In Flutter i have a simple slider to select latitude and longitude, like this.

 import 'package:flutter/material.dart';
    
    class FormPage extends StatefulWidget {
      const FormPage({Key? key}) : super(key: key);
    
      @override
      _FormPage createState() => _FormPage();
    }
    
    class _FormPage extends State<FormPage> {
      @override
      Widget build(BuildContext context) {
        final _name = TextEditingController();
        final _email = TextEditingController();
        final _password = TextEditingController();
        String _language = 'ES';
        final _city = TextEditingController();
        double _latitude = 90.0;
        double _longitude = 180.0;
    
        final form = Column(mainAxisSize: MainAxisSize.min, children: [
          Text('New user', style: Theme.of(context).textTheme.headline4),
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: TextFormField(
              controller: _name,
              decoration: const InputDecoration(labelText: 'Name'),
            ),
          ),
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: TextFormField(
              controller: _email,
              decoration: const InputDecoration(labelText: 'Email'),
            ),
          ),
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: TextFormField(
              controller: _password,
              obscureText: true,
              enableSuggestions: false,
              autocorrect: false,
              decoration: const InputDecoration(labelText: 'Password'),
            ),
          ),
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: TextFormField(
              controller: _city,
              decoration: const InputDecoration(labelText: 'City'),
            ),
          ),
          DropdownButton<String>(
            value: _language,
            icon: const Icon(Icons.arrow_downward),
            elevation: 16,
            style: const TextStyle(color: Colors.deepPurple),
            underline: Container(
              height: 2,
              color: Colors.deepPurpleAccent,
            ),
            onChanged: (String? newValue) {
              setState(() {
                _language = newValue!;
              });
            },
            items:
                <String>['ES', 'EN'].map<DropdownMenuItem<String>>((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
          ),
          Slider(
            value: _latitude,
            max: 90.0,
            min: -90.0,
            divisions: 1,
            label: _latitude.toString(),
            onChanged: (double newValue) {
              setState(() {
                _latitude = newValue; <- Doesn´t update value
              });
            },
          ),
          Slider(
            value: _longitude,
            max: 180.0,
            min: -180.0,
            divisions: 1,
            label: _longitude.toString(),
            onChanged: (double newValue) {
              setState(() {
                _longitude = newValue; <- Doesn´t update value
              });
            },
          )
        ]);
    
        return Scaffold(
          body: SafeArea(child: form),
          floatingActionButton: FloatingActionButton(
            onPressed: () => null,
            tooltip: 'Save',
            child: const Icon(Icons.check),
          ),
        );
      }
    }

The problem: The slider doesn´t update the state, so can´t change value. So, anybody see what´s wrong?

CodePudding user response：

You have to put your variables outside of you widget build function, otherwise everytime you call setstate your latitude and longtitude are reset to 90 and 180

import 'package:flutter/material.dart';

class FormPage extends StatefulWidget {
  const FormPage({Key? key}) : super(key: key);

  @override
  _FormPage createState() => _FormPage();
}

class _FormPage extends State<FormPage> {
  final _name = TextEditingController();
  final _email = TextEditingController();
  final _password = TextEditingController();
  String _language = 'ES';
  final _city = TextEditingController();
  double _latitude = 90.0;
  double _longitude = 180.0;
  
  @override
  Widget build(BuildContext context) {

    final form = Column(mainAxisSize: MainAxisSize.min, children: [
      Text('New user', style: Theme.of(context).textTheme.headline4),
      Padding(
        padding: const EdgeInsets.all(8.0),
        child: TextFormField(
          controller: _name,
          decoration: const InputDecoration(labelText: 'Name'),
        ),
      ),
      Padding(
        padding: const EdgeInsets.all(8.0),
        child: TextFormField(
          controller: _email,
          decoration: const InputDecoration(labelText: 'Email'),
        ),
      ),
      Padding(
        padding: const EdgeInsets.all(8.0),
        child: TextFormField(
          controller: _password,
          obscureText: true,
          enableSuggestions: false,
          autocorrect: false,
          decoration: const InputDecoration(labelText: 'Password'),
        ),
      ),
      Padding(
        padding: const EdgeInsets.all(8.0),
        child: TextFormField(
          controller: _city,
          decoration: const InputDecoration(labelText: 'City'),
        ),
      ),
      DropdownButton<String>(
        value: _language,
        icon: const Icon(Icons.arrow_downward),
        elevation: 16,
        style: const TextStyle(color: Colors.deepPurple),
        underline: Container(
          height: 2,
          color: Colors.deepPurpleAccent,
        ),
        onChanged: (String? newValue) {
          setState(() {
            _language = newValue!;
          });
        },
        items:
        <String>['ES', 'EN'].map<DropdownMenuItem<String>>((String value) {
          return DropdownMenuItem<String>(
            value: value,
            child: Text(value),
          );
        }).toList(),
      ),
      Slider(
        value: _latitude,
        max: 90.0,
        min: -90.0,
        divisions: 1,
        label: _latitude.toString(),
        onChanged: (double newValue) {
          setState(() {
            _latitude = newValue; <- Doesn´t update value
          });
        },
      ),
      Slider(
        value: _longitude,
        max: 180.0,
        min: -180.0,
        divisions: 1,
        label: _longitude.toString(),
        onChanged: (double newValue) {
          setState(() {
            _longitude = newValue; <- Doesn´t update value
          });
        },
      )
    ]);

    return Scaffold(
      body: SafeArea(child: form),
      floatingActionButton: FloatingActionButton(
        onPressed: () => null,
        tooltip: 'Save',
        child: const Icon(Icons.check),
      ),
    );
  }
}