Global key is not restoring state sometimes of child widget - flutter

Question

Basically, in this example, I am using global key to restore value of by child,
This work,
But sometimes, it appears that the global key does not restore the value of the child items.
To make it clear, I have added a checkbox widget and a slider widget to the first and last pages of this code (green and red).
When I change the slider value on the first page and try to see it on the third page, it works fine.
Same thing if I change checkbox value (like true) on first page and then click on third page it shows mostly true, but sometimes does not.
I am unable to understand the reason behind this issue. Please refer to the following gif to better understand my question.
import 'package:flutter/material.dart';
void main() {
runApp(MaterialApp(home: MyApp()));
}
class MyApp extends StatelessWidget {
final _key = GlobalKey();
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text("Global Key Example")),
body: PageView.builder(
itemCount: 3,
itemBuilder: (context, index) {
switch (index) {
case 0:
return Container(
color: Colors.green,
child: ActionPage(_key),
);
break;
case 1:
return Container(
color: Colors.blue,
child: Text("Blank Page"),
);
break;
case 2:
return Container(
color: Colors.red,
child: ActionPage(_key),
);
break;
default:
throw "Not found";
}
},
),
);
}
}
class ActionPage extends StatefulWidget {
#override
_ActionPageState createState() => _ActionPageState();
ActionPage(key) : super(key: key);
}
class _ActionPageState extends State<ActionPage> {
bool _switchValue = false;
double _sliderValue = 0.5;
#override
Widget build(BuildContext context) {
return Column(
children: [
Switch(
value: _switchValue,
onChanged: (v) {
setState(() => _switchValue = v);
},
),
Slider(
value: _sliderValue,
onChanged: (v) {
setState(() => _sliderValue = v);
},
)
],
);
}
}

Answer 1

I'd recommened you to not use a GlobalKey here. Make sure to pass the needed data not the widget sate. If you take a look at the documentation the following pitfalls are listed:
GlobalKeys should not be re-created on every build. They should
usually be long-lived objects owned by a State object, for example.
Creating a new GlobalKey on every build will throw away the state of
the subtree associated with the old key and create a new fresh subtree
for the new key. Besides harming performance, this can also cause
unexpected behavior in widgets in the subtree. For example, a
GestureDetector in the subtree will be unable to track ongoing
gestures since it will be recreated on each build.
Instead, a good practice is to let a State object own the GlobalKey,
and instantiate it outside the build method, such as in
State.initState.
So you have to make sure that two widgets with the same key are never rendered on the screen at the same time. With a PageView this obviously can happen since you can display (eventhough there might be a transition state) two of your widgets with the same key at the same time. This can mess things up. Also they changed quite a bit in the past implementation wise if I recall correctly.
You should either pass the value directly to your widget or use something like an InheritedWidget to be safe.
Generally speaking I'd always try to avoid using GlobalKeys because they are quite complex and can have side effects which might not be always fully comprehensible. Also there are good alternatives.