widget communication using void callback flutter - flutter

Question

I have a two widget classes , one is the main. I have a class widget have a slider and I want to change a value in the main widget which its contain a row have this slider widget as a child and that text.
here is what I'm trying to do
class MyHomePage extends StatelessWidget {
final double distance ;
final VoidCallback distanceChanged;
const MyHomePage({#required this.distanceChanged,this.distance});
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Progress Widget'),
centerTitle: true,
),
body:
Column(
children: [
DistanceSlider(),
Container(height: 120,
),
Text('$distance',
),
],
),
);
}
}
and here is the slider widget class
class DistanceSlider extends StatefulWidget {
#override
_DistanceSliderState createState() => _DistanceSliderState();
}
class _DistanceSliderState extends State<DistanceSlider> {
double _currentSliderValue = 10;
#override
Widget build(BuildContext context) {
return
Container(
height: 150,
child:
Card(
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(15),
),
child: Padding(
padding: EdgeInsets.all(5),
child: Column(
children: <Widget> [
Expanded(child:
Padding(padding: EdgeInsets.all(20),
child: Row(
children: [Text('Max Distance',
style: TextStyle(
fontSize: 18,
color: Colors.black26,
),
),
Spacer(),
Text(_currentSliderValue.toInt().toString()+'Kms',
style: TextStyle(
fontSize: 18,
),
),
],
),
),
),
SliderTheme(
data: SliderTheme.of(context).copyWith(
inactiveTrackColor: Colors.black12,
trackHeight: 3.0,
),
child: Slider(
value: _currentSliderValue,
min: 0,
max: 100,
divisions: 5,
// _currentSliderValue.round().toString(),
onChanged: (double value) {
setState(() {
_currentSliderValue = value;
}
);
MyHomePage(distance: value,);
},
),
),
],
),
),
),
);
}
}
how can I change the value of distance in the first class based on the slider value ?

Answer 1

I think what you're looking for is a state management solution. Get started here.

Answer 2

I tend to use value holders (if you mix them with ChangeNotifier you can easily react to changes) that I pass to child widgets.
In your case something like this:
class ValueHolder with ChangeNotifier {
double _value;
double get value => _value;
void set(double newValue) {
_value = newValue;
notifyListeners();
}
}
And then instead of declaring _currentSliderValue in the child widget, pass the ValueHolder to the child and use this instead. In the parent widget, attach a listener to the ValueHolder which calls setState on the parent (or update only the Text widget using a StreamBuilder).