Flutter - Why slider doesn't update in AlertDialog?

Question

Asked by Helio S on July 10, 2018 (source).

I doing a AlertDialog, so when I tried to insert Slider widget inside the state of value sound realy stranger, and this doesn't happens if Slider is outside of AlertDialog

new Slider(
  onChanged: (double value) {
    setState(() {
      sliderValue = value;
    });
  },
  label: 'Oi',
  divisions: 10,
  min: 0.0,
  max: 10.0,
  value: sliderValue,
)

The complete widget code of AlertDialog

Future<Null> _showDialog() async {
  await showDialog<Null>(
      context: context,
      builder: (BuildContext context) {
        return new AlertDialog(
          title: const Text('Criar novo cartão'),
          actions: <Widget>[
            new FlatButton(onPressed: () {
              Navigator.of(context).pop(null);
            }, child: new Text('Hello'))
          ],
          content: new Container(
            child: new Column(
              mainAxisAlignment: MainAxisAlignment.start,
              mainAxisSize: MainAxisSize.min,
              children: <Widget>[
                new Text('Deseja iniciar um novo cartão com quantos pedidos ja marcados?'),
                new Slider(
              onChanged: (double value) {
                setState(() {
                  sliderValue = value;
                });
              },
              label: 'Oi',
              divisions: 10,
              min: 0.0,
              max: 10.0,
              value: sliderValue,
            )
              ],
            ),
          ),
        );
      }
  );
}

and everything is under State class of StatefullWidget.

Its look like doesn't update the value and when try to change the value keep in same position.

Update 1

The problem is there are 2 required parameters in Slider (onChanged, value), So I shoud update this or UI keep quite, see the video how the aplication is running

Video on Youtube

Update 2

I've also opened a issue to get help with this at Github repository, if someone wants to get more information can go to issue #19323

Answer

Question answered by Rémi R (source).

The problem is that it's not your dialog that holds the state. It's the widget that called showDialog. Same goes for when you call setState, you are calling in on the dialog creator.

The problem is, dialogs are not built inside build method. They are on a different widget tree. So when the dialog creator updates, the dialog won't.

Instead, you should make your dialog stateful. Hold the data inside that dialog. And then use Navigator.pop(context, sliderValue) to send the slider value back to the dialog creator.

The equivalent in your dialog would be

FlatButton(
  onPressed: () => Navigator.of(context).pop(sliderValue),
  child: Text("Hello"),
)

Which you can then catch inside the showDialog result :

final sliderValue = await showDialog<double>(
  context: context,
  builder: (context) => MyDialog(),
)
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