I have a simple question. I have a list of three integers. In my app, I have a floating action button. Each time when button is pressed the 'next' element of list is displayed. I have to reset to the first element once the looping is completed. I achieved this in a hard way;
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
home: MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
List nums = [1, 2, 3];
void _incrementCounter() {
if (_counter <= 1) {
setState(() {
_counter++;
});
} else {
setState(() {
_counter = 0;
});
}
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Text('${nums[_counter]}'),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
child: Icon(Icons.add),
),
);
}
}
I wish to know if there are some easier ways to achieve this. Thanks in advance for any help.
You can use remainder (%) to get back to the first index after a loop:
void _incrementCounter() {
setState(() {
_counter = (_counter + 1) % nums.length;
});
}
Related
I have a registration form in my application and I want to check if the DropDown value is empty or not. So I will give a warning to the screen. But I couldn't use DropDown value on checkFieldStatus function. How can I get this?
These are my codes that i used for my app:
class Register extends StatefulWidget {
#override
_RegisterState createState() => _RegisterState();
}
class _RegisterState extends State<Register> {
List listGender = ["Erkek", "Kız"];
List listTeacher = ["Oğulcan Baybars", "Kübra Yeşilkazak"];
final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
String genderHolder;
String teacherHolder;
var _imageFile = null;
#override
Widget build(BuildContext context) {
return SingleChildScrollView(
child: Form(
key: _formKey,
child: Column(
children: [
CustomDropDownField(
list: listGender,
hintText: "Cinsiyet",
value: genderHolder,
),
CustomDropDownField(
list: listTeacher,
hintText: "Öğretmeniniz",
value: teacherHolder,
),
ElevatedButton(
onPressed: () {
checkFieldStatus();
},
child: Text("Kayıt Ol")),
],
),
),
);
}
Future<void> checkFieldStatus() async {
if (_imageFile != null) {
showDialog(
context: context,
builder: (context) {
return ErrorAlertDialog(
message: "Resim yüklendi",
);
});
} else {
**Where I want to do the checks**
? registerUser()
: displayDialog("Lütfen formdaki bütün alanları doldurun.";
}
}
}
My CustomDropDownField like this:
import 'package:flutter/material.dart';
class CustomDropDownField extends StatefulWidget {
final List list;
final String hintText;
String value;
CustomDropDownField({
Key key,
this.list,
this.hintText,
this.value,
}) : super(key: key);
#override
_CustomDropDownFieldState createState() => _CustomDropDownFieldState();
}
class _CustomDropDownFieldState extends State<CustomDropDownField> {
#override
Widget build(BuildContext context) {
return Container(
child: DropdownButton(
isExpanded: true,
hint: Text(widget.hintText),
items: widget.list.map((valueItem) {
return DropdownMenuItem(value: valueItem, child: Text(valueItem));
}).toList(),
value: widget.value,
onChanged: (newValue) {
setState(() {
widget.value = newValue;
});}),);}
CustomDropDownField only changes the String value in its own state it does not reflect to the _RegisterState screen you can do a few different things:
Pass a callback function that updates the value in the _RegisterState screen
or even better
Use a state management like Provider or Bloc to update the value.
Hi how i can Load this list in a ListView or ListViebuilder?
Future<List<bool>> getBoolList() async{
List<bool> prefList = [];
var sharedPreferences = await SharedPreferences.getInstance();
Set<String> keys = sharedPreferences.getKeys();
for(int i=0; i<keys.length ; i++){
bool value = sharedPreferences.getBool(keys.elementAt(i));
prefList.add(value);
}
return prefList;
}
List<bool> list = await getBoolList();
how I got there
Flutter SharedPreferences how to load all saved?
Edit: my favorite.dart
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
// ignore: must_be_immutable
class Favoriten extends StatefulWidget {
#override
_FavoritenState createState() => _FavoritenState();
}
class _FavoritenState extends State<Favoriten> {
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Favorites'),
),
body: // MyList
);
}
}
You have to use stream builder. It observes the stream. (Best way to implement is using bloc pattern)
class Favoriten extends StatefulWidget {
#override
_FavoritenState createState() => _FavoritenState();
}
class _FavoritenState extends State<Favoriten> {
final _boolList = PublishSubject<List<bool>>();
Observable<List<bool>> get boolList => _boolList.stream;
loadList() async{
List<bool> prefList = [];
var sharedPreferences = await SharedPreferences.getInstance();
Set<String> keys = sharedPreferences.getKeys();
for(int i=0; i<keys.length ; i++){
bool value = sharedPreferences.getBool(keys.elementAt(i));
prefList.add(value);
}
_boolList.sink.add(prefList);
}
#override
Widget build(BuildContext context) {
loadList();
return StreamBuilder(
stream: boolList,
builder: (context, snapshot) {
if (snapshot.hasData) {
return root(snapshot.data);
} else {
return Container(
child: YourLoader(),// display loader
);
}
}
);
}
Widget root(List<bool> list){
return ListView.builder(
itemBuilder: (context, index) {
return Container(); // your design here
}
itemCount: list.length,
);
}
}
Note :- You have to add rxdart: ^0.22.0 plugin in your pubspec.yaml
and then import 'package:rxdart/rxdart.dart';
What I've already done
I have a stateful widget that generates a ListView on screen.
class AppListView extends StatefulWidget {
final ValueChanged onChange;
final List<MatchList> matchList;
final ValueChanged finalBetList;
AppListView({this.onChange, this.matchList, this.finalBetList});
#override
_AppListViewState createState() => _AppListViewState();
}
class _AppListViewState extends State<AppListView> {
int counter = 0;
List<MatchList> betList = List<MatchList>();
I have a Home Screen that calls this Stateful Widget. In here I am using a callback function (onChange) to get the counter value from the widget. Everything is working perfect.
Stack(children: [
AppListView(
matchList: matchList,
//callback function brings the counter value from ListView class
onChange: (value) {
setState(() {
counter = value;
});
},
),
Positioned(
child: Padding(
padding: const EdgeInsets.only(top: 280.0, left: 330.0),
child: Container(
width: 60,
height: 60,
child: FloatingActionButton(
onPressed: () {
counter > 0
? Navigator.pushNamed(context, BetScreen.id)
: print('you shall not pass');
},
child: Text(
counter.toString(),
style: kTextStyleAppBarTitle,
),
),
),
),
)
])
What is the problem
But when I am trying to call that widget from another screen with a similar callback function(finalBetList), I got "The method 'call' was called on null. Receiver: null" error. Actually everything that I do is the same as the other example that works fine. I can't find what I'm missing. Is it something about Lists?
class _BetScreenState extends State<BetScreen> {
List<MatchList> betList = List<MatchList>();
int counter = 0;
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: CustomAppBar(
currentBalance: '\u0024' + '200.00',
userLevel: 'Level 30',
userName: 'Mark',
),
body: Container(
child: AppListView(
finalBetList: (value) {
setState(() {
//counter = value;
betList = value;
//print(betList);
});
},
matchList: betList,
),
),
);
}
}
After Edit
I added the cod of AppListView widget
class AppListView extends StatefulWidget {
final ValueChanged onChange;
final List<MatchList> matchList;
final ValueChanged finalBetList;
AppListView({this.onChange, this.matchList, this.finalBetList});
#override
_AppListViewState createState() => _AppListViewState();
}
class _AppListViewState extends State<AppListView> {
int counter = 0;
List<int> betList = List<int>();
#override
Widget build(BuildContext context) {
children: <Widget>[
AppButton.buildAppButton(
context,
AppButtonType.BETSELECTION,
widget.matchList[index].homeOdds,
kCategoryButtonDimensions,
color: widget.matchList[index].homeSelected
? Colors.yellow
: Colors.white,
onPressed: () {
if (widget.matchList[index].drawSelected ||
widget.matchList[index].awaySelected) {
widget.matchList[index].drawSelected =
false;
widget.matchList[index].awaySelected =
false;
counter--;
//betList part
if (betList.length > 0)
betList
.remove(widget.matchList[index].id);
}
widget.matchList[index].homeSelected =
!widget.matchList[index].homeSelected;
if (widget.matchList[index].homeSelected) {
counter++;
betList.add(widget.matchList[index].id);
} else {
counter--;
if (betList.length > 0)
betList.remove(widget.matchList[index]
.id); //if selected, add to betList to send BetScreen
}
widget.onChange(counter);
print(betList);
widget.finalBetList(betList);
setState(() {});
},
),
Here is my point: I have two pages. The first page has a textFormField and it's value I send to the second screen. In the second screen I want to create a list of what I received from the first page. But the problem is that the list is never created. It just shows the last element received.
import 'package:flutter/material.dart';
class PageTwo extends StatefulWidget {
final String descricao;
const PageTwo({Key key, this.descricao}) : super(key: key);
#override
_PageTwoState createState() => _PageTwoState();
}
class _PageTwoState extends State<PageTwo> {
final List<String> listDescricao = [];
#override
void initState() {
listDescricao.add(widget.descricao);
super.initState();
}
//List images items
Widget imgListItem() {
return ListView.builder(
itemCount: listDescricao.length,
itemBuilder: (BuildContext context, int index) {
return widget.descricao == null
? Container()
: Text(listDescricao[index]);
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: imgListItem(),
);
}
}
As required this is the PAGE ONE:
import 'package:flutter/material.dart';
class PageOne extends StatefulWidget {
#override
_PageOneState createState() => _PageOneState();
}
class _PageOneState extends State<PageOne> {
final GlobalKey<ScaffoldState> _scaffoldkey = new GlobalKey<ScaffoldState>();
final inputDescricaoController = TextEditingController();
//send image
doSend() async {
String descricao = inputDescricaoController.text ??= "";
Navigator.of(context).pushAndRemoveUntil(
MaterialPageRoute(
builder: (BuildContext context) => PageTwo(descricao: descricao)),
(Route<dynamic> route) => false);
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: RaisedButton(
child: Column(
children: <Widget>[
TextField(
controller: inputDescricaoController,
),
Text(
'send',
),
],
),
onPressed: () async {
doSend();
},
),
);
}
}
I am migrating my application from android to flutter and till now I have used ListView in a flutter. my question is, is there any specialized technique to handle a large amount of data in the flutter? for reference, you can look at android RecyclerView. it handles in-memory views and recycles its runtime. so how to achieve functionality like RecyclerView in Flutter? or it's not necessary for the flutter?
The easiest way is to use a ListView.builder without specifying the itemCount parameter.
Here is the simplest example:
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new MaterialApp(
home: new MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
#override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text("Infinite List"),
),
body: ListView.builder(
itemBuilder: (context, index) {
return Text("$index");
},
),
);
}
}
Later, you can enhance this by fetching real data. You could show a 'CircularProgressIndicator' in the last item of the list while waiting for the new data.
body: ListView.builder(
itemBuilder: (context, index) {
if (index < data.length) {
// Show your info
return Text("$index");
} else {
getMoreData();
return Center(child: CircularProgressIndicator());
}
},
itemCount: data.length + 1,
),
You can see that we trick the list by adding an index, and calling for more data when displaying that final index.
getMoreData() would include a call to setState() to force a rebuild and to take into account the new data.
Below is a simple infinite list widget based on chemamolins's answer. It accepts an itemBuilder to build the current item and onRequest callback to request more data when the user scrolls to the bottom.
import 'package:flutter/material.dart';
typedef Future<List<T>> RequestFn<T>(int nextIndex);
typedef Widget ItemBuilder<T>(BuildContext context, T item, int index);
class InifiniteList<T> extends StatefulWidget {
final RequestFn<T> onRequest;
final ItemBuilder<T> itemBuilder;
const InifiniteList(
{Key? key, required this.onRequest, required this.itemBuilder})
: super(key: key);
#override
_InifiniteListState<T> createState() => _InifiniteListState<T>();
}
class _InifiniteListState<T> extends State<InifiniteList<T>> {
List<T> items = [];
bool end = false;
_getMoreItems() async {
final moreItems = await widget.onRequest(items.length);
if (!mounted) return;
if (moreItems.isEmpty) {
setState(() => end = true);
return;
}
setState(() => items = [...items, ...moreItems]);
}
#override
Widget build(BuildContext context) {
return ListView.builder(
itemBuilder: (context, index) {
if (index < items.length) {
return widget.itemBuilder(context, items[index], index);
} else if (index == items.length && end) {
return const Center(child: Text('End of list'));
} else {
_getMoreItems();
return const SizedBox(
height: 80,
child: Center(child: CircularProgressIndicator()),
);
}
},
itemCount: items.length + 1,
);
}
}
Usage
child: InifiniteList<String>(
onRequest: requestItems,
itemBuilder: (context, item, index) => Container(
padding: const EdgeInsets.all(30),
color: index % 2 == 0 ? Colors.purple.shade100 : Colors.lime.shade100,
child: Text(item, style: Theme.of(context).textTheme.headline6),
),
),
// normally this is the place where you request the next batch of items
// on the network.
Future<List<String>> requestItems(int nextIndex) {
const pageSize = 15;
var result = List<String>.generate(pageSize, (i) => "Item: ${nextIndex + i + 1}");
return Future<List<String>>.delayed(
const Duration(milliseconds: 500),
() => result,
);
}
Live Demo
Displaying lists of data is a fundamental pattern for mobile apps. Flutter includes the ListView widget to make working with lists a breeze.
I have solved the issue by doing the following steps
Use the ListView Widget
There are four constructors of ListView Class
You have to use Builder Constructor (ListView.builder)
Builder Constructor is used when you have to make a list of elements on demand
It is appropriate for list views with a large (or infinite) number of children
HERE you can have Solution Video CLICK HERE