Flutter DorpDownMenu selected item don't change - flutter

Question

in this below code when i try change item on DropDownMenu, selected item don't change and selected item is first item of list
for example after selecting bbbbbbb i have aaaaaaa or selecting ccccccc i have aaaaaaa
import 'package:flutter/material.dart';
void main()=>runApp(
MaterialApp(
home: _MyApp(),
),
);
class _MyApp extends StatefulWidget {
#override
State<StatefulWidget> createState()=>_MyAppState();
}
class _MyAppState extends State<_MyApp> {
SessionsEntity sessionData;
#override
Widget build(BuildContext context) {
List<DropdownMenuItem<SessionsEntity>> _dropdownMenuItems;
_dropdownMenuItems = buildDropdownMenuItems();
sessionData = _dropdownMenuItems[0].value;
return Scaffold(
body: DropdownButtonHideUnderline(
child: Theme(
data: Theme.of(context).copyWith(
canvasColor: Colors.white,
),
child: Container(
child: Center(
child: DropdownButton(
items: _dropdownMenuItems,
isDense: true,
value: sessionData,
onChanged: onChangeDropdownItem,
isExpanded: true,
hint: Text('please select item'),
),
),
),
),
),
);
}
List<DropdownMenuItem<SessionsEntity>> buildDropdownMenuItems() {
List<SessionsEntity> sessions = [
SessionsEntity(1, 'aaaaaaa', 1, 'a-a-a-a-a'),
SessionsEntity(2, 'bbbbbbb', 2, 'b-b-b-b-b'),
SessionsEntity(3, 'ccccccc', 2, 'c-c-c-c-c'),
];
List<DropdownMenuItem<SessionsEntity>> items = List();
for (SessionsEntity session in sessions) {
items.add(
DropdownMenuItem(
value: session,
child: Text(session.sessionName),
),
);
}
return items;
}
onChangeDropdownItem(SessionsEntity selectedSession) {
setState(() {
sessionData = selectedSession;
});
}
}
class SessionsEntity {
final int id;
String sessionName;
int sessionType;
String dateTime;
SessionsEntity(this.id, this.sessionName,this.sessionType, this.dateTime);
}

Answer 1

The problem lies at the below line.
sessionData = _dropdownMenuItems[0].value;
Since your are initializing sessionData variable every time in the build() method, the call to setState() has no effect.
Basically setState() calls the build() method again with the new data but when you initialize sessionData insiside build() it gets overwritten with same value every time build() is called.
You should declare the use the _dropdownMenuItems list as a class variable instead of inside build() and use the initState() method to initialize the _dropdownMenuItems and sessionData variables only once. Here is an example -
class _MyApp extends StatefulWidget {
#override
State<StatefulWidget> createState()=>_MyAppState();
}
class _MyAppState extends State<_MyApp> {
//Remove below line from build method
List<DropdownMenuItem<SessionsEntity>> _dropdownMenuItems;
SessionsEntity sessionData;
#override
void initState() {
super.initState();
_dropdownMenuItems = buildDropdownMenuItems();
sessionData = _dropdownMenuItems[0].value;
}
#override
Widget build(BuildContext context) {
//Rest of the code remains same
}
}

Answer 2

Following will help you. The sample widget is made for selecting a reason.
Sample widget code :
Widget dropdownReasons() {
return new DropdownButton<String>(
value: _currentReason,
items: _dropDownMenuItems,
onChanged: changedDropDownItem);
}
}
The function below handles what to do when the drop down value is changed :
void changedDropDownItem(String selectedReason) {
setState(() => _currentReason = selectedReason; // <-- This is the most important line
}

Answer 3

Try below code, this will work fine
import 'package:flutter/material.dart';
void main() {
runApp(MaterialApp(home: new MyApp(),),
);
}
class MyApp extends StatefulWidget {
#override
_MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
SessionsEntity sessionData;
List<DropdownMenuItem<SessionsEntity>> _dropdownMenuItems;
// Build method is called whenever there is change in the state or ui, so instead of initializing the dropdown menu items and initial selected value of dropdown menu in build method you should initialize it in initState method which is called only once.
#override
void initState() {
// TODO: implement initState
_dropdownMenuItems = buildDropdownMenuItems();
sessionData = _dropdownMenuItems[0].value;
super.initState();
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: DropdownButtonHideUnderline(
child: Theme(
data: Theme.of(context).copyWith(
canvasColor: Colors.white,
),
child: Container(
child: Center(
child: DropdownButton(
items: _dropdownMenuItems,
isDense: true,
value: sessionData,
onChanged: onChangeDropdownItem,
isExpanded: true,
hint: Text('please select item'),
),
),
),
),
),
);
}
List<DropdownMenuItem<SessionsEntity>> buildDropdownMenuItems() {
List<SessionsEntity> sessions = [
SessionsEntity(1, 'aaaaaaa', 1, 'a-a-a-a-a'),
SessionsEntity(2, 'bbbbbbb', 2, 'b-b-b-b-b'),
SessionsEntity(3, 'ccccccc', 2, 'c-c-c-c-c'),
];
List<DropdownMenuItem<SessionsEntity>> items = List();
for (SessionsEntity session in sessions) {
items.add(
DropdownMenuItem(
value: session,
child: Text(session.sessionName),
),
);
}
return items;
}
onChangeDropdownItem(SessionsEntity selectedSession) {
setState(() {
sessionData = selectedSession;
});
}
}
class SessionsEntity {
final int id;
String sessionName;
int sessionType;
String dateTime;
SessionsEntity(this.id, this.sessionName,this.sessionType, this.dateTime);
}