Flutter Slider in BottomNavigationBar does not update - slider

Question

Using Flutter. I create a Slider Widget in a child of a BottomNavigationBar-Widget. There is its onChanged function defined, which should change the value of the Slider.
If I put the Slider in a simple Scaffold Widget it works fine.
How do I make this work in a BottomNavigationBar?
Is there anything missing (a listener/callback or something) I have to declare in the BottomNavigationBar-Widget?
I am using the Flutter stable channel with Android Studio 3.1.4.
It follows the non-working code. There is also a widget whithout the BottomNavigationBar, where the slider works as intended.
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter Issue Slider in BottomNavigationBar',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new SliderInNavigationBar(),
// home: new MyHomePage(),
);
}
}
class SliderInNavigationBar extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return new _SliderInNavigationBarScreenState();
}
}
class _SliderInNavigationBarScreenState extends State<SliderInNavigationBar> {
int _currentIndex = 0;
List<Widget> _children;
int _period = 0;
#override
void initState() {
super.initState();
_children = [
new Slider(
value: _period.toDouble(),
min: 0.0,
max: 100.0,
onChanged: (double value) {
print('OnChanged');
setState(() {
_period = value.round();
});
}),
];
}
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text('Slider in BottomNavigationBar'),
),
body: _children[_currentIndex],
bottomNavigationBar: new BottomNavigationBar(
currentIndex: _currentIndex,
type: BottomNavigationBarType.fixed,
items: [
new BottomNavigationBarItem(
icon: Icon(Icons.timelapse),
title: Text('Page 1'),
),
new BottomNavigationBarItem(
icon: Icon(Icons.message),
title: Text('Page 2'),
),
],
),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key}) : super(key: key);
#override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _period = 0;
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text("Slider in AppBar"),
),
body: new Center(
child: new Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
new Slider(
value: _period.toDouble(),
min: 0.0,
max: 100.0,
onChanged: (double value) {
print('OnChanged');
setState(() {
_period = value.round();
});
}),
],
),
),
);
}
}
Short Flutter doctor output:
flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel beta, v0.5.1, on Microsoft Windows [Version 10.0.17134.228], locale de-DE)
[√] Android toolchain - develop for Android devices (Android SDK 27.0.3)
[√] Android Studio (version 2.3)
[√] Android Studio (version 3.1)
[!] VS Code, 64-bit edition (version 1.24.1)
[√] Connected devices (1 available)
! Doctor found issues in 1 category.

Answer 1

Good afternoon,
You have to declared the Slider directly in the body not as a List<Widget> instantiated in the initState().
The following code should work :)
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter Issue Slider in BottomNavigationBar',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new SliderInNavigationBar(),
// home: new MyHomePage(),
);
}
}
class SliderInNavigationBar extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return new _SliderInNavigationBarScreenState();
}
}
class _SliderInNavigationBarScreenState extends State<SliderInNavigationBar> {
int _currentIndex = 0;
//List<Widget> _children;
int _period = 0;
#override
void initState() {
super.initState();
/**
_children = [
new Slider(
value: _period.toDouble(),
min: 0.0,
max: 100.0,
onChanged: (double value) {
print('OnChanged');
setState(() {
_period = value.round();
});
}),
];**/
}
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text('Slider in BottomNavigationBar'),
),
body: new Slider(
value: _period.toDouble(),
min: 0.0,
max: 100.0,
onChanged: (double value) {
print('OnChanged');
setState(() {
_period = value.round();
});
}),
bottomNavigationBar: new BottomNavigationBar(
currentIndex: _currentIndex,
type: BottomNavigationBarType.fixed,
items: [
new BottomNavigationBarItem(
icon: Icon(Icons.timelapse),
title: Text('Page 1'),
),
new BottomNavigationBarItem(
icon: Icon(Icons.message),
title: Text('Page 2'),
),
],
),
);
}
}
I commented the code that was wrong.