I'm confused about widget.snapshot.data["info"] and snapshot.data["info"]. Can anyone explain me this.
widget.snapshot.data["info"] should be used when a StatefulWidget is used and the snapshot is from the StatefulWidget
Example
class ViewDocuments extends StatefulWidget {
DocumentSnapshot snapshot;
ViewDocuments(this.snapshot);
#override
_ViewDocumentsState createState() => _ViewDocumentsState();
}
class _ViewDocumentsState extends State<ViewDocuments> {
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Text("${widget.snapshot.data["info"]}"),
),
);
}
}
Basically you use widget.--- to access fields in a StatefulWidget class State, it is a way for a StatefulWidget and its State to communicate
For snapshot.data["info"], it should be used in StatelessWidget since it doesn't have a State
For Example
class ViewDocuments extends StatelessWidget {
DocumentSnapshot snapshot;
ViewDocuments(this.snapshot);
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Text("${snapshot.data["info"]}"),
),
);
}
}
I hope this helps you.
Related
I'm using Provider and a ChangeNotifier to access some data across multiple screens in my app. I'm now wanting to implement restoration, so that even if that app dies in the background the user will come back to the same page with the same data. I have managed to make the app go back to the same page, but I can't figure out how to implement a restorable version of my ChangeNotifier. I found a RestorableChangeNotifier class on the Flutter API but am struggling to use it. I've put a simple code below that demonstrates the issue that I'm trying to achieve - any help would be appreciated!
Super simplified example
Class that uses ChangeNotifier, that stores the data:
class ItemClass extends ChangeNotifier {
final List<String> _itemNames = [];
List<String> get itemNames => _itemNames;
void addItem(String newItem) {
_itemNames.add(newItem);
notifyListeners();
}
}
Top level widget with ChangeNotifierProvider:
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return ChangeNotifierProvider(
create: (context) => ItemClass(),
child: const MaterialApp(
home: HomeScreen(),
restorationScopeId: 'root',
),
);
}
}
The screen I want to be at, showing the info from the ItemClass:
class Screen1 extends StatefulWidget {
const Screen1({Key? key}) : super(key: key);
#override
State<Screen1> createState() => _Screen1State();
}
class _Screen1State extends State<Screen1> {
final TextEditingController _textEditingController = TextEditingController();
#override
void dispose() {
_textEditingController.dispose();
super.dispose();
}
#override
Widget build(BuildContext context) {
ItemClass itemClass = context.watch<ItemClass>();
return Scaffold(
appBar: AppBar(),
body: Center(
child: Column(
children: [
Text('Items: ${itemClass.itemNames}'),
TextField(
controller: _textEditingController,
),
TextButton(
child: const Text('Add item'),
onPressed: () =>
itemClass.addItem(_textEditingController.text)),
],
)));
}
}
I don't know where or how to implement the restoration for this ItemClass.
right widget has gesterdetector that adds a String ("ZzZ") to List;
left widget shows all String there in String list by List view Buildder,
right widget adds "ZzZ" to list after pressing the button successfully but it dosent sets ui state...
in android studio after hot reload it shows all added "ZzZ"
import 'package:flutter/material.dart';
List<String> ListOfZzZ=[];
class homescreen extends StatefulWidget {
#override
_homescreenState createState() => _homescreenState();
}
class _homescreenState extends State<homescreen> {
#override
Widget build(BuildContext context) {
return Material(
child: Scaffold(
body: Row(children: [
Expanded(child:RightSidewidget()),
Expanded(child:LeftSidewidget())
],
)),
);
}
}
class RightSidewidget extends StatefulWidget {
#override
_RightSidewidgetState createState() => _RightSidewidgetState();
}
class _RightSidewidgetState extends State<RightSidewidget> {
#override
Widget build(BuildContext context) {
return GestureDetector(
child: Container(child:Text("add new ZzZ"),),
**onTap: (){
setState(() {
ListOfZzZ.add("ZzZ");
});},);**
}
}
class LeftSidewidget extends StatefulWidget {
#override
_LeftSidewidgetState createState() => _LeftSidewidgetState();
}
class _LeftSidewidgetState extends State<LeftSidewidget> {
#override
Widget build(BuildContext context) {
return Container(child:
ListView.builder(
itemCount: ListOfZzZ.length,
itemBuilder: (context,index)=>Text(ListOfZzZ[index])),);
}
}
check the Provider package it can help you achieve what you want, ere is a really good tutorial by the flutter devs showing how to use manage the state of your app and notify widgets of the changes other widgets have.
setState rebuild in very specyfic way. you can read about this in here:
https://api.flutter.dev/flutter/widgets/State/setState.html
in simple world setState call the nearest build (I think this is not full true, but this intuitions works for me)
In your code when you tap right widget and call setState only rightwidget will be rebuild.
So this is the easy solutions:
Make left and right widget statless.
In homescreen in row add gestureDetector(or textButton like in my example) and here call setState. When you do that, all homeSreen will be rebuild so left and right widget too. and your list will be actual. Here is example:
List<String> ListOfZzZ = [];
class homescreen extends StatefulWidget {
#override
_homescreenState createState() => _homescreenState();
}
class _homescreenState extends State<homescreen> {
#override
Widget build(BuildContext context) {
return Material(
child: Scaffold(
body: Row(
children: [
Expanded(
child: TextButton(
onPressed: () => setState(() {
ListOfZzZ.add("ZzZ");
}),
child: RightSidewidget())),
Expanded(child: LeftSideWidget())
],
)),
);
}
}
class RightSidewidget extends StatelessWidget {
const RightSidewidget({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return Container(
color: Colors.amber[50],
child: Text("add new ZzZ"),
);
}
}
class LeftSideWidget extends StatelessWidget {
const LeftSideWidget({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return Container(
child: ListView.builder(
itemCount: ListOfZzZ.length,
itemBuilder: (context, index) => Text(ListOfZzZ[index])),
);
}
}
The hard way, but more elegant and better is to use some state manager like bloc. Here is official site: https://bloclibrary.dev/#/gettingstarted
there is a lot of tutorials and explanations. But this is not solutions for 5 minutes.
Edit: I make some solution with BLoC. I hope this help. I use flutter_bloc and equatable packages in version 7.0.1
void main() {
EquatableConfig.stringify = kDebugMode;
Bloc.observer = SimpleBlocObserver();
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: Scaffold(
appBar: AppBar(
title: Text('myList'),
),
body: BlocProvider(
create: (context) => MylistBloc()..add(AddToList('Start')),
child: Row(
children: [
Expanded(flex: 1, child: buttonsPanel()),
Expanded(flex: 1, child: ListOfZzZ()),
],
),
),
),
);
}
}
class ListOfZzZ extends StatefulWidget {
const ListOfZzZ({Key? key}) : super(key: key);
#override
_ListOfZzZState createState() => _ListOfZzZState();
}
class _ListOfZzZState extends State<ListOfZzZ> {
late MylistBloc _mylistBloc;
#override
Widget build(BuildContext context) {
return BlocBuilder<MylistBloc, MylistState>(
//builder: (context, state) {return ListView.builder(itemBuilder: (BuildContext context,int index){return ListTile(title: state.positions[index];)},);},
builder: (context, state) {
if (state.positions.isEmpty) {
return const Center(child: Text('no posts'));
} else {
return ListView.builder(
itemBuilder: (BuildContext context, int index) {
return ListTile(title: Text(state.positions[index]));
},
itemCount: state.positions.length,
);
}
},
);
}
}
class buttonsPanel extends StatefulWidget {
const buttonsPanel({Key? key}) : super(key: key);
#override
_buttonsPanelState createState() => _buttonsPanelState();
}
class _buttonsPanelState extends State<buttonsPanel> {
late MylistBloc _mylistBloc;
#override
void initState() {
super.initState();
_mylistBloc = context.read<MylistBloc>();
}
#override
Widget build(BuildContext context) {
return Column(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: [
TextButton(
onPressed: () => {_mylistBloc.add(AddToList('Spam'))},
child: Text('Spam')),
TextButton(
onPressed: () => {_mylistBloc.add(AddToList('Ham'))},
child: Text('Ham')),
],
);
}
class SimpleBlocObserver extends BlocObserver {
#override
void onTransition(Bloc bloc, Transition transition) {
super.onTransition(bloc, transition);
print(transition);
}
#override
void onError(BlocBase bloc, Object error, StackTrace stackTrace) {
print(error);
super.onError(bloc, error, stackTrace);
}
}
class MylistState extends Equatable {
final List<String> positions;
final int lenght;
const MylistState({this.positions = const <String>[], this.lenght = 0});
#override
List<Object> get props => [positions];
#override
String toString() => 'Lenght: {$lenght} Positions: {$positions}';
#override
MylistState copyWith(List<String>? positions) {
return MylistState(positions: positions ?? this.positions);
}
}
abstract class MylistEvent extends Equatable {
const MylistEvent();
#override
List<Object> get props => [];
}
class AddToList extends MylistEvent {
final String posToAdd;
#override
AddToList(this.posToAdd);
}
class MylistBloc extends Bloc<MylistEvent, MylistState> {
MylistBloc() : super(MylistState(positions: const <String>[]));
#override
Stream<MylistState> mapEventToState(
MylistEvent event,
) async* {
if (event is AddToList) {
yield await _mapListToState(state, event.posToAdd);
}
}
Future<MylistState> _mapListToState(
MylistState state, String posToAdd) async {
List<String> positions = [];
positions.addAll(state.positions);
positions.add(posToAdd);
return MylistState(positions: positions, lenght: positions.length);
}
}
}
So inherited widget is useful for passing data down the tree, but how do I set that data in the first place if inherited widgets are immutable?
I'm trying to set a phone number for OTP auth and then display that number on another screen. Provider is kind of advanced for me at the moment, how do I approach this?
thank you
You have to rebuild somewhere your InheritedWidget.
You can use any stage management for it, for example you can use StatefulWidget:
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
class MyInheritedWidget extends InheritedWidget {
final int counter;
MyInheritedWidget({Key key, this.counter, Widget child})
: super(key: key, child: child);
#override
bool updateShouldNotify(MyInheritedWidget oldWidget) {
return oldWidget.counter != counter;
}
static MyInheritedWidget of(BuildContext context) {
return context.dependOnInheritedWidgetOfExactType<MyInheritedWidget>();
}
}
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Home(),
);
}
}
class Home extends StatefulWidget {
#override
_HomeState createState() => _HomeState();
}
class _HomeState extends State<Home> {
int _counter = 0;
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: MyInheritedWidget(counter: _counter, child: CounterWidget()),
),
floatingActionButton: FloatingActionButton(
onPressed: () {
setState(() {
_counter++;
});
},
),
);
}
}
class CounterWidget extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Text("${MyInheritedWidget.of(context).counter}",
style: TextStyle(fontSize: 100));
}
}
Firstly you would use a StreamProvider for your stream of data (The same as you would using a StreamBuilder):
class Widget1 extends StatelessWidget {
#override
Widget build(BuildContext context) {
return StreamProvider<User>.value(
value: AuthService().user,
child: Wrapper(),
);
}
}
Next widget has no required data
class Widget2 extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Container(
child: Widget3(),
);
}
}
Access your data via Provider.of
class Widget3 extends StatelessWidget {
#override
Widget build(BuildContext context) {
final user = Provider.of<User>(context);
if (user == null) {
return Login();
} else {
return Dashboard();
}
}
}
With this method, you still need to access the data somewhere down the widget tree. You can't go up, if you want to have the ability to have a widget up the tree listen to something that happens down the tree, you will want to look at ChangeNotifier
Below is the minimal version of what i'm attempting to do with my app that maintains the issue. The slider still responds to being interacted with by showing it's animation, however it does not change the value or actually move. This only seems to occur because the Scaffold has been separated out to it's own widget.
I know that changing it to a stateless widget would fix it in this case, however the version in my app requires state changes.
I've tried adding UniqueKeys up and down the widget tree with no luck, though i'll admit I don't fully understand their use in this case.
Any advice, or a point in the right direction? Should I keep playing around with keys, or am I going about this in the wrong way?
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Scaffold Test',
home: MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage();
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
double _value = 20.0;
#override
Widget build(BuildContext context) {
return MyScaffold(
child: Center(
child: Slider(
min: 0,
max: 100,
value: _value,
onChanged: (double value) {
setState(() {
_value = value;
});
},
),
),
);
}
}
class MyScaffold extends StatefulWidget {
final Widget child;
MyScaffold({this.child});
#override
State<StatefulWidget> createState() => new MyScaffoldState(child);
}
class MyScaffoldState extends State<MyScaffold> {
Widget child;
MyScaffoldState(this.child);
#override
Widget build(BuildContext context) {
return Scaffold(
body: child,
);
}
}
As commented by user #pskink, removing the state constructor fixes the issue.
class MyScaffold extends StatefulWidget {
final Widget child;
MyScaffold({this.child});
#override
State<StatefulWidget> createState() => MyScaffoldState();
}
class MyScaffoldState extends State<MyScaffold> {
#override
Widget build(BuildContext context) {
return Scaffold(
body: widget.child,
);
}
}
In flutter, you should never have constructors for states.
if you need to initialize state properties, use initState.
I've create simple PageView app to test multiple pages.
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(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
#override
Widget build(BuildContext context) {
final firstPage = FirstPage(key: Key("FirstPage"));
final secondPage = SecondPage(key: Key("SecondPage"));
debugPrint("_MyHomePageState.build");
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: PageView(
children: <Widget>[
firstPage,
secondPage,
],
),
);
}
}
class FirstPage extends StatelessWidget {
FirstPage({Key key}) : super(key: key);
#override
Widget build(BuildContext context) {
debugPrint("FirstPage.build");
return Container(
child: Center(
child: Text("First Page"),
),
);
}
}
class SecondPage extends StatelessWidget {
SecondPage({Key key}) : super(key: key);
#override
Widget build(BuildContext context) {
debugPrint("SecondPage.build");
return Container(
child: Center(
child: Text("Second Page"),
),
);
}
}
Even thought _MyHomePageState.build has been shown only once, FirstPage.build and SecondPage.build were printed on every page changes.
What I'd like to prevent unnecessary page draw, how can I accomplish this?
You can achieve so by using
1. const keyword
Make your widgets accept to be const:
class FirstPage extends StatelessWidget {
const FirstPage({Key key}) : super(key: key);
#override
Widget build(BuildContext context) {
debugPrint("FirstPage.build");
return Container(
child: Center(
child: Text("First Page"),
),
);
}
}
and call it with const keyword:
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: PageView(
children: <Widget>[
const firstPage(),
const secondPage(),
],
),
);
2. AutomaticKeepAliveClientMixin
Convert your StatelessWidget to StatefullWidget.
class FirstPage extends StatefulWidget {
FirstPage({Key key}) : super(key: key);
#override
_FirstPageState createState() => _FirstPageState();
}
class _FirstPageState extends State<FirstPage> {
#override
Widget build(BuildContext context) {
debugPrint("FirstPage.build");
return Container(
child: Center(
child: Text("First Page"),
),
);
}
}
Extends AutomaticKeepAliveClientMixin on StatefullWidget created State.
class _FirstPageState extends State<FirstPage> with AutomaticKeepAliveClientMixin {
Call super on the build method.
#override
Widget build(BuildContext context) {
super.build(context);
debugPrint("FirstPage.build");
return Container(
child: Center(
child: Text("First Page"),
),
);
}
Override wantKeepAlive getter with true returned value.
#override
bool get wantKeepAlive => true;
And then your widget tree won't dispose of this widget so it won't rebuild over and over.
Code Example:
class FirstPage extends StatefulWidget {
FirstPage({Key key}) : super(key: key);
#override
_FirstPageState createState() => _FirstPageState();
}
class _FirstPageState extends State<FirstPage>
with AutomaticKeepAliveClientMixin {
#override
Widget build(BuildContext context) {
super.build(context);
debugPrint("FirstPage.build");
return Container(
child: Center(
child: Text("First Page"),
),
);
}
#override
bool get wantKeepAlive => true;
}
3. MVVM Architecture with any State-management solution you like
It will save your state on ViewModel away from the View, so your UI can rebuild itself anytime it wants with no worries about your State because the ViewModel is still the same.
You should always imagine that your build() methods (for both StatefulWidget and StatelessWidget) are being called 60 times per second, so they should be simple and idempotent. Anything else should be moved into a StatefulWidget initState() and friends.
It's easy!
pageController can help you.
Just in your _MyHomePageState
Declare final pageController = PageController(keepPage: false);
And in your PageView
PageView(
controller: pageController,
children: <Widget>[
firstPage,
secondPage,
],
)
Good Luck.