose();
assert(() {
if (state._debugLifecycleState == _StateLifecycle.defunct)
return true;
throw FlutterError.fromParts(<DiagnosticsNode>[
ErrorSummary('${state.runtimeType}.dispose failed to call super.dispose.'),
ErrorDescription(
'dispose() implementations must always call their superclass dispose() method, to ensure '
'that all the resources used by the widget are fully released.'
),
]);
}());
// This is the key
state._element = null;
}
This basically means the state can't be updated and setState can no longer be called. So when you check if a widget is mounted, you're checking if its state can still be updated.
Use case:
Going back to our example Stateful Widget example, let's say we had a number that we wanted to update 30 seconds after the Widget is created.
class Example extends StatefulWidget {
#override
_ExampleState createState() => _ExampleState();
}
class _ExampleState extends State<Example> {
int count = 0;
#override
void initState() {
Future.delayed(const Duration(seconds: 30), () {
setState(() => count = 5);
});
super.initState();
}
#override
void dispose() {
super.dispose();
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Text('count $count'),
));
}
}
Our code will work fine, as long as the widget is disposed of or closed. If it is disposed of, we will get the famous error:
setState() called after dispose()
To prevent this, all we have to do is check if our widget still has state before updating it.
#override
void initState() {
Future.delayed(const Duration(seconds: 30), () {
if (mounted) setState(() => count = 5);
});
super.initState();
}
It represents whether a state is currently in the widget tree.
https://api.flutter.dev/flutter/widgets/State/mounted.html
You shouldn't call setState() on a state that is not currently in the tree.
Edit: The other answer provides a simple example. I should also mention that the described behavior is evident from the StatefulWidget lifecycle: https://flutterbyexample.com/lesson/stateful-widget-lifecycle
It's opinionated, but as far as I can see, it's a rare ocasion when you have to check for mounted, because you unsubscribe from outside events in dispose(). Even the Future from the example could be wrapped in CancelableOperation to cancel it in dispose(), which is before mounted == false
i need some help understanding how to obtain data from inherited widget.
I usually get the parameter from my widget directly from the build method using
#override
Widget build(BuildContext context) {
//THIS METHOD
var data = StateContainer.of(context).data;
return Container(child:Text("${data.parameter}"));
}
But this method cant be called from initState since there is no buildContext yet.
I need in the initState method to have that parameter (i call my fetch from server in that and i need to pass that data to my function), so, how should i do it?
#override
void initState() {
otherData = fetchData(data);
super.initState();
}
I tried using didChangeDipendencies() but it is called every time the view is rebuilt (popping from screen, etc.) so it is not what i want to use and neither the FutureBuilder widget.
Any suggestion?
First, note that you probably do want to use didChangeDependencies. But you can't just do your call there without any check. You need to wrap it in an if first.
A typical didChangeDependencies implementation should look similar to:
Foo foo;
#override
void didChangeDependencies() {
super.didChangeDependencies();
final foo = Foo.of(context);
if (this.foo != foo) {
this.foo = foo;
foo.doSomething();
}
}
Using such code, doSomething will be executed only when foo changes.
Alternatively, if you are lazy and know for sure that your object will never ever change, there's another solution.
To obtain an InheritedWidget, the method typically used is:
BuildContext context;
InheritedWidget foo = context.inheritFromWidgetOfExactType(Foo);
and it is this method that cannot be called inside initState.
But there's another method that does the same thing:
BuildContext context;
InheritedWidget foo = context.ancestorInheritedElementForWidgetOfExactType(Foo)?.widget;
The twist is:
- this method can be called inside initState
- it won't handle the scenario where the value changed.
So if your value never changes, you can use that instead.
1, If you only need InheritedWidget as a Provider of parameter for Widget.
You can using on initState as bellow:
#override
void initState() {
super.initState();
var data = context.ancestorInheritedElementForWidgetOfExactType(type)?.widget;
}
2, If you need listener to re-render widget when data of InheritedWidget change. I suggest you wrapper your StatefulWidget insider a StatelessWidget,
parameter of StatefulWidget is passed from StatelessWidget, when InheritedWidget change data, it will notify to StatelessWidget, on StatefulWidget we will get change on didChangeDependencies and you can refresh data.
This is code guide:
class WrapperDemoWidget extends StatelessWidget {
#override
Widget build(BuildContext context) {
DemoData data = StateContainer.of(context).data;
return Container();
}
}
class ImplementWidget extends StatefulWidget {
DemoData data;
ImplementWidget({this.data});
#override
_ImplementWidgetState createState() => _ImplementWidgetState();
}
class _ImplementWidgetState extends State<ImplementWidget> {
#override
void initState() {
super.initState();
//TODO Do sth with widget.data
}
#override
void didChangeDependencies() {
super.didChangeDependencies();
//TODO Do change with widget.data
}
#override
Widget build(BuildContext context) {
return Container();
}
}
I prefer the solution with didChangeDependencies because Future.delayed solution is a bit hack, looks unprofessional and unhealthy. However, it works out of the box.
This is the solution I prefer:
class _MyAppState extends State<MyApp> {
bool isDataLoaded = false;
#override
void didChangeDependencies() {
if (!isDataLoaded) {
otherData = fetchData(data).then((_){
this.isDataLoaded = true;
});
}
super.didChangeDependencies();
}
...
You can also get the context in initState, try using a future with duration zero. You can find some examples here
void initState() {
super.initState();
Future.delayed(Duration.zero,() {
//use context here
showDialog(context: context, builder: (context) => AlertDialog(
content: Column(
children: <Widget>[
Text('#todo')
],
),
actions: <Widget>[
FlatButton(onPressed: (){
Navigator.pop(context);
}, child: Text('OK')),
],
));
});
}
i use it to make loading screens using inherited widgets and avoid some global variables
I have widget with data that changes regularly and I'm using a Timer.periodic to rebuild the widget. This starts out working smoothly but becomes choppy pretty quickly is there a better way to do this?
class _MainScreenState extends State<MainScreen> {
static const Duration duration = Duration(milliseconds: 16);
update(){
system.updatePos(duration.inMilliseconds/1000);
setState(() {});
}
#override
Widget build(BuildContext context) {
Timer.periodic(duration, (timer){
update();
});
return PositionField(
layoutSize: widget.square,
children: system.map
);
}
}
You are making a big mistake:
The build method must never have any side effects, because it is called again whenever setState is called (or when some higher up widget changes, or when the user rotates the screen...).
Instead, you want to create your Timer in initState, and cancel it on dispose:
class TimerTest extends StatefulWidget {
#override
_TimerTestState createState() => _TimerTestState();
}
class _TimerTestState extends State<TimerTest> {
Timer _timer;
int _foo = 0;
// this is only called once when the widget is attached
#override
void initState() {
super.initState();
_timer = Timer.periodic(Duration(seconds: 1), (timer) => _update());
}
// stop the timer when the widget is detached and destroyed
#override
void dispose() {
_timer.cancel();
super.dispose();
}
void _update() {
setState(() {
_foo++;
});
}
#override
Widget build(BuildContext context) {
return Text('Foo: ${_foo}');
}
}
Since flutter calls the build method many times in different condition, to avoid getting the data many times, I initialize the data in initState.
I want to re-build the widget when the data is ready.
Here is my code :
class Test extends StatefulWidget {
#override
_TestState createState() => new _TestState();
}
class _TestState extends State<Test> {
Data data;
bool dataReady = false;
#override
void initState() {
super.initState();
getData(context).then((Data data) async {
setState(() {
dataReady= true;
});
});
}
#override
Widget build(BuildContext context) {
if (dataReady) {
return createMainContent(context);
} else {
return new Container();
}
}
}
However, it results in following exception :
inheritFromWidgetOfExactType(_InheritedProvider) or inheritFromElement() was called before _TestState.initState() completed.
May I know am I doing something wrong here?
When I add the following line to implementation of getData(context)
await Future.delayed(new Duration(milliseconds: 300));
the exception does not happen.
For everyone coming here at a later point
It is best to use the #override void didChangeDependencies () method of the State class.
From the docs
This method is also called immediately after initState. It is safe to call BuildContext.inheritFromWidgetOfExactType from this method.
But make sure to check if you have already performed your initialization
#override
void didChangeDependencies() {
super.didChangeDependencies();
if (bloc == null) { // or else you end up creating multiple instances in this case.
bloc = BlocProvider<MyBloc>.of(context);
}
}
Edit: Better answer below.
Apparently, you cannot access getData(context) during initState (more concrete: before it completed).
The reason, so I believe, is that getData tries to look up an InheritedWidget ancestor up in the tree, but the tree is just now being built (your widget is created during the parent widget's build).
The obvious solution would be to delay getData's lookup to a later point in time. There are several ways to achieve that:
Delay the lookup to a later time. scheduleMicrotask should work fine.
Look it up during the first build call. You could have an isInitialized field set to false and in you build, something like:
if (!isInitialized) {
isInitialized = true;
// TODO: do the getData(...) stuff
}
an alternative is to put it inside PostFrameCallback which is between initState and Build.
#override
void initState() {
WidgetsBinding.instance.addPostFrameCallback((_) => getData());
super.initState();
}
getData() async {
}
I moved my code to my build method from initState and it worked
class _TestState extends State<Test> {
Data data;
bool dataReady = false;
#override
void initState() {
super.initState();
}
#override
Widget build(BuildContext context) {
getData(context).then((Data data) async {
setState(() {
dataReady= true;
});
});
if (dataReady) {
return createMainContent(context);
} else {
return new Container();
}
}
}