I need to update a widget whenever the widget that's nested to it is updated.
Let's say widget A is nested in widget B, by the way we can have GlobalKey of widget A (if it can help to detect if the widget A is updated). Here, I need to update widget B whenever the widget A is updated, and in order to do it I need to check if the widget A is updated, if it is then I'll update widget B too.
You could do this for example:
import 'package:flutter/material.dart';
void main() {
runApp(const MaterialApp(home: B()));
}
class B extends StatefulWidget {
const B({super.key});
#override
State<B> createState() => BState();
}
class BState extends State<B> {
#override
Widget build(BuildContext context) {
print('B updates');
return Scaffold(
body: Column(
children: [A(updater: update)],
));
}
void update() {
setState(() {
});
}
}
class A extends StatefulWidget {
final Function? updater;
const A({Key? key, this.updater}) : super(key: key);
#override
State<A> createState() => _AState();
}
class _AState extends State<A> {
#override
void setState(VoidCallback fn) {
if (widget.updater != null) widget.updater!();
super.setState(fn);
}
#override
Widget build(BuildContext context) {
return TextButton(
onPressed: () {
setState(() {
print('clicked button');
});
},
child: const Text('click'));
}
}
Whenever the button is clicked it prints both "clicked button" as "B updates". So B rebuilds on each click. If you leave out the setState override in A you will see only the "clicked button" prints.
Related
I have two files
Parent.dart and Chid.dart
Parent.dart:
------------
class Parent extends StatefulWidget {
const Parent ({super.key});
#override
State<Parent > createState() => _ParentState();
}
class _ParentState extends State<Parent> {
#override
Widget build(BuildContext context) {
return IconButton(
icon: Icon(Icons.addbox),
onPressed: (){//Call Method from the child},
),
}
}
and
Child.dart:
------------
class Child extends StatefulWidget {
const Child ({super.key});
#override
State<Child> createState() => _ChildState();
}
class _ChildState extends State<Child> {
void myFunciton()
{
//do something cool.
}
#override
Widget build(BuildContext context) {
return Container();
}
}
I can send data from parent to child and call methods from parent to child but what I was looking for is a way to access a child method from the parent. Is there a way to call the method from the child when the button is pressed?
First you need to change _ChildState to ChildState (remove the underlined) for outside dart file can access it
Then in parent.dart, declare a GlobalKey, pass it to Child widget and call function like this:
class _ParentState extends State<Parent> {
final key = GlobalKey<ChildState>();
#override
Widget build(BuildContext context) {
return Column(
children: [
Child(key: key), //I assume you have a child widget here
IconButton(
icon: Icon(Icons.addbox),
onPressed: () { //Call Method from the child},
key.currentState?.myFunciton();
}
),
],
);
}
}
In flutter,
How can a parent widget know if a child among many children widgets has received focus? For example, Can we know if a child in a Row widget's children has received focus?
Can I detect this focus before the child widget receives it?
It actually depends on your take and which architecture you wanna follow.
This snippet that I'm posting uses NotificationListener, a custom notification and a custom child widget. This might work for an application like a print or a callback, but you might need to change the architecture and use a state management tool to achieve greater things.
Parent Widget class:
class MyParentWidget extends StatelessWidget {
const MyParentWidget({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return NotificationListener<FocusNotification>(
onNotification: (notification) {
print("New widget focused: ${notification.childKey.toString()}");
return true;
},
child: Row(
children: List.generate(
5,
(index) => MyChildWidget(
Key('widget-$index'),
),
),
),
);
}
}
Child Widget class:
class MyChildWidget extends StatefulWidget {
const MyChildWidget(Key key) : super(key: key);
#override
_MyChildWidgetState createState() => _MyChildWidgetState();
}
class _MyChildWidgetState extends State<MyChildWidget> {
final node = FocusNode();
#override
initState() {
node.addListener(() {
if (node.hasFocus) {
final notification = FocusNotification(widget.key!);
notification.dispatch(context);
}
});
super.initState();
}
#override
dispose() {
node.dispose();
super.dispose();
}
#override
Widget build(BuildContext context) {
return TextField(
focusNode: node,
);
}
}
Custom Notification class:
class FocusNotification extends Notification {
const FocusNotification(this.childKey);
final Key childKey;
}
I am making a list of stateless widget as shown below and passing the id as the parameter to the widgets.
Code for cartPage:-
class Cart extends StatefulWidget {
#override
_CartState createState() => _CartState();
}
class _CartState extends State<Cart> {
bool loading=true;
List<CartTile> cartTiles=[];
#override
void initState() {
super.initState();
if(currentUser!=null)
getData();
}
getData()async
{
QuerySnapshot snapshot=await cartReference.doc(currentUser.id).collection('cartItems').limit(5).get();
snapshot.docs.forEach((doc) {
cartTiles.add(CartTile(id: doc.data()['id'],index: cartTiles.length,));
});
setState(() {
loading=false;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.transparent,
body: loading?Center(child:CircularProgressIndicator():SingleChildScrollView(
child: Column(
children: cartTiles,
),
),
);
}
}
Code for CartTile:-
class CartTile extends StatelessWidget {
final String id;
CartTile({this.id,});
#override
Widget build(BuildContext context) {
return StreamBuilder(
stream: productReference.doc(id).snapshots(),
builder: (context,snapshot)
{
//here am using the snapshot to build the cartTile.
},
);
}
}
So, my question is whenever I will call setState in my homepage then will the stateless widget be rebuilt and increase my document reads. Because i read somewhere that when we pass the same arguments or parameters to a stateless widget then due to its cache mechanism it doesn't re build. If it will increase my reads then is there any other way to solve this problem?
I have a Flutter where I display a list of elements in a Column, where the each item in the list is a custom widget. When I update the list, my UI doesn't refresh.
Working sample:
class Test extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return TestState();
}
}
class TestState extends State<Test> {
List<String> list = ["one", "two"];
final refreshKey = new GlobalKey<RefreshIndicatorState>();
#override
Widget build(BuildContext context) {
return Scaffold(
body: Container(
padding: EdgeInsets.all(40),
child: Row(
children: <Widget>[
Container(
child: FlatButton(
child: Text("Update"),
onPressed: () {
print("Updating list");
setState(() {
list = ["three", "four"];
});
},
)
),
Column(
children: list.map((s) => ItemView(s)).toList(),
)
],
),
)
);
}
}
class ItemView extends StatefulWidget {
String s;
ItemView(this.s);
#override
State<StatefulWidget> createState() => ItemViewState(s);
}
class ItemViewState extends State<ItemView> {
String s;
ItemViewState(this.s);
#override
Widget build(BuildContext context) {
return Text(s);
}
}
When I press the "Update" button, my list is updated but the UI is not. I believe this has something to do with using a custom widget (which is also stateful) because when I replace ItemView(s) with the similar Text(s), the UI updates.
I understand that Flutter keeps a track of my stateful widgets and what data is being used, but I'm clearly missing something.
How do I get the UI to update and still use my custom widget?
You should never pass parameters to your State.
Instead, use the widget property.
class ItemView extends StatefulWidget {
String s;
ItemView(this.s);
#override
State<StatefulWidget> createState() => ItemViewState();
}
class ItemViewState extends State<ItemView> {
#override
Widget build(BuildContext context) {
return Text(widget.s);
}
}
I am trying to refresh the parent widget from sub children widget. Actually, there are a number of widgets in between like A uses B and B uses C. I would like to refresh A widget on an event in C widget.I researched a lot but couldn't find an exact answer. A code snipped will be really helpful. Thanks in advance
There are a few solutions:
A pass a callback that does a setState to B, which then pass it to C:
class A extends StatefulWidget {
#override
_AState createState() => _AState();
}
class _AState extends State<A> {
#override
Widget build(BuildContext context) {
return B(
onSomething: () => setState(() {}),
);
}
}
class B extends StatelessWidget {
final VoidCallback onSomething;
const B({Key key, this.onSomething}) : super(key: key);
#override
Widget build(BuildContext context) {
return C(onSomething: onSomething);
}
}
class C extends StatelessWidget {
final VoidCallback onSomething;
const C({Key key, this.onSomething}) : super(key: key);
#override
Widget build(BuildContext context) {
return RaisedButton(
onPressed: onSomething,
);
}
}
use NotificationListener in A, and dispatch a Notification from C:
class MyNotification extends Notification {}
class A extends StatefulWidget {
#override
_AState createState() => _AState();
}
class _AState extends State<A> {
#override
Widget build(BuildContext context) {
return NotificationListener<MyNotification>(
onNotification: (_) {
setState(() {});
},
child: B(),
);
}
}
class C extends StatelessWidget {
#override
Widget build(BuildContext context) {
return RaisedButton(
onPressed: () {
MyNotification().dispatch(context);
},
);
}
}
Why do you want to refresh? You updated any data and wanted the new ones to be displayed?
You could try to use Provider widget. With it you can modify any data and notify everyone interested in that data that it changed.
In your setup you could put the provider on the A widget, on the C widget you could get the value, updated and notify everyone. When doing that, A widget will automatically rebuild with the updated information.
The code would be something like this:
class AppState with ChangeNotifier {
AppState();
YourData _data;
void setData(YourData data) {
_data = data;
notifyListeners();
}
}
ChangeNotifierProvider.value(
value: AppState(),
child: WidgetA()
)
WidgetC() {
Provider.of<AppState>(context).setData(yourChangedDataHere);
}