How to get a value from a dialog in flutter?

What is missing is that you need to return the value when closing the dialog, like this:

return AlertDialog(
  title: Slider(
    value: value,
    min: 0,
    max: 100,
    onChanged: (va) {
      setState(() {
        value = va;
      });
    },
  ),
  actions: [
    ElevatedButton(
        onPressed: () => Navigator.pop(context),
        child: Text('cancel')),
    ElevatedButton(
        onPressed: () => Navigator.pop(context, value),
        child: Text('confirm')),
  ],
);

It will then be returned by the showDialog function as expected as a Future